JPH10178552A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent disappearance of gradation due to saturation in a high density area and to prevent generation of a pseudo contour. SOLUTION: A read part 1 measures a color patch density of a color patch print for correction set on a platen of the read part 1 and sends out current gradation to an image density control part 16. The image density control part 16 calculates a conversion table which is an inverse function of a density of an objective reference patch, after covering the current gradation density and the objective gradation density of the reference patch with the conversion table, compares by matching the density of the reference patch with the density of the objective reference patch to generate correction data to correct the gradation characteristics of the image and to prepare a density conversion table 15. In the case of outputting an image, after an image processing part 2 applies color conversion and gradation conversion processing to original image data from the image read part 1, the image processing part 2 converts the result by using the density conversion table 15 to make the gradation coincide with the objective gradation.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an image forming apparatus for forming an image to be output from input color image data or gradation image data.

[0002]

2. Description of the Related Art Conventionally, in an image forming apparatus for forming an image to be output from input color image data or gradation image data, when outputting the input color image data or gradation image data, In addition, the density (gradation) of the input image data is corrected in order to reproduce the density (gradation) of the input image data more faithfully according to the characteristics of the device (input system or output system).

For example, in Japanese Patent Application Laid-Open No. Hei 5-33667,
LUT (lookup table) including spline interpolation
Is created, and gradation correction is performed using the LUT. Further, as another conventional technique, there is a technique in which the density (gradation) of input image data is checked and gradation correction is performed in order to further improve reproducibility.

In Japanese Patent Application Laid-Open No. Hei 5-14728, in order to improve the reproducibility of input image data at low and high densities, the density value of "0" and A gradation correction table in which the vicinity of "255" is approximated by a polygonal line is provided to correct the density (gradation) of the input image data.

[0005]

However, Japanese Unexamined Patent Publication No. Hei 5-33667 and Japanese Patent Application Laid-Open No. Hei 5-33667 check the density (gradation) of input image data and perform gradation correction. In the prior art, a diffuse reflection type density sensor is used. The density sensor, the output sensitivity for the highlight area
The output sensitivity in the high density range is quite low. Therefore, for example, when a digital value is converted by an A / D converter having an 8-bit resolution, the value in the high-density region is saturated depending on the state of the image forming apparatus, and the gradation in the corresponding range disappears, or There is a problem that a pseudo contour occurs at an end point of the section, which leads to deterioration of image quality.

Further, in the case where the density (gradation) of the input image data is checked and the gradation is corrected, the curve of the relationship between the density (gradation) of the input image data and the target gradation density is greatly curved. If the input / output characteristics have an undulating shape unique to the device, noise erroneous detection will occur in that part, resulting in erroneous correction, and the data check tolerance must be increased. There was a problem that would occur.

Further, Japanese Patent Application Laid-Open No. Hei 5-14728 has a problem that it is not possible to remove various noises contained in the output of the density sensor, which is indispensable for performing high-precision gradation correction. There is a problem that it is not possible to prevent the generation of a pseudo contour at the time of interpolation.

The present invention has been made in view of the above circumstances, and has as its object to provide an image forming apparatus capable of preventing the disappearance of gradation due to saturation in a high-density region and the occurrence of a false contour. .

[0009]

According to a first aspect of the present invention, there is provided a reference patch forming means for forming a reference patch on an image carrier, and a reference patch forming means for forming the reference patch on the image carrier. Storage means for storing the target density value of the reference patch to be read, reading means for reading the density value of the reference patch formed by the reference patch forming means, and reading by the reading means and the target density value stored in the storage means. Calculating means for calculating a value which is an inverse function of the density value of the reference patch obtained, and forming the value based on a difference between the inverse function value of the target density value and the inverse function value of the reference patch calculated by the calculating means. Generating means for generating correction data for correcting the density gradation characteristics of the image to be obtained.

