JP2001309178A - Image processing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the operating efficiency of an image forming device by preventing excess toner consumption, earlier wear of a photoreceptor, and load increase in a cleaner or the like while maintaining prescribed image quality so as to reduce the time for image correction processing. SOLUTION: A density of a gradation test pattern formed by a simple medium tone process control with a small forming number of pattern images is measured (101, 102), and approximated data are obtained by applying approximation processing to detected data (103). A rate of change at preceding correction is compared with a rate of change at this correction (104), a detailed medium tone process control with many forming numbers of pattern images is shifted only when a difference is 0.1 or over (105). The detailed medium tone process control is selectively performed on the basis of a gradient of an approximated straight line obtained by the simple medium tone process control and the detailed medium tone process control is not conducted under a state where a sufficiently accurate gradation correction curve can be obtained by the simple medium tone process control.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing method for performing image processing on image data to be subjected to image forming processing, and more particularly to a method for converting input image data into output image data during image forming processing. The present invention relates to an improvement in a correction process for correcting a corrected tone correction curve.

[0002]

2. Description of the Related Art In an image forming apparatus such as a copying machine, a printer, and a facsimile apparatus, which performs an electrophotographic image forming process, a density stability for stably reproducing a predetermined image density is obtained.
It is necessary to achieve both gradation reproducibility for faithfully reproducing the gradation from highlight to shadow of a reproduced image. However, the density and gradation of the formed image fluctuate due to a change in the density of the toner as a developer or a change in the setting conditions of the image forming process.

Therefore, maximum density correction processing is performed at predetermined timing as means for improving density stability, and gradation correction (γ correction) processing is performed at predetermined timing as means for improving gradation reproducibility. The method has been adopted.
Here, the maximum density correction processing is for preventing a density fluctuation from occurring in a formed image that should have the same density, and is performed as follows. First, a plurality of test pattern latent images are formed on the photoreceptor at a constant exposure intensity, and when these test pattern latent images are developed by the developing device, the linear velocity of the developing roller of the developing device is changed for each test pattern latent image. Develop while changing. Thereafter, the toner density of each developed test pattern image is detected by a density detecting device, and the linear velocity of the developing roller at which an optimum density is obtained is obtained based on the detection result.

[0004] The gradation correction process is for faithfully reproducing the gradation of an image serving as a document, and is performed as follows. First, a plurality of test pattern latent images are formed on the photoconductor at different exposure intensities corresponding to predetermined different densities. Next, these test pattern latent images are developed with the linear velocity of the developing roller kept constant. Then, the toner density of each developed test pattern image is detected, and a tone correction curve is created based on the detected toner density data.

When the image forming apparatus is actually used, the deterioration of the developing performance,
Deterioration of image forming conditions such as a decrease in image density due to a change in the potential of the photosensitive layer of the photoreceptor and a loss of gradation balance occurs.
Therefore, it is necessary to perform an image correction process including a maximum density correction process and a gradation correction process every time the image forming operation is performed a certain number of times. In particular, in the gradation correction process, it is necessary to appropriately correct the gradation correction curve to be used.

[0006]

However, since a large number of test patterns are formed when performing the maximum density correction processing and the gradation correction processing, the image correction processing such as the maximum density correction processing and the gradation correction processing is performed. If a long time is required and the frequency of executing the image correction process is increased to improve the image quality, the time required for the image correction process other than the original image formation process becomes longer, and the operation efficiency of the image forming apparatus is reduced. Further, frequent image correction processing means that a large number of test pattern images are formed many times, which results in excessive consumption of toner and accelerated consumption of the photoconductor,
There is a problem that the load on the cleaner increases. On the other hand, if the frequency of execution of the image correction process is reduced, it is suitable for a subtle change in image density or a change in gradation due to a change in the internal environment of the image forming apparatus, a change in development characteristics, etc. occurring during that time. Correction cannot be performed, and there is a problem that image quality is deteriorated.

SUMMARY OF THE INVENTION It is an object of the present invention to reduce the number of test pattern images created during image correction processing while maintaining a predetermined image quality, resulting in excessive consumption of toner, early consumption of a photoconductor, and burden on a cleaner. An object of the present invention is to provide an image processing method capable of preventing an increase or the like and shortening the image correction processing to improve the operation efficiency of the image forming apparatus.

[0008]

The present invention has the following arrangement as means for solving the above-mentioned problems.

(1) A gradation correction curve that defines a correction amount according to the image density when correcting image data is converted to a test pattern image composed of a plurality of pattern images formed based on preset density data. In the image processing method including the correction processing using the correction processing, the correction processing is two types of correction processing in which the number of pattern images constituting the test pattern image is different, and the simple correction processing and the pattern image in which the number of pattern images is smaller. A large number of detailed correction processes can be executed, and an approximate straight line representing the relationship between the detection data and the density data for the test pattern image formed in the simple correction process is obtained. Based on the state of change of the approximation straight line at the time of this correction process,
It is characterized in that continuation of the simple correction process or transition to the detailed correction process is selected.

