JP2001309173A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device that can prevent gradation jump or the like caused when a gradation conversion characteristic has a saturation region. SOLUTION: The image forming device is provided with a storage means that stores either tables used for dither processing, a detection means that detects the gradation characteristic, and a selection means that selects a dither table corresponding to the gradation characteristic detected by the detection means, and conducts the dither processing by using the selected dither table.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus used in an image forming apparatus such as a copying machine for forming an image by a digital method. The present invention relates to an image forming apparatus that forms a smooth image without a jump.

[0002]

2. Description of the Related Art Conventionally, in a subsequent stage of gradation processing (dither processing), changes in gradation characteristics such as development characteristics and latent image characteristics of an image forming section are corrected to eliminate or suppress changes in gradation characteristics. Image forming apparatus such as a copying machine that changes a gradation conversion table (hereinafter, referred to as a second gradation conversion table) for the image forming unit according to a temporal change of the gradation characteristic of the image forming unit.

In this image forming apparatus, when the amount of toner adhering to a transfer body or an image carrier is large, image bleeding due to scattering of toner or toner on the image carrier is sufficiently transferred. Color reproducibility is deteriorated due to insufficient transfer or the like, which is not transferred to the medium. In order to prevent this, the second gradation is used to limit the emission energy of the laser, which is the writing energy for writing to the photoconductor (latent image carrier), so as not to be larger than a certain value. The gradation conversion characteristic of the conversion table is not proportional to the output signal from the image processing unit, and may be changed so as not to change (saturate) at a certain output value.

[0004]

[Problems to be solved by the invention]

[0005] In this case, however, an image defect such as a gradation jump may occur in relation to the dither table. For example, as shown in FIG. 11, the output value of the dither table as the gradation processing means is 8 bits (0
0h to FFh in hexadecimal notation), as an example, a case where the pixels a00 to a11 forming the dither have a positional relationship in the main scanning direction and the sub-scanning direction as shown in FIG. In this case, as shown in FIG. 13A, the gradation characteristics of the image forming unit at the subsequent stage of the dither table are not linear, and the output characteristics of the image forming unit change with respect to the output values C0h to FFh. Absent. Therefore, the input value to the dither table using this output value range, that is, the output value of the printer γ table, does not change the image density. As a result, as shown in FIG. 14, problems such as loss of gradation and occurrence of gradation skipping occur.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and has the advantage that the set values of the dither table are changed in accordance with the conversion characteristics of the gradation conversion means after the gradation processing means so that the gradation is not skipped. An object of the present invention is to provide an image forming apparatus capable of reproducing a smooth image.

[0007]

In order to achieve the above object, according to the present invention, a storage means for storing a plurality of dither tables used for dither processing, a detection means for detecting gradation characteristics, Selecting means for selecting a dither table corresponding to the gradation characteristic detected by the means,
The dither processing is performed using the selected dither table.

According to a second aspect of the present invention, in the image forming apparatus of the first aspect, the image forming apparatus performs a tone conversion process after performing the dither process, and the detecting means includes a tone conversion device in the tone conversion process. It is characterized by detecting characteristics.

According to a third aspect of the present invention, in the device according to the second aspect, when the gradation characteristic has a saturation region in which the output signal does not change in accordance with the input signal, the selecting means may select the saturation region. A dither table that corresponds to any one of the area of the write value and the area of the latent image formation energy value of the saturation area and that converts the gradation characteristic into one having no saturation area is selected. It is characterized by.

According to a fourth aspect of the present invention, in the device according to the third aspect, the selecting means includes a dither table for increasing a write value not included in the saturated region and a latent image forming energy value not included in the saturated region. It is a feature that is selected.

According to a fifth aspect of the present invention, in the apparatus according to any one of the first to fourth aspects, the image forming apparatus performs dither processing on the gradation pattern when forming the gradation pattern image after performing the dither processing. It is characterized by performing processing.