[0010] According to the second aspect of the present invention, in the first aspect,
The image processing apparatus according to claim 1, further comprising: a conversion table that is an inverse function of a relationship between the density value read by the reading unit and the target density value stored in the storage unit. It is characterized in that a value that is an inverse function of the target density value and the density value of the reference patch is calculated.

In order to solve the above-mentioned problems,
In the invention according to claim 6, a reference patch forming means for forming a reference patch on the image carrier, a storage means for storing a target density value of the reference patch formed by the reference patch forming means, and the reference patch forming means Reading means for reading the density value of the reference patch formed by the method, and generating correction data for correcting the density gradation characteristics of the image to be generated based on the density value of the reference patch read by the reading means. And correction means for performing interpolation on the correction data in the high-density region by linear interpolation connecting a predetermined two points with a straight line, among the correction data generated by the generation means. Features.

According to the present invention, the reference patch is formed on the image carrier by the reference patch forming means, the density value of the reference patch is read by the reading means, and the target density value stored in the storage means is read by the calculating means. And calculating a value which is an inverse function of the density value of the reference patch read by the reading means, and by the generation means, the inverse function value of the target density value and the inverse function value of the reference patch calculated by the calculation means. The correction data for correcting the density gradation characteristics of the formed image is generated based on the difference of the image, so that the disappearance of the gradation due to the saturation of the high density region can be prevented, and the pseudo contour can be prevented. Can be prevented from occurring.

[0013]

Embodiments of the present invention will now be described with reference to the drawings. A. Configuration of Embodiment A-1. FIG. 1 is a schematic diagram showing a configuration of a color copying machine to which an image forming apparatus according to an embodiment of the present invention is applied, and FIG.
FIG. 2 is a block diagram illustrating a configuration of the image forming apparatus. In the figure, a color copying machine is roughly divided into a reading section 1 for reading a document, an image processing section 2 for processing read image data, and driving a laser in accordance with the processed image data to irradiate a light beam to a photosensitive member. It comprises an ROS optical unit 3 and an image forming unit 4 for forming an image.

The reading section 1 irradiates the original 5 with an exposure lamp 6, reads the reflected light with a CCD 7, reads the read light, amplifies it to a predetermined level with an amplifier 8, and then converts the 8-bit digital image data with an A / D converter. Convert to After shading correction is performed by the shading correction unit 10 and gap correction is performed by the gap correction unit 11, the density conversion unit 12 converts the reflectance data into density data, and supplies the data to the image processing unit 2.

The image processing unit 2 includes a color conversion unit 13 shown in FIG.
The basic image processing as a color copying machine, that is, color signal conversion, black reproduction (UCR), MTF processing, and the like are performed to convert the image data into four color image data of yellow, magenta, cyan, and black. Next, the gradation converting unit 14 reads the reading unit 1
Then, conversion of each color gradation is performed according to the gradation property of the image forming unit 4. Further, the image processing section 2 includes a density conversion table 15 described later, creates the density conversion table 15 under the control of an image density control section 16 described later, and performs density control of image data.

The D / A converter 17 converts the image data into analog data and supplies the analog data to one input terminal of a selector 19. Further, the patch signal generation unit 18 generates a plurality of reference patch image signals having different densities, which are image density control patches, and supplies the same to the other input terminal of the selector 19.
The selector 19 selects one of the analog image data and the patch image signal and supplies it to the comparator 21. The selector 19 selects analog image data at the time of normal copying, and when an instruction to create a patch is issued by the arithmetic unit 39 of the image forming unit 4 and an instruction to create a patch is issued from the patch signal generating unit 18, the patch signal generation The patch image signal from the section 18 is selected and supplied to the comparator 21 for binarization.

The triangular wave generator 19 supplies a triangular wave signal having a predetermined period to the comparator 21. The comparator 21 compares a signal of a predetermined cycle supplied from the triangular wave generator 19 with one of the analog image data and the patch image signal, performs pulse width modulation, and converts the image data into binary image data. . FIG. 3 is a waveform diagram illustrating binarization of image data by pulse width modulation in the comparator 21. In the figure, the input analog image data is compared with a triangular wave. A portion where the analog image data is larger than the triangular wave is set to “0”, ie, laser OFF, and a portion where the analog image data is small is set to “1”, ie, laser ON. The value image data is supplied from the comparator 21 to the ROS optical unit 3.