In this configuration, in the simple correction processing in which a smaller number of pattern images are formed than in the detailed correction processing, the approximate straight line in the previous correction processing and the approximate straight line in the current correction processing are compared. Based on the result, a detailed correction process in which a larger number of pattern images are formed than in the simple correction process is selectively executed. Therefore, when the approximate straight line at the time of the current correction process does not greatly change from the approximate straight line at the time of the previous correction process, and the tone correction curve can be appropriately corrected by the simple correction process, the detailed correction process is performed. And a large number of pattern images are not formed unnecessarily. Therefore, the time required for image processing is reduced, and the operation efficiency of the image forming apparatus is improved. Also,
The toner is not wasted by unnecessary pattern images, and the running cost of the image forming apparatus is reduced. On the other hand, when the approximate straight line at the time of the current correction process is significantly different from the approximate straight line at the time of the previous correction process, and the tone correction curve cannot be properly corrected by the simple correction process, the detailed correction process is performed. . For this reason, the gradation correction curve is corrected in detail, and image quality with excellent gradation reproducibility is maintained.

(2) Using the difference between the slope of the approximate straight line at the time of the previous correction process and the slope of the approximate straight line at the time of the current correction process as the change state, the continuation of the simple correction process or the transition to the detailed correction process is selected. It is characterized by the following.

In this configuration, the detailed correction processing is selectively executed based on a comparison result between the inclination of the approximate straight line at the time of the previous correction processing and the inclination of the approximate straight line at the time of the current correction processing. Therefore, the gradation correction curve is accurately corrected by the processing according to the degree of change between the previous correction processing and the current correction processing determined based on the inclination of the approximate straight line as a reference.

(3) The difference between the approximate straight line in the previous correction processing and the approximate straight line in the current correction processing at two points representing the low-density area and the high-density area is calculated. The continuation of the simple correction process or the shift to the detailed correction process is selected by setting the ratio to the difference in the high density region as the change state.

In this configuration, the difference between the approximate straight line in the previous correction process and the approximate straight line in the current correction process at two points representing the low density region and the high density region is determined by The inclination of the approximate straight line is compared with the inclination of the approximate straight line at the time of the current correction processing, and the detailed correction processing is selectively executed based on the comparison result. Therefore, the degree of change between the previous correction processing based on the slope of the approximate straight line and the current correction processing can be easily and accurately determined.

(4) When the transition to the detailed correction process is not selected, the difference between the approximate straight line at the previous correction process and the approximate straight line at the current correction process at the density of the preset representative point is determined. Based on this, it is characterized in that the user selects to stop or continue the simple correction process.

In this configuration, based on the comparison result between the approximate straight line at the time of the previous correction process and the approximate straight line at the time of the current correction process, the process proceeds to the detailed correction process, the continuation of the simple correction process, or the simple correction process. Either stop of the process is executed. Therefore, the content of the correction process of the gradation correction curve is selected more finely according to the degree of change between the previous correction process and the current correction process, and while maintaining a predetermined image quality, excessive consumption of toner In addition, early wear of the photoconductor and increase in the load on the cleaner can be prevented more accurately.

(5) The density at the representative point is a density representing a low density range. In this configuration, the suspension or continuation of the simple correction process is selected based on the result of comparing the density in the low density range between the approximate straight line at the time of the previous correction process and the approximate straight line at the time of the current correction process. Therefore, the image formed based on the image data after the correction using the corrected gradation correction curve does not have the background fog.

(6) The simple correction processing is characterized in that a gradation correction curve is corrected using a weighting coefficient preset for each of a plurality of density levels.

In this configuration, at the time of the simple correction processing,
The gradation correction curve is corrected for each of a plurality of density levels based on a predetermined weighting coefficient. Therefore, the gradation correction curve is more realistically corrected in consideration of the density of the image to be formed and the effects of other correction processes.

[0020]

FIG. 1 is a schematic diagram showing a configuration of an image forming section of a digital color copying machine which is an image forming apparatus in which an image processing method according to an embodiment of the present invention is executed. The present invention can be similarly applied to other image forming apparatuses such as a printer and a facsimile apparatus for forming an electrophotographic image other than the digital color copying machine.

The digital color copier reads a color image from a document in a scanner unit, performs predetermined image processing on the color image, supplies the image data to the image forming unit 10, and reads the color image read from the document on a recording medium such as paper. Reproduce the image.

Image forming section 10 of digital color copying machine
Is provided with a transfer / transport belt 17 which is stretched in a state where a horizontal portion is formed vertically between two rollers 17a and 17b and rotates in the direction of arrow A. While being located at the upper horizontal portion, the transfer transport belt 17 rotates the paper placed on the upper surface by rotating in the direction of arrow A to form the image on the image forming station 1.
0a to 10d. Each of the image forming stations 10a to 10d performs electrophotographic image formation using toner of three primary colors (cyan, magenta, and yellow) of black and subtractive color.