[0012]

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

As an embodiment of the present invention, an example in which the image forming apparatus according to the present invention is an electrophotographic copying machine (hereinafter referred to as a copying machine) is shown. 1 to 53
1 shows an embodiment of an image forming apparatus to which the present invention is applied. In addition, the numerical value used below and ending with h is a numerical value in hexadecimal notation. For example, if it is indicated as C0h, it means "C0" in hexadecimal and 10
In radix, it means “192”.

Hereinafter, the configuration of a copying machine applied to the present invention will be described. According to the configuration diagram of the copying machine shown in FIG. 2, a photosensitive member around an organic photoconductor (OPC) drum 102 of φ120 [mm] serving as an image carrier, which is arranged substantially at the center of the copying machine main body 101, is provided. A charger 103 for charging the surface of the body, a laser optical system 104 for irradiating a semiconductor laser beam onto the surface of the uniformly charged photosensitive drum 102 to form an electrostatic latent image, and applying a toner of each color to the electrostatic latent image. A black developing device 105 that supplies and develops and obtains a toner image for each color, 3 each for yellow Y, magenta M, and cyan C
Color developing devices 106, 107, and 108; an intermediate transfer belt 109 for sequentially transferring toner images of each color formed on the photosensitive drum 102; a bias roller 110 for applying a transfer voltage to the intermediate transfer belt 109; The cleaning device 111 for removing the toner remaining on the surface of the photosensitive drum 102, the charge removing unit 112 for removing the charge remaining on the surface of the photosensitive drum 102 after transfer, and the like are sequentially arranged along the rotation of the photosensitive drum 102. Are arranged.

A transfer bias roller 113 for applying a voltage for transferring the transferred toner image to a transfer medium is provided on the intermediate transfer belt 109, and a transfer bias roller 113 for removing the toner image remaining on the intermediate transfer belt 109 after the transfer. A belt cleaning device 114 is provided. A fixing device 116 for fixing the transferred toner image by heating and pressurizing is disposed at an end of the transfer belt 115 that conveys the transfer medium separated from the intermediate transfer belt 109 by heating and pressing. A paper output tray 117 is attached to a transfer medium outlet 116.

On the laser optical system 104, a contact glass 118 serving as a document placing table placed on the copying machine main body 101, an exposure lamp 119 for irradiating a document on the contact glass 118 with scanning light, and an exposure lamp 119 Mirror 121 that guides the reflected light illuminated by the original and reflected by the original to an imaging lens 122 that forms an image of the reflected light guided by the reflecting mirror 121
There are an image sensor array 123 of a CCD (Charge Coupled Device) for converting the reflected light imaged by the imaging lens 122 into an electric signal, and an image forming apparatus not shown in FIG. The image sensor array 12
The image signal converted into the electric signal in 3 controls the laser oscillation of the semiconductor laser in the laser optical system 104 via the image forming apparatus.

Hereinafter, a control system built in the copying machine will be described. FIG. 3 is a diagram showing a control system built in the copying machine. According to FIG. 3, a main control unit (CPU) 130 exists in the control system.
32 and an interface I /
Via O133, laser optical system control unit 134, power supply circuit 135, optical sensor 136 (a, b, c), toner density sensor 137, environment sensor 138, photosensitive drum surface potential sensor 139, toner supply circuit 140, intermediate transfer The belt drive unit 141 and the operation unit 142 are connected.

A laser optical system controller 134 adjusts the laser output, a power supply circuit 135 applies a predetermined discharge voltage to the charger 103, and a predetermined voltage to the black developing device 105 and the color developing devices 106, 107 and 108. And a predetermined transfer voltage is applied to the bias roller 110 and the transfer bias roller 113.

The optical sensor 136 is connected to the photosensitive drum 102
And a light-emitting element such as a light-emitting diode and a light-receiving element such as a photosensor arranged in the vicinity of the area after transfer of the toner. The amount of toner adhering to the background of the medium is detected for each color, and the residual potential of the photosensitive drum 102 after static elimination is detected.