The ROS optical section 3 is provided with a laser light quantity varying device 22 and a laser driving circuit 23 which are controlled by the arithmetic unit 39 of the image forming section 4 and vary the laser light quantity. The laser drive circuit 23 controls ON / OFF of the laser 24 based on the binary data supplied from the comparator 21.
The laser light is deflected by the polygon mirror 25 and fθ
The light is guided to the photoconductor 28 of the image forming unit 4 via the lens 26 and the reflection mirror 27.

The image forming section 4 includes a charging device 29, a rotary developing device 30, a transfer device 31, a cleaner device 32, and a discharging lamp 33 around a photoreceptor 28, and a rotary developing device 30, and a developing device for each color. The apparatus includes a toner dispensing device 34 for supplying toner, a fixing device 35, and a sheet conveying device 36. Rotary developing device 30
Are a transfer drum 30a, a transfer corotron 31b disposed around the transfer drum 30a, and a peeling corotron 31
c, and a discharging corotron 31d. The electrometer 37 measures the potential on the photoconductor 28 to control the photoconductor potential, and the optical sensor 38 measures the patch density on the photoconductor 28 to control the toner dispensing. Further, the arithmetic unit 39 controls the entire image formation, and controls image formation conditions in accordance with a patch creation instruction and outputs of the electrometer 37 and the optical sensor 38. Variable developing bias device 40
Is controlled by the arithmetic unit 39 to change the developing bias. The charge amount varying device 41 is controlled by the arithmetic device 39 and changes the charge amount of the charging device 29.

B. Next, an operation of the image forming apparatus according to the present embodiment will be described.

B-1. Image Forming Control In the above image forming apparatus, an image is formed according to a well-known xerographic process. That is, the rotating photoconductor 28 is uniformly negatively charged by the charging device 29, and a first color latent image is first formed by the laser beam.
The latent image is the first color (Blac) of the rotary developing device 30.
The portion written by the laser beam with the negatively charged black toner in the developing device of k) is developed, and the developed image is transported from the paper tray by the paper transporting device 36 and wound around the transfer drum 31a. The paper is transferred to the paper not to be transferred by the transfer corotron 31b. The image remaining on the photoconductor 28 without being transferred is removed by the cleaner device 32.

Next, after the photosensitive member 28 is neutralized by the neutralizing lamp 33, it is again uniformly negatively charged by the charging device 29, and the image of the second color (yellow) is formed in the same manner as described above. Continued. Thus, up to the third color (magenta) and the fourth color (cyan)
When the developed images of the four colors are sequentially transferred to the sheet on the transfer drum 31a, the sheet is separated from the transfer drum 31a by the separation corotron 31c, and a color copy fixed by the fixing device 35 is formed. Around the transfer drum 31a, there is a charge removing corotron 31d, which removes excess charges on paper and the film of the transfer drum 31a after transfer of each color or after peeling of the paper.

B-2. Basic Control (Toner Dispensing Control, Photoconductor Potential Control) Next, toner dispensing control by an optical sensor 38 that measures a known patch density on the photoconductor 28,
Charge amount variable control by an electrometer 37 for measuring the above potential,
The control of the photoconductor potential by the developing bias variable device 40 and the laser light amount variable device 22 will be described.

The control of the toner dispensing device 34 is performed by measuring the toner dispensing control patch density on the photoreceptor 28 with the optical sensor 38. In this example, the operation is performed immediately after the device is turned on and every 10 copies thereafter. A signal for generating a patch is sent from the device 39 to the patch signal generator 18, and each color selector 19 selects a patch image signal for toner dispensing control with an image area ratio of 50% from the patch signal generator 18, and a comparator. 21. Hereinafter, in the above-described process of the color copying machine, a control patch having an image area ratio of 50% is formed on a non-image portion on the photosensitive member in the same procedure as image formation.