The transfer / conveying belt 17 faces the density detecting sensor 1 while being located at the lower horizontal portion. Further, a fixing device 18 is arranged on the downstream side of one roller 17a of the transfer conveyance belt 17. Fixing device 18
Is constituted by a pair of rollers, heats and presses the sheet that has passed through each of the image forming process units 10a to 10d, melts the toner image transferred onto the sheet, and fixes the toner image on the surface of the sheet.

Each of the image forming stations 10a to 10d has the same configuration except for the amount of stored toner. As an example, the image forming station 10a forms a photosensitive layer on the surface of a cylindrical conductive substrate, and surrounds a photosensitive drum 11a rotating in the direction of arrow B, a charger 12a, an exposure unit 13a, a developing unit 14a, and a transfer unit. Vessel 15a
And the cleaner 16a and the like are arranged in this order.

The charger 12a uniformly charges the surface of the photosensitive drum 11a with a charge of a predetermined polarity. The exposure unit 13a forms an electrostatic latent image by exposing the surface of the photosensitive drum 11a with image light. The developing unit 14a supplies the toner contained therein to the surface of the photosensitive drum 11a, and visualizes the electrostatic latent image into a toner image. Transfer device 15a
Is opposed to the peripheral surface of the photosensitive drum 11a with the transfer / transport belt 17 interposed therebetween, and transfers the toner image carried on the surface of the photosensitive drum 11a to the surface of a sheet placed on the transfer / transport belt 17. I do. The cleaner 16 removes the toner remaining on the peripheral surface of the photosensitive drum 11a after the completion of the transfer process.

The developing unit 14a includes the photosensitive drum 11
and a developing roller that rotates in opposition to the peripheral surface of FIG.
The developing roller supplies the toner carried on the surface to the peripheral surface of the photosensitive drum 11a by rotation. By changing the peripheral speed of the developing roller, that is, the rotational speed, the amount of toner supply to the peripheral surface of the photosensitive drum 11a can be increased or decreased, and the density of the toner image can be adjusted.

Each of the exposure units 12a to 12d provided in the image forming stations 10a to 10d is supplied with image data of each color of black, cyan, magenta and yellow, and is also provided with developing units 14a to 14d.
Contains toner of each color of black, cyan, magenta, and yellow. Accordingly, in each of the image forming stations 10a to 10d, a toner image of each color of black, cyan, magenta, and yellow is sequentially transferred onto the paper, and a full color image is formed on the paper passing through the fixing device 18 by subtractive color mixing of the toner images of each color. An image is formed.

The density detecting sensor 1 includes a light-emitting element 2 and a light-receiving element 3, and irradiates light from the light-emitting element 2 to the surface of the transfer conveyance belt 17 on which a test pattern image is formed during image correction processing to be described later. The reflected light is received by the light receiving element 3, and an electric signal corresponding to the amount of received light is output as a toner density detection signal.

After the test pattern image formed on the surface of the transfer / transport belt 17 is opposed to the density detecting sensor 1, the transfer / transport belt 1 is cleaned by a cleaning means (not shown).
7 is removed from the surface.

Each of the image forming stations 10a to 10a
0d, the density detection sensor 1 is disposed at a position facing the surface of the photoconductor drum 11 after the development process to detect the density of the test pattern image before being transferred to the transfer / conveyance belt 17. The invention can be implemented as well.

FIG. 2 is a block diagram showing the configuration of the image processing unit of the digital color copying machine. The image processing unit 20 of the digital color copier includes an image data input unit 4
0, image data processing unit 41, image data output unit 42, gradation correction unit 46, density recognition unit 47, memory 49, and CPU
44.

The image data input unit 40 converts read signals of three additive primary colors (RGB) read from a color image of a document by the scanner unit into digital data. The image data processing unit 41 generates image data of three subtractive primary colors and black (YMCK system) from the RGB image data, and performs zoom processing and the like according to the set copy magnification. The tone correction section 46 is a YMC
A gradation correction process described later is performed on the K-system image data. The image output unit 42 is a YMC that has been subjected to gradation correction processing.
The driving data generated based on the K-system image data is output to the exposure units 18a to 18d.

The memory 49 stores data for forming a test pattern on the surface of the transfer / conveyance belt 17 during an image correction process described later. This data is supplied to the image output unit 42 by the CPU 44 during the image forming process. The density recognition unit 47 recognizes a density signal output from the density detection sensor 1.

The operation of each unit in the image processing unit 20 is generally controlled by the CPU 44. Also, C
The PU 44 controls the operation of the photosensitive drum 11 and the like in the image forming process unit 10 in synchronization with the operation of the image output unit 42. Further, the CPU 44 optimizes the correction conditions in the tone correction unit 46 and the process conditions in the image forming process unit 10 based on the density signal of the test pattern recognized by the density recognition unit 47 during the image correction process.