The optical sensors 136a, 136b and 136c are arranged in the same order as the photosensitive drum 102 and the intermediate transfer belt 109, respectively.
The optical sensor 136a detects the amount of toner attached to the photosensitive drum 102, the optical sensor 136b detects the amount of toner attached to the intermediate transfer belt 109, The optical sensor 136c detects the amount of toner attached to the transport belt 155. Practically, the above three optical sensors 136
May be detected by any one of them.

The detection output signal from the optical sensor 136 is
The voltage is applied to an optical sensor control unit (not shown). The optical sensor control unit calculates the ratio between the toner adhesion amount in the toner image of the detection pattern latent image and the toner adhesion amount in the background portion of the transfer medium, compares the ratio value with a predetermined reference value, and detects the change in image density. Upon detection, the toner density sensor 137 corrects the control value of the toner supply signal applied to the toner supply circuit 140.

Further, the toner density sensor 137 includes a black developing device 105, color developing devices 106, 107 and 10
8, the toner density is detected based on the change in the magnetic permeability of the developer, and the detected toner density value is compared with a predetermined reference value. If the toner density falls below a certain value, the toner density falls. The toner supply circuit 140 has means for applying a toner supply signal indicating that the developer corresponding to the minute is supplied. The photosensitive drum surface potential sensor 139 detects the surface potential of the photosensitive drum 102, and the intermediate transfer belt driving unit 141 controls driving of the intermediate transfer belt.

Hereinafter, the developing device will be described.
Since each developing device has the same configuration and performs the same operation,
The magenta developing device 107 will be described with reference to the example of FIG. A developer containing a magenta toner and a carrier is contained in the magenta developing device 107, and the developer is stirred by the rotation of the developer stirring regulating member 202M, and the developer stirring regulating member 20 on the developing sleeve 201M.
The amount of developer pumped to the developing sleeve 201M is adjusted by 2M. The supplied developer is magnetically carried on the developing sleeve 201M, and rotates in the rotation direction of the developing sleeve 201M as a magnetic brush.

FIG. 1 is a block diagram for explaining the internal configuration of the image forming apparatus according to the embodiment of the present invention. According to FIG. 1, the image forming apparatus includes a scanner 42.
0, shading correction circuit 401, area processing circuit 4
23, scanner γ correction circuit 402, image memory 403,
Image processing circuit 404, MTF filter 405, color conversion U
CR processing circuit 406, pattern generation circuit 421, scaling circuit 407, image processing circuit 408, image processing printer γ
Correction circuit 409, gradation processing circuit 410, I / F selector 411, pattern generation circuit 422, image forming printer gamma correction circuit 412, printer 413, system controllers 417 and 419, serial I / F 425, ROM
414, CPU 415, RAM 416, and BUS4
26.

The R, G, and B image signals obtained by scanning the original with the scanner 420 are subjected to shading correction by a shading correction circuit 401.
The unevenness in the main scanning direction is corrected. This image signal is 10
It is output by a bit signal. Area processing circuit 423
Generates an area signal indicating an area in image data on which image processing is being performed. As described above, by performing image processing for each region, it is possible to set optimal image processing parameters such as a color correction coefficient, a spatial filter, and a gradation table for each region. Examples of the type of the area include a character area, a silver halide photograph (photographic paper) area, a print manuscript area, an area printed by an inkjet printer, an area where a highlighter is used, a map area, There are areas printed by a thermal transfer printer.

The scanner γ correction circuit 402 converts the image data from reflectance data to brightness data. The image memory 403 stores the image data converted by the scanner γ correction circuit 402. The image separation circuit 404
Determining whether the area is a text area or a photo area,
Then, it is determined whether the area is a chromatic area or an achromatic area.