The optical sensor 38 is, as shown in FIG.
The light from D38a is applied to the toner patch on the photoconductor 28, the reflected light is measured by the photodiode 38b, and the patch density on the photoconductor 28 is measured. If the patch density measured here is lower than the target, the toner dispensing device 34 is driven to increase the toner density to bring the patch density closer to the target. Conversely, if the measured patch density is higher than the target, the toner dispensing device 34 is stopped and the patch density approaches the target.

B-2. Photoconductor Potential Control Next, the operation of the photoconductor potential control will be described with reference to the flowchart shown in FIG. In this example, the photoconductor potential control is performed in accordance with this flowchart in accordance with an instruction from the arithmetic unit 39 of the image forming unit 4 before the start of copying immediately after the power of the apparatus is turned on and before the start of copying after every 30 minutes.

Target dark potential VHS, target exposure partial potential VL
S, from the target dark potential VHS to the developing bias potential VB
The anti-fogging potential difference VC is stored in the arithmetic unit of the image forming unit in advance. First, in step Sa1, the grid voltage of the charging device 29 is
The dark potentials VH1 and VH2 at G1 and VG2 are detected by the electrometer 37, and at step Sa2, a grid voltage VGS for obtaining the target dark potential VHS is calculated. Next, step S
At 3, the photoconductor 28 is charged with the grid voltage VGS obtained at step Sa2.

In response to an instruction from the arithmetic unit 39, the laser light quantity varying device 22 drives the laser drive circuit 23 with two kinds of laser light quantities LD1 and LD2, and puts two kinds of laser light quantities on the photoreceptor 28. Exposure patches in LD1 and LD2 are created, and respective exposure partial potentials VL1 and VL
2 is detected with an electrometer. Next, in step S4, a laser light amount LDS for obtaining the target exposure partial potential VLS is calculated.
Next, in step S5, the developing bias potential VB is calculated from the difference between the target dark potential VHS and the fog prevention potential difference VC, and in step S6, the grid voltage VGS and the laser light amount LD are calculated.
S, the developing bias potential VB is set by each variable device, and the process ends.

B-3. Image density control Next, a plurality of reference patches having different densities are created, a density conversion table is created based on the density measurement results, and image density control for converting density characteristics of image data is performed.
This will be described with reference to the flowchart shown in FIG. 6 and the conceptual diagram shown in FIG. When the image density control is executed, the arithmetic unit 39 sends a signal for creating a correction color patch to the patch signal generation unit 18 in step Sb1, and each color selector 19 outputs the correction color patch from the patch signal generation unit 18. A patch image signal is selected and sent to the comparator 21, and a correction color patch print is output on a sheet of paper in the same procedure as that of image formation in the above-described color copier process (procedure 1). Here, FIG. 7 is a color patch print for correction in the present embodiment, and is a conceptual diagram showing 24 gradation patches of different densities for each color.

Next, in this embodiment, since the image reading section of the color copying machine is used as a density measuring device for the color patch print for correction, the color patch print for correction is read on the platen of the reading section 1 in step Sb2. (Step 2). As a density control device for correction color patch printing, a density type (gradation type) may be used in addition to the image reading unit of the color copying machine.

Next, in step Sb3, the reading unit 1 measures the density of 24 color patches of each color, obtains the current gradation, and sends the density measurement result to the image density control unit 16 (procedure 3). If there is no problem in the measurement result in step Sb4, the process proceeds to step Sb5, where the image density control unit 16
The current gradation is compared with a predetermined target gradation, and a density conversion table 15 is created and set (procedure 4). At this time,
If there is a problem in the measurement result, it is considered that the mounting direction of the correction color patch print is defective, and the like.
At b6, a warning is displayed and the processing is stopped.