With the above configuration, the image forming process section 1
In order to reproduce the shading of the image according to the image data when forming a copy image or a test pattern image at 0, the shading of the image data was reproduced on the peripheral surface of the photosensitive drum 11 via the exposure unit 13. It is necessary to form an electrostatic latent image. As a method for this, there are a pulse width modulation method and a power modulation method. In the pulse width modulation method, the on / off time (pulse width) of the laser beam emitted from the exposure unit 13 is controlled according to the density of the image. In the power modulation method, the intensity of a laser beam emitted from the exposure unit 13 is controlled according to the density of an image.

In the image processing section 40, the maximum density correction processing and the gradation correction processing are sequentially performed on each of the YMCK system image data during the image correction processing. Each correction process will be described below. Here, the gradation correction unit 4 is transmitted from the image data input unit 40 via the image data processing unit 41.
6 is the image input data and the gradation correction unit 4
6 is output to the image data output unit 42 as image output data and the CPU 44 is used to form a test pattern.
Read from the memory 49 by the image data output unit 4
2 is referred to as test pattern data, and the data recognized by the density recognition unit 47 is referred to as detection data (see FIG. 2).

1) Maximum Density Correction Processing The maximum density correction processing is performed to suppress a change in the overall density of an image to be subjected to image formation processing. In the maximum density correction process, among the test pattern data stored in the memory 49, the CPU 44 reads out test pattern data for forming a plurality of test pattern images having the highest toner density (maximum density). Are supplied to the image data output unit 42. Thus, a plurality of test patterns having the maximum density are formed on the respective surfaces of the photoconductor drums 11a to 11d.

As described above, the photosensitive drums 11a to 11a
For each electrostatic latent image formed plurally for each d, CP
U44 turns the developing rollers in the developing units 14a to 14d at different rotational speeds to visualize the toner images into toner images. Therefore, the photosensitive drums 11a to 11d
A plurality of electrostatic latent images formed under the same exposure conditions on the surface are developed with different toner densities.

The toner images of the test patterns formed on the surfaces of the photosensitive drums 11a to 11d are transferred to transfer devices 15a to 15d.
After being transferred to the surface of the transfer / conveying belt 17 by 5d, the density detection sensor 1 and the density recognition unit 47 detect and recognize the toner density. The CPU 44 has a memory 4
9 is compared with the toner density target value for the test pattern image having the maximum density stored in advance and the toner density recognized by the density recognition unit 44 from each test pattern image actually formed. The developing condition (rotational speed of the developing roller) of the test pattern image in which the close toner density is detected is set as the developing condition in the subsequent image forming process.

2) Gradation Correction Processing The gradation correction processing is performed in order to faithfully reproduce the gradation of the original image in the copy image by suppressing the fluctuation in the gradation of the toner image. In the gradation correction process, the CPU 44 selects data (gradation test pattern data) for forming a plurality of test patterns (gradation test patterns) having different densities from the test pattern data stored in the memory 49. ),
It is supplied to the image data output unit 42. As a result, an electrostatic latent image of a gradation test pattern is formed on the photosensitive drums 11a to 11d.

Thus, the photosensitive drums 11a to 11a
The CPU 44 develops the electrostatic latent image of the gradation test pattern formed in d under the development conditions (rotational speed of the developing roller) set in the above-described maximum density correction processing. The toner images of the gradation test pattern formed on each of the photosensitive drums 11a to 11d are transferred to transfer devices 15a to 15d.
After being transferred to the surface of the transfer conveyance belt 17 by d,
The density detection sensor 1 and the density recognition section 47 detect and recognize the toner density.

The CPU 44 compares the target density of each tone test pattern previously stored in the memory 49 with the toner density recognized by the density recognition unit 47 from the tone test pattern image actually formed. A tone correction curve is determined based on the result.

Here, the gradation correction curve is a reference for performing appropriate gradation correction on the image input data in the gradation correction section 46. Are associated one-to-one. This gradation correction curve is represented as a curve on coordinates where the horizontal axis represents the density of image input data and the vertical axis represents the density of image output data. The gradation correction curve is stored as a look-up table in the memory 49 or the like, and is updated in a halftone process control (hereinafter, referred to as a halftone process control) which will be described later. Hereinafter, the process of correcting the gradation correction curve by the halftone process controller is simply referred to as a correction process.

As a method of determining or correcting the gradation correction curve, there are two methods of a simple halftone process control and a detailed halftone process control.
There are types. In order to update the previously corrected tone correction curve in the halftone process control, the detected data at the previous correction and the detected data at the current correction are compared.
Therefore, data such as an approximate straight line obtained based on the detection data at the time of the previous correction is stored in the memory 49.

A. Simple halftone process control In the correction process using the simple halftone process control, first, a tone test pattern image is formed on the transfer / conveyance belt 17 in the same manner as described above. Here, as the density of the formed tone test pattern, that is, the density defined by the tone test pattern data, for example, values of four points equally divided from the lowest density to the highest density are used. The rotation speed of the developing roller as a developing condition when developing the electrostatic latent image uses a value set at the time of the maximum density correction processing.