The MTF filter 405 performs processing such as smoothing to change the frequency characteristics of the image signal. Also, MTF
The filter 405 performs an adaptive edge enhancement process according to the edge degree of the image data. The adaptive edge enhancement processing is, for example, processing for performing edge enhancement in a character area and not performing edge enhancement in a halftone area. This adaptive edge enhancement processing is performed on all R, G, and B image data.

FIG. 5 is a block diagram showing the internal configuration of the MTF filter. As shown in FIG. 5, the image signal input from the scanner γ correction circuit 402 is smoothed by a smoothing filter 1101. For this smoothing, for example, coefficients as shown in Table 1 below are used.

[0029]

[Table 1]

The Laplacian filter 1102 extracts a differential component of the image data. As the Laplacian filter, for example, coefficients as shown in Table 2 below are used.

[0031]

[Table 2]

The edge amount detection filter 1103 detects an edge from the image data of the area not subjected to the γ correction by the scanner γ correction circuit 402. This edge detection is performed, for example, only for the 8-bit component from the head.
6 to 9 show an example of an edge detection filter in the sub-scanning direction, an example of an edge detection filter in the main scanning direction, a first example of an edge detection filter in an oblique direction, and a second example of an edge detection filter in an oblique direction, respectively. This is an example. Of the edge amounts obtained using these edge filters, the maximum value is the edge degree. This edge degree is smoothed by the smoothing filter 1104. This smoothing is performed in order to reduce the sensitivity difference between the even pixels and the odd pixels in the scanner 420. When the sensitivity difference is small, it is not necessary to perform the process. For example, coefficients as shown in Table 3 below are used.

[0033]

[Table 3]

The table conversion circuit 1105 specifies the density of lines and points (contrast, density, etc.) and the smoothness of halftone dots by converting the degree of edge into a table. This edge degree is largest when black lines or dots are drawn on a white background, and smaller when pixel boundaries are smooth such as fine halftone dots in printing, silver halide photographs, thermal transfer originals, etc. Become.

The MTF filter 405 multiplies the product of the edge degree (image signal C shown in FIG. 5) converted by the table conversion circuit 1105 and the output value (image signal B shown in FIG. 5) of the Laplacian filter 1102 (FIG. 5). And an image signal (image signal E shown in FIG. 5) obtained by adding the image signal D shown in FIG. 5 and the image signal smoothed by the smoothing filter 1101 (image signal A shown in FIG. 5) to the image processing circuit 404. I do.

The color conversion UCR processing circuit 406 has a color correction processing section and a UCR processing section.

This color correction processing section corrects the difference between the color separation characteristics of the input system and the spectral characteristics of the color system of the output system, and determines the amount of the color material YMC required to reproduce a faithful color. calculate. This amount is obtained by performing a matrix operation as shown in Equation 1 below.

[0038]

(Equation 1)

R, G and B shown in the above formula 1 are
It is a complement of the values of red R, green G, and blue B of the image signal. The matrix coefficient aij is determined by the spectral characteristics of the input system and the output system (color material). Although the above equation 1 is a primary masking equation, color correction may be performed with higher accuracy by using higher-order terms such as quadratic terms such as B2 and BG. Further, the arithmetic expression may be changed depending on the hue, or the Neugebauer equation may be used.

The UCR processing unit replaces a portion where three colors of YMC overlap with Bk (black). The arithmetic expression used for this UCR process is shown in Expression 2 below. Y ′ = Y−α · min (Y, M, C) M ′ = M−α · min (Y, M, C) C ′ = C−α · min (Y, M, C) Bk = α · min (Y, M, C) (2) In the above equation (2), α is a coefficient for determining the amount of UCR, and when α is 1, UCR processing is performed on all signals. Note that α may be a constant value. For example, by setting α close to 1 in the high-density section and setting α close to 0 in the low-image-density section, it is possible to smooth the image in the low-image-density section.