B-4. Processing for Creating a Density Conversion Table Next, processing for creating a density conversion table will be described.
Here, FIG. 8 is a conceptual diagram for explaining the density conversion table. FIGS. 9 and 10 are flowcharts for explaining the process of creating the density conversion table.

Step 1) First, "density of target reference patch (target gradation density)" is stored. The “density of the target reference patch (target gradation density)” is stored in advance as a constant, or a gradation pattern is created / read and stored in the image forming apparatus after image quality setup.

Step 2) Next, a “conversion table that is an inverse function of the density of the target reference patch” is calculated from the “density of the target reference patch”.

As a calculation method, a conversion table which is an inverse function of "the relationship between Cin (input area gradation ratio) and target gradation density" is prepared. "A conversion table optimized by characteristics unique to the image forming apparatus" Conversion table based on “Log conversion” Conversion table obtained by “pure Log conversion using mathematical formula” A conversion table based on a combination of an OP amplifier and the like for the analog output of the density sensor is considered.

Step 3) The “conversion table” is used to convert “reference patch density (current tone density)” and “target reference patch density (target tone density)”.

Step 4) The “density of the reference patch (current tone density)” and the “density of the target reference patch (target tone density)” (24 points each) converted by the “conversion table” are It is extended to 256 points by linear interpolation. As a result, “density of reference patch (current gradation density)” and “density of target reference patch (target gradation density)” are lines L1 and L2 shown in FIG. 8, respectively. The interpolation method is not limited to linear interpolation, but may be spline interpolation or linear / linear interpolation.
The accuracy can be further improved by using interpolation by least squares due to non-linearity or the like.

Step 5) Density conversion for correcting the gradation characteristics of an image by comparing the "reference patch density" and the "target reference patch density" expanded to 256 points by linear interpolation. Create Table 15.

As a result, even if noise is included in the output of the density sensor in the image forming process, such noise can be removed, so that gradation correction can be performed in a short time and with high accuracy. Further, in the created density conversion table 15, compared to the density conversion table according to the related art shown in FIG. Can be prevented, and the generation of a false contour can be prevented.

FIG. 9 is a conceptual diagram showing the procedure of image density control. When correction is performed, a density conversion table 15 is created according to the image density control execution flowchart shown in FIG. The image processing unit 2 performs color conversion and gradation conversion processing on the original image data supplied from the image reading unit 1,
, The gradation is matched with the target gradation. Similarly, in the case of a printer, external image data is converted by the density conversion table 15, and the gradation is made to match the target gradation.

C. Modification Next, a modification of the present invention will be described. In the above-described embodiment, the density conversion table 15 is a “conversion table that is an inverse function of the density of the target reference patch”. Desired density conversion table 15
Is to try to create. Hereinafter, a method of creating the density conversion table 15 according to the present modification will be described.

Step 1) The "target reference patch density" is stored in advance.
The "target reference patch density" may be stored in advance by storing / reading a gradation pattern by an image forming apparatus after image quality setup, in addition to a method of storing the density as a constant in advance.

Step 2) The "reference patch density" and the "target reference patch density" (24 points each) are expanded to 256 points by linear interpolation.

Step 3) Density for correcting the gradation characteristic of the image by abuttingly comparing “density of reference patch” and “density of target reference patch” expanded to 256 points by linear interpolation. A conversion table 15 is created. Note that the interpolation method is not limited to linear interpolation as in the above-described embodiment, and it is possible to further improve accuracy by using spline interpolation or interpolation using least squares such as linear / nonlinear. .

Step 4) In the density conversion table, a straight line passing through two preset Cin (E) points is determined by linear regression, and a final density conversion table is set (see FIG. 13). 7).

In the density conversion table, the density conversion table in a range higher than the “first set Cin” is
A straight line passing through two points of the “preset first Cin” value and the “preset second Cin” value is obtained by linear interpolation, and a final density conversion table is set (FIG. 14). , Corresponding to claim 8).