Next, the density of the formed gradation test pattern image is detected by the density detection sensor 1 for each pattern image, and is recognized by the density recognition section 47. here,
FIG. 3 shows the density (duty) of the tone test pattern data and the density (ID) of the detection data recognized by the density recognition unit 47 from the tone test pattern image actually formed.
There is a relationship shown in In FIG. 3, the horizontal axis represents the density (duty) of the gradation test pattern data, and the vertical axis represents the density (ID) of the detection data. The solid line in the figure is an approximate straight line obtained by linearly approximating the relationship between the densities of the four points at the time of the current correction, and the broken line in the figure shows the approximate straight line at the time of the previous correction.

Here, in order to update and correct the previously corrected gradation correction curve based on the detection data at the time of the current correction, an approximate straight line of the detection data at the time of the previous correction and the detection data at the time of the current correction are used. The correction amount may be determined based on the difference from the approximate straight line. As a method for determining this correction amount,
There are a parallel shift method and a weighted shift method.

A. Determination of Correction Amount by Parallel Shift Method In this method, correction processing is performed by moving a gradation correction curve in parallel. Therefore, the amount of shift of the approximate line of the detected data at the time of the current correction with respect to the approximate line of the detected data at the time of the previous correction is measured.

Specifically, as shown in FIG.
specific concentration in the duty (here, duty = a)
, The value of the ID (here, ID = A) on the approximate straight line (broken line) at the time of the previous correction, and the value of duty on the approximate straight line (solid line) of the current correction at this time (here, the duty) = A '), and a fixed concentration (duty =
The variation (a'-a) for a) is calculated.

The amount of change (a'-
The correction process can be performed by moving the gradation correction curve in parallel using (a) (see FIG. 4B).
Here, the fixed density is preferably set in a density range from a halftone region to a highlight region (low-density region) where it is relatively easy to visually determine the color change of the image.

B. Determination of Correction Amount by Weight Shift Method In this method, the tone correction curve is weighted according to the density and moved. Generally, in a low-density region and a high-density region of an image, a change in gradation is small. This is because, in the low-density region, the toner density itself is low, so that the fluctuation of the density due to the state of the toner and the developing conditions is small.
This is because gradation is corrected in the high density region by the maximum density correction process.

Therefore, as shown in FIGS. 5A and 5B, for each of the plurality of divided density levels from the low density level to the high density level, the density level of the fixed density in the parallel shift method is set to 100. As a percentage, a weighting coefficient is set which is high at the medium density level and low at the low density level and low at the high density level. Move the curve.

That is, as shown in FIG. 6A, the approximate straight line at the time of the previous correction and the approximate straight line at the time of the current correction are compared, and the variation (a'-a) for the fixed density (duty = a) is obtained. Is calculated for the density levels including the fixed density, and for the other density levels, the amount of movement is obtained using a preset weighting coefficient. As shown in FIG. The gradation correction curve is moved by the movement amount for each of the density levels.

As described above, by moving the gradation correction curve by the movement amount determined for each density level, more accurate correction processing can be performed. In particular, by making the movement amount at the medium density level relatively large and making the movement amount at the low density level and the high density level relatively small, the correction processing is more accurate than the correction processing by the parallel shift method. It can be performed.

In the correction processing by the simple half tone process control,
The gradation correction curve is moved by using any one of the parallel shift method and the weighted shift method.

B. Detailed halftone process control FIG. 7 is a diagram for explaining the processing contents in the detailed halftone process control. In the process of the detailed halftone process control, first, a tone test pattern divided into a larger number than the number of divisions of the tone test pattern formed in the simple halftone process control is formed on the transfer conveyance belt 17. That is, in the detailed halftone process control, the range from the lowest density to the highest density is divided into finer density areas than in the simple halftone process control, and a gradation test pattern including a pattern corresponding to each density area is formed.

Next, the density of each pattern corresponding to each density area in the gradation test pattern is determined by the density detection sensor 1.
And the density is recognized by the density recognition unit 47. At this time, in order to reduce the influence of noise and obtain sufficient sensor sensitivity, only the density range where the density ID of the detection data is 5% or more is recognized on the low density side.

Furthermore, the detection data of, for example, five patterns formed in a range (low density range) where the density ID of the detection data is 5% or more and 35% or less, are linearly approximated, and the rising point of the detection data at which ID = 0 is obtained. Pattern density (d
uty) (see FIG. 7A). Thereafter, data of a total of six points including the obtained data of the rising point is approximated by a curve such as a quadratic curve.

Thereafter, the saturation point of the detection value of the density detection sensor 1 where the ID is larger than 35% (point E in FIG. 7C)
In the following range (medium density range), an approximate curve is obtained using test pattern data and detection data of a pattern formed in this density range. In the transition region from the low-density region to the medium-density region, a straight line is obtained by interpolating between points on approximate data (points C and D in FIG. 7B) in each region.

In this way, a single tone curve is created by connecting approximate curves created for each density area using the density data of a plurality of patterns constituting the tone test pattern. Further, a duty value that can obtain the same output as the target value of each density level stored in advance is read from the created tone curve, and the difference between the read value and the initial value is used as a correction amount to set the tone of each density level. Correct the curve.