The scaling circuit 407 performs vertical and horizontal scaling. The image processing circuit 408 performs a repeat process and the like.

The image processing printer γ correction circuit 409
The image signal is corrected according to the image quality mode of characters, photos, and the like. In addition, background removal and the like are also performed. Further, there are a plurality of gradation conversion tables that can be switched in accordance with the area signal generated by the area processing circuit 423. In this gradation conversion table, optimal image processing parameters are set for each original such as a character, a silver halide photograph (photographic paper), a print original, an inkjet original, a fluorescent pen original, a map original, and a thermal transfer original. .

The gradation processing circuit 410 performs dither processing. Dither processing is performed from 1 × 1 dither-less processing to m ×
This is performed by selecting an arbitrary size up to dither processing including n pixels (m and n are integers, the same applies hereinafter). For example,
When the dither processing can be performed on up to 36 pixels, the dither processing can be performed in a size of 6 pixels in the main scanning direction and 6 pixels in the sub scanning direction, or 18 pixels in the main scanning direction and 2 pixels in the sub scanning direction. For example, a pixel area.

FIG. 49 shows an example in which 36 pixels of 66 pixels in the main scanning direction and 6 pixels in the sub-scanning direction are used for dither processing. FIG. 50 is an example of an index table. This index table is a table in which the correspondence between each pixel and the number of the gradation table applied to this pixel is recorded. 51 to 53 show main scanning direction 2
It is an example of a gradation processing table of two pixels in the sub-scanning direction. The horizontal axis shown here is the image signal input to the pixel, and the vertical axis shows the output value from the pixel. The index table and the gradation processing table are stored in the gradation processing circuit 41.
It is stored in a register within 0. The setting values in these tables are rewritten under the control of the CPU 415. FIG.
4 is an example in which the value is set so that the pixel number is shifted by one pixel in the main scanning direction. Although not shown, it is also possible to set to shift in the sub-scanning direction. As described above, by setting the shift amount in the main scanning direction and the shift amount in the sub-scanning direction, it is possible to set gradation processing in which each screen is different for each YMCK color. FIG. 55 is an example of an index table corresponding to dither of two pixels in the main scanning direction and two pixels in the sub-scanning direction.

The output of the gradation processing circuit 410 can reduce the pixel frequency to 、, and the gradation processing circuit 410
The image data bus to the selector 411 has a width of 16 bits (two pieces of 8-bit image data) so that image signals of two pixels are simultaneously transferred to the printer unit.

The respective image processing circuits constituting the printer section of the image processing section will be described below. Even if used independently of the scanner / IPU, it functions as a printer unit.

The I / F selector 411 is connected to the scanner 42
0 so that the image signal read at 0 can be connected to be processed by an external image forming apparatus or the like, or the image signal from an external host computer or an image processing unit can be connected to be output by the printer 413. Switching function and the function of an interface when connected as described above.

Image forming processing printer γ correction circuit 412
Converts the image signal from the I / F selector 411 using the gradation conversion table, and outputs it to the laser modulation circuit in the printer 413 shown in FIG.

When an image signal is input from the external computer 418,
/ F · selector 411, gradation-converted by an image forming printer γ correction circuit 412,
An image is formed by the device 3 so as to function as a printer unit.

The above-described image processing circuits of the scanner / IPU unit and the printer unit are controlled by the CPU 415.
The BUS 426 is connected to the CPU 415.
The CPU 415 is further connected to a ROM 414 and a RAM 416 by a BUS 426, and is connected to a system controller 417 through a serial I / F 424. A command from an operation unit (not shown) or the like is transmitted through the system controller 417.
Various parameters are set in each image processing circuit based on the transmitted image quality mode, density information, area information, and the like.