Next, a method of creating the density conversion table 15 according to the above-described modified example will be described with reference to FIGS. First, in step Sc1, a reference image data correction table for creating an image density detection reference pattern,
And an image density detection reference pattern is created. Next, in step Sc2, a reference patch density (current gradation density) is read according to the detection reference pattern. And step S
In c3, the reference patch density (current gradation density, 12 points) is set to 2
Expand to 56 gradations. Next, in step Sc4, the density conversion table 15 is created according to the reference patch density after expansion (current tone density) and the target reference patch density (target tone density). The details of the density conversion table 15 will be described later.

Next, the density conversion table 15 is transferred to the image forming section 4 in step Sc5. Next, the image forming unit 4 forms an image according to the transferred density conversion table 15 in step Sc6. Then, in step Sc7, it is determined whether or not it is time to create the density conversion table 15;
And the above processing is performed again.

Next, in preparing the density conversion table 15, in the process corresponding to claim 4, first, in step Sd1, the reference patch density after expansion (current gradation density) and the target reference patch density ( A density conversion table is calculated based on the target gradation density, and in step Sd2,
Correct the correction amount in the high density area. That is, in the density conversion table, Cin (D) in which the correction amount departs from “255”
And a value obtained by subtracting the offset value e (constant 0) from the Cin (D), that is, in a range higher than Cin (E), Cin (D)
Based on the correction amount of (E), a value equal to the correction amount of the density conversion table is set.

In preparing the density conversion table 15, in the process corresponding to claim 5, first, in step Se1, the reference patch density after expansion (current tone density) and the reference patch density (target A correction amount table is calculated based on the gradation density), and in step Se2,
Correct the correction amount in the high density area. That is, in the density conversion table, Cin (C) in which the correction amount departs from “255”
And a value obtained by subtracting an offset value (constant) from Cin (C), that is, in a range higher than Cin (D),
It is determined by linear regression passing through two points, the correction amount of (D) and the correction amount of Cin0 (= 255).

In the above-described embodiment, the density conversion table 15 is created based on the measurement results of a plurality of reference patches having different densities on the paper. The density conversion table may be created by measuring the above plurality of reference patches having different densities.

[0052]

As described above, according to the first aspect of the present invention, the reference patch is formed on the image carrier by the reference patch forming means, and the density value of the reference patch is read by the reading means, and the calculation is performed. Means for calculating a value which is an inverse function of the target density value stored in the storage means and the density value of the reference patch read by the reading means, and the target density calculated by the calculation means by the generation means. Since the correction data for correcting the density gradation characteristics of the formed image is generated based on the difference between the inverse function value of the value and the inverse function value of the reference patch, the gradation of the gradation due to the saturation of the high density region is generated. The advantage is obtained that the disappearance can be prevented and the generation of the false contour can be prevented.

According to a second aspect of the present invention, in the image processing apparatus according to the first aspect, an inverse function of a relationship between a density value read by the reading means and a target density value stored in the storage means. With a conversion table,
The calculation means calculates a value that is an inverse function of the target density value and the density value of the reference patch according to the conversion table. Therefore, it is possible to more easily prevent the disappearance of the gradation due to saturation in the high density range. At the same time, the advantage that the generation of the false contour can be prevented can be obtained.

According to the sixth aspect of the present invention, the reference patch is formed on the image carrier by the reference patch forming means.
The density value of the reference patch is read by the reading means,
Based on the density value of the reference patch read by the reading unit by the generating unit, the correction unit generates correction data for correcting the density gradation characteristic of the image to be generated. Correction data in the high-density region is interpolated by linear interpolation connecting two predetermined points with a straight line, so that the loss of gradation due to saturation in the high-density region can be prevented more easily, The advantage is obtained that contouring can be prevented.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating a configuration of a color copying machine to which an image forming apparatus according to an embodiment of the invention is applied.

FIG. 2 is a block diagram illustrating a configuration of the image forming apparatus.