The range (high density range) where the ID is larger than the saturation point of the detection value of the density detection sensor 1 and equal to or less than the maximum value of the detection data (point F in FIG. The same processing is performed.

Comparing the simple halftone process control described above with the detailed halftone process control, the number of test patterns to be created is small in the simple halftone process control, so that the processing time is shorter and the toner consumption is smaller than in the detailed halftone process control. However, the accuracy of the correction is poor. On the other hand, since the number of test patterns to be created is large in the detailed halftone process control, it can be corrected more accurately than in the simple halftone process control, but the processing time becomes longer and the toner consumption increases.

Therefore, when correcting the gradation correction curve,
By appropriately using the simple halftone process control and the detailed halftone process control as appropriate, it is possible to perform the optimum correction according to the situation. Hereinafter, a method for selecting the simple halftone process control and the detailed halftone process control will be described.

FIG. 8 is a flowchart showing a process of selecting a simple halftone process control and a detailed halftone process control in the image processing method according to the first embodiment of the present invention. In the selection processing in the image processing method according to this embodiment,
At the execution timing of a predetermined process control, first, a simple halftone process control is started to form a gradation test pattern (101). Next, the density of the gradation test pattern is measured and recognized (102). Further, approximation data is obtained by performing approximation processing on the detected data (103).

The approximate data is data representing a straight line approximating the detected data, and the approximate data represented by the approximate straight line is called an approximate straight line. In this approximation process, based on the approximation straight line, the gradient of the straight line, that is, the change rate (ΔID / Δduty) of the detection data (ID) with respect to the density (duty) of the gradation test pattern is calculated.

Next, the change rate at the time of the last correction and the change rate at the time of the current correction are compared (104), and if the difference exceeds a predetermined value (for example, 0.1 or more), The difference between the slope of the approximate straight line and the correction at this time is large, that is, the change in the gradation of the toner image is relatively large.
The process proceeds to the detailed halftone process control (105). On the other hand, if the difference does not exceed the predetermined value, the difference between the slope of the approximate straight line between the previous correction and the current correction is small, and it can be determined that the change in the gradation of the toner image is relatively small. Continue the tone control.

With the above processing, based on the slope of the approximate straight line obtained in the simple halftone process computer, the change in the gradation of the toner image between the last correction and the current correction is determined according to whether the change is large. Selectively execute the detailed halftone process control. As a result, the detailed halftone process control is not performed in a situation in which the gradation change of the toner image is small between the previous correction and the current correction, and a sufficiently accurate tone correction curve can be obtained by the simple halftone process control. In this way, while suppressing the decrease in the operation efficiency of the apparatus and the increase in the toner consumption,
By appropriately correcting the gradation correction curve, it is possible to reliably prevent the fluctuation of the gradation of the toner image.

FIG. 9 is a flowchart showing a process of selecting a simple halftone process control and a detailed halftone process control in the image processing method according to the second embodiment of the present invention. In the selection processing in the image processing method according to this embodiment,
When the difference between the rate of change at the previous correction and the rate of change at the current correction does not exceed a predetermined value in the process of 104 in FIG. 8, at a fixed density (duty = a) included in the low density area,
The absolute value | A'-A | of the difference between the value A 'on the approximate straight line at the previous correction and the value A on the approximate straight line at the current correction is calculated (11
1) The calculated absolute value | A'-A | is compared with a reference value (112).

The reference value is, for example, a value of 10% of the value A on the approximate straight line at the time of correction this time. In this comparison, if the absolute value | A'-A | is equal to or larger than the reference value, the movement amount of the approximate straight line is large between the previous correction and the current correction (the state of FIG. 10A). Then, the correction process by the simple intermediate program is continued (113). Absolute value |
If A'-A | is less than the reference value, the amount of movement of the approximate straight line between the previous correction and the current correction is small (the state shown in FIG. 10B). The correction processing is stopped (114).

With the above processing, the simple halftone process control is selectively stopped based on the movement amount of one point on the approximate straight line obtained in the simple halftone process control. As a result, in the situation where the amount of movement of the approximate straight line between the previous correction and the current correction is small and the image input data can be accurately converted to the image output data by the gradation correction curve updated at the previous correction, the correction by the halftone processing computer is performed. By not performing the process, it is possible to further suppress a decrease in the operation efficiency of the apparatus and an increase in the toner consumption.

FIG. 11 is a flowchart showing a process of selecting a simple halftone process control and a detailed halftone process control in the image processing method according to the third embodiment of the present invention.
In the selection processing in the image processing method according to this embodiment, processing of 121 to 123 is executed instead of the processing of 104 in FIG. That is, the fixed concentration (d
uty = a) and a fixed concentration (dut) included in the high concentration region.
y = b), the absolute values | A′−A |, | B′− of the differences between the values A ′ and B ′ on the approximate straight line at the time of the previous correction and the values A and B on the approximate straight line at the time of the current correction B |
1) Further, the ratio of the difference between the absolute value | A'-A | and | B'-B | with respect to the calculated absolute value | A'-A | is calculated (122), and the calculated ratio is used as a reference value ( For example, 10%)
(123).