The pattern generation circuit 421 and the pattern generation circuit 422 generate gradation patterns used in the scanner / IPU section and the printer section, respectively. FIG.
According to the method, the area signal is generated from the area processing circuit 423 after comparing the designated area information on the document with the reading position information at the time of image reading. The scanner γ correction circuit 402 and the MTF filter 4
05, color conversion UCR processing circuit 406, image processing circuit 40
8. Parameters used in the image processing printer γ correction circuit 409 and the gradation processing circuit 410 are changed. FIG.
At 0, the printer gamma correction circuit 409 for image processing and the gradation processing circuit 410 are particularly shown.

In the image processing printer γ correction circuit 409, the area signal from the area processing circuit 423 is decoded by the decoder 1 and is input by the selector 1 from among a plurality of gradation conversion tables such as characters or ink jet. An appropriate one is selected depending on the situation. FIG.
In the example of the original, the area 0 of the character, the area 1 of the photographic paper,
An example in which an ink jet region 2 exists is shown. For the character area 0, the character gradation conversion table 1 is selected. For the photographic paper area 1, the photographic paper gradation conversion table 3 is selected.
, The gradation conversion table 2 for inkjet is selected.

The signal subjected to gradation conversion by the image processing printer γ correction circuit 409 is again made to correspond to the area signal in the gradation processing circuit 410, and the selector is selected based on the area signal decoded by the decoder 2. By two
The gradation processing to be used is switched. Examples of usable gradation processing include processing in which dither is not used, processing in which dither is used, and error diffusion processing. The error diffusion process is performed on the ink jet document.

Whether the image signal after the gradation processing is transmitted on line 1 or line 2 is determined by the decoder 3 according to the reading position information. Whether transmission is performed on line 1 or transmission on line 2 is switched every time one pixel differs in the sub-scanning direction. The image signal of line 1 is transmitted to a FIFO (Firs
t In First Out) Temporarily stored in memory, line 1
And the image signal of line 2 are output in synchronization. By doing so, the pixel frequency is reduced by half,
It is input to the I / F selector 411.

FIG. 4 is a block diagram of the laser modulation circuit. As an example, the writing frequency is 18.6 MHz, and the scanning time of one pixel is 53.8 MHz. The 8-bit image data is stored in a look-up table (LUT) 4
51 may perform γ conversion. Next, a pulse width modulation circuit (PWM) 341 converts the image signal into an 8-level pulse width based on the upper 3 bits of the 8-bit image signal.
The power modulation circuit (PM) 453 performs 32-level power modulation on the lower 5 bits. The laser diode (LD) 454 emits light based on the modulated signal. A photodetector (PD) 455 monitors the light emission intensity and performs correction for each dot. The maximum value of the laser light intensity can be set to 8 bits (256 steps) independently of the image signal. The beam diameter in the main scanning direction is, for example, 600 DP with respect to the size of one pixel.
In I, 50 μm in the main scanning direction per 42.3 μm of one pixel,
Those having a sub scanning direction of 60 μm are used. For example, FIG.
The laser modulation circuit is prepared for each of the five line 1 and line 2 image data. The image data of line 1 and line 2 are synchronized, and scan the photoconductor in parallel with the main scanning direction.

An image reading system based on the block diagram of FIG. 45 will be described below. The image reading system is C
CD (Charge Coupled Device) 5401, amplifier circuit 54
02, S / H circuit 5403, A / D conversion circuit 5404,
Black correction circuit 5405, shading correction circuit 540
6, CCD driver 5409, pulse generator 54
10, clock generator 5411, ROM 541
3, the CPU 5414 and the RAM 5415. The image processing unit 5407 and the printer 5412
Denotes an image processing unit and an image forming unit of the copying machine.

Each image reading system circuit is a BUS54
20 and is controlled by the CPU 5414. Data necessary for the control is stored in the ROM 5413 and R
AM5415. Meanwhile, the CPU 5414
Is a system controller 54 for controlling the entire copying machine.
19 and a serial I / F 5421. Further, the CPU 5414 also controls a scanner driving device (not shown), and controls the scanner 420 shown in FIG