FIG. 3 is a waveform diagram illustrating binarization of image data by pulse width modulation in a comparator 21.

FIG. 4 is a conceptual diagram for explaining a method of measuring a patch density on a photoconductor by an optical sensor.

FIG. 5 is a flowchart showing the operation of photoconductor potential control in FIG.

FIG. 6 is a flowchart for explaining an operation of image density control for creating a density conversion table and converting density characteristics of image data.

FIG. 7 is a conceptual diagram illustrating a correction color patch print according to the present embodiment, showing tone patches having different densities of 24 colors.

FIG. 8 is a conceptual diagram illustrating a density conversion table.

FIG. 9 is a conceptual diagram showing a procedure of image density control.

FIG. 10 is a flowchart illustrating an operation of an image forming apparatus according to a modification of the present invention.

FIG. 11 is a flowchart illustrating an operation of an image forming apparatus according to a modification.

FIG. 12 is a flowchart illustrating another operation of the image forming apparatus according to the modification.

FIG. 13 is a conceptual diagram (corresponding to claim 7) for explaining an operation of interpolating by linear interpolation that connects two points of a maximum density value and a preset value by a straight line according to a modified example.

FIG. 14 is a conceptual diagram (corresponding to claim 8) for explaining an operation of performing interpolation by linear interpolation that connects two points of a first value and a second value with a straight line according to a modified example.

[Explanation of symbols]

28 Photoconductor (Image Carrier) 18 Patch Signal Generating Unit (Reference Patch Forming Unit) 15 Density Conversion Table (Storage Unit) 1 Reading Unit (Reading Unit) 16 Image Density Control Unit (Computing Unit, Generating Unit, Interpolating Expansion Unit, Interpolation means) 39 arithmetic unit (thinning means, regeneration means)

Claims (8)

[Claims] 1. A reference patch forming means for forming a reference patch on an image carrier, a storage means for storing a target density value of the reference patch formed by the reference patch forming means, and a reference patch forming means. Reading means for reading the density value of the read reference patch; calculating means for calculating a value which is an inverse function of the target density value stored in the storage means and the density value of the reference patch read by the reading means; Generating means for generating correction data for correcting a density gradation characteristic of an image to be formed based on a difference between the inverse function value of the target density value and the inverse function value of the reference patch, calculated by the arithmetic means. An image forming apparatus. 2. A conversion table which is an inverse function of a relationship between a density value read by the reading means and a target density value stored in the storage means, wherein the calculating means calculates a target density according to the conversion table. 2. The image forming apparatus according to claim 1, wherein a value that is an inverse function of the value and the density value of the reference patch is calculated. 3. A conversion table optimized by characteristics unique to the image forming apparatus, wherein the calculating means calculates a value which is an inverse function of a target density value and a density value of a reference patch according to the conversion table. The image processing apparatus according to claim 1, wherein: 4. An interpolating means for interpolating and expanding by interpolation the values which are the inverse functions of the target density value and the density value of the reference patch calculated by the calculating means. 2. The image forming apparatus according to 1. 5. The image forming apparatus according to claim 4, wherein said interpolation expanding means performs interpolation expansion by linear interpolation. 6. A reference patch forming means for forming a reference patch on an image carrier, a storage means for storing a target density value of the reference patch formed by the reference patch forming means, and a reference patch forming means. Reading means for reading the density value of the read reference patch, and generating correction data for correcting density gradation characteristics of an image to be generated based on the density value of the reference patch read by the reading means. Means, for the correction data in the high-density region among the correction data generated by the generation means, interpolating means for interpolating by linear interpolation connecting a predetermined two points with a straight line. Image forming apparatus. 7. The image forming apparatus according to claim 6, wherein said interpolating means interpolates by linear interpolation connecting two points of a maximum density value and a preset value with a straight line. 8. The image forming apparatus according to claim 6, wherein said interpolating means performs interpolation by linear interpolation connecting two points of the first value and the second value with a straight line.