In this comparison, when the ratio is equal to or more than the reference value, the slope of the approximate straight line changes greatly between the previous correction and the current correction (FIG. 12A).
State), the process shifts to a detailed halftone process control (105), and more detailed correction processing is performed. On the other hand, when the ratio is less than the reference value, the slope of the approximate straight line does not change so much between the previous correction and the current correction (FIG. 1).
2 (B)), the correction process is performed by the simple halftone process control.

With the above processing, based on the detected data at the fixed density at two points on the approximate straight line, the slope from the previous correction to the approximate straight line at the current correction is calculated based on the change in the slope between the approximate straight line at the previous correction and the approximate straight line at the current correction. Depending on the tonality change state,
It is possible to appropriately correct the gradation correction curve and reliably prevent the gradation of the toner image from changing, while suppressing a decrease in the operation efficiency of the apparatus and an increase in the toner consumption.

FIG. 13 is a flowchart showing a process of selecting a simple halftone process control and a detailed halftone process control in the image processing method according to the fourth embodiment of the present invention.
In the selection processing in the image processing method according to the present embodiment, when the ratio is less than the reference value in 123 in FIG. 11, the fixed density (duty = a) included in the low density area is set on the approximate straight line at the time of the previous correction. Is calculated (131), and the calculated absolute value | A'-A | is compared with a reference value. (132).

In this comparison, when the absolute value | A'-A | is equal to or larger than the reference value, the movement amount of the approximate straight line is large between the previous correction and the current correction (FIG. 1).
0 (A) state, the correction process is performed by the simple intermediate process control (133). If the absolute value | A'-A | is less than the reference value, the amount of movement of the approximate straight line between the previous correction and the current correction is small (the state of FIG. 10B), The correction process by the key control computer is stopped (134).

With the above processing, the simple halftone process control is selectively stopped based on the movement amount of one point on the approximate straight line obtained in the simple halftone process control. Due to this, in the situation where the amount of movement of the approximate straight line at the time of this correction with respect to the approximate straight line at the previous correction is small and the image input data can be accurately converted to the image output data by the gradation correction curve updated and corrected at the previous correction. Correction processing by the halftone processing computer is not performed, and it is possible to further suppress a decrease in the operation efficiency of the apparatus and an increase in the toner consumption.

[0077]

According to the present invention, the following effects can be obtained.

(1) In the simple correction processing in which a smaller number of pattern images are formed than in the detailed correction processing, the approximate straight line in the previous correction processing and the approximate straight line in the current correction processing are compared. Based on the detailed correction process in which a large number of pattern images are formed as compared with the simple correction process, the approximate straight line at the time of the current correction process is larger than the approximate straight line at the time of the previous correction process. In a state where the gradation correction curve has not been changed and the gradation correction curve can be appropriately corrected by the simple correction processing, the detailed correction processing is not performed, and the time required for the image processing can be reduced.
The operation efficiency of the image forming apparatus can be improved. Further, waste of toner due to unnecessary pattern images can be prevented, and the running cost of the image forming apparatus can be reduced. On the other hand, when the approximate straight line at the time of the current correction process is significantly different from the approximate straight line at the time of the previous correction process and the tone correction curve cannot be properly corrected by the simple correction process, the detailed correction process is performed. By correcting the gradation correction curve in detail, it is possible to maintain image quality excellent in gradation reproducibility.

(2) The detailed correction process is selectively executed based on the comparison result between the slope of the approximate straight line at the time of the previous correction process and the slope of the approximate straight line at the time of the current correction process, thereby obtaining the approximate straight line. By processing according to the degree of change between the previous correction processing and the current correction processing determined based on the inclination,
The gradation correction curve can be accurately corrected.

(3) The difference between the approximate straight line in the previous correction processing and the approximate straight line in the current correction processing at the two points representing the low-density area and the high-density area is approximated in the previous correction processing. By comparing the inclination of the straight line with the inclination of the approximate straight line at the time of the current correction processing, and selectively executing the detailed correction processing based on the comparison result, the previous correction processing based on the inclination of the approximate straight line is performed. The degree of change between the time and the time of the current correction processing can be easily and accurately determined.

(4) Based on the comparison result between the approximate straight line at the time of the previous correction process and the approximate straight line at the time of the current correction process, shift to the detailed correction process, continue the simple correction process, or execute the simple correction process. By executing one of the cancellations, the content of the correction processing of the gradation correction curve can be selected more finely according to the degree of change between the previous correction processing and the current correction processing, and the predetermined image While maintaining the quality, it is possible to more appropriately prevent excessive consumption of the toner, early consumption of the photoconductor, increase in the load on the cleaner, and the like.

(5) By selecting the suspension or continuation of the simple correction processing based on the result of comparing the density in the low density region between the approximate straight line at the previous correction processing and the approximate straight line at the current correction processing. In addition, the image formed based on the image data after the correction using the corrected gradation correction curve does not have a background fog, and can maintain good image quality.

(6) At the time of the simple correction processing, the gradation correction curve is corrected based on a predetermined weighting coefficient for each of a plurality of density levels, so that the density of an image to be formed and the effects of other correction processing are taken into consideration. Thus, the gradation correction curve can be more realistically corrected.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating a configuration of an image forming unit of a digital color copying machine that is an image forming apparatus that executes an image processing method according to an embodiment of the present invention.

FIG. 2 is a block diagram illustrating a configuration of an image processing unit of the digital color copying machine.

FIG. 3 is a diagram showing an approximate straight line at the time of a gradation correction curve correction process in the image processing method.

FIG. 4 is a diagram for explaining a method of correcting a gradation correction curve by a simple halftone process controller in the image processing method.

FIG. 5 is a diagram showing an example of weighting coefficients used for a simple halftone process control by a weighting shift method in the image processing method.

FIG. 6 is a diagram for explaining a method of correcting a tone correction curve in a simple halftone process computer using the same weighted shift method.

FIG. 7 is a diagram illustrating a method of correcting a gradation correction curve by a detailed halftone process control in the image processing method.

FIG. 8 is a flowchart showing a processing procedure when correcting a gradation correction curve in the image processing method according to the first embodiment of the present invention.

FIG. 9 is a flowchart showing a processing procedure when correcting a gradation correction curve in an image processing method according to a second embodiment of the present invention.

FIG. 10 is a diagram for explaining a method of comparing approximate straight lines in a gradation correction curve correction process of the image processing method.

FIG. 11 is a flowchart showing a processing procedure when correcting a gradation correction curve in an image processing method according to a third embodiment of the present invention.

FIG. 12 is a diagram for explaining a method of comparing approximate straight lines in the gradation correction curve correction process of the image processing method.

FIG. 13 is a flowchart showing a processing procedure when correcting a gradation correction curve in an image processing method according to a fourth embodiment of the present invention.

[Explanation of symbols]

 1-Density Detection Sensor 10-Image Forming Process Section 10a-10d-Image Forming Station 17-Transfer Conveying Belt

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) // G03G 15/01 B41J 3/00 A 9A001 (72) Inventor Shinji Imagawa 22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka, Japan No. 22 Sharp Corporation (72) Inventor Toshiaki Ino 22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka Prefecture Inside Sharp Corporation (72) Takashi Kitagawa 22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka (72) Inventor Yasutaka Maeda Inventor Yasutaka Maeda 22-22 Nagaikecho, Abeno-ku, Osaka City, Osaka F-term (reference) 2C262 AA24 AA26 AA27 AB07 AB11 AC02 AC04 BA02 BA09 BB03 BB30 BB36 BC07 BC10 EA04 EA16 FA13 GA02 2H027 DA09 DB01 EB03 EB04 EC03 EC06 EC20 EF09 2H030 AA02 AA03 AB02 AD12 AD16 BB02 BB36 5B057 CA01 CA07 CB01 CB07 CE11 CE13 CH11 DB02 DB06 5C077 MM27 MP01 MP08 PP15 PP33 PP43 PP47 PQ08 PQ20 RR19 SS01 9A001 GG15 HH23 JJ35 KZ42

Claims (6)

[Claims] 1. A test pattern image comprising a plurality of pattern images formed based on density data set in advance, wherein a tone correction curve defining a correction amount according to image density when correcting image data is used. An image processing method including a correction process for correcting the number of pattern images, wherein the correction process is two types of correction processes in which the number of pattern images constituting the test pattern image is different, and the simple correction process and the number of pattern images are smaller. The detailed correction process can be executed more frequently, and an approximate straight line representing the relationship between the detection data and the density data of the test pattern image formed in the simple correction process is obtained. Selecting a continuation of the simple correction process or a shift to the detailed correction process based on a change state of the approximate straight line at the time of the correction process. Image processing method. 2. The method according to claim 1, wherein the difference between the slope of the approximate straight line at the time of the previous correction processing and the slope of the approximate straight line at the time of the current correction processing is set as the change state, and the continuation of the simple correction processing or the transition to the detailed correction processing is selected. The image processing method according to claim 1, wherein: 3. A difference between an approximate straight line at the time of the previous correction process and an approximate straight line at the time of the current correction process at two points representing the density of the low density region and the high density region is determined. 2. The image processing method according to claim 1, wherein a continuation of a simple correction process or a shift to a detailed correction process is selected as a ratio of a difference in a density range to the change state. 4. When the transition to the detailed correction process is not selected, the density of a preset representative point is determined based on the difference between the approximate straight line in the previous correction process and the approximate straight line in the current correction process. 4. The image processing method according to claim 1, wherein a selection is made to stop or continue the simple correction process. 5. The image processing method according to claim 4, wherein the density at the representative point is a density representing a low density area. 6. The image according to claim 1, wherein said simple correction processing corrects a gradation correction curve using a weighting coefficient preset for each of a plurality of density levels. Processing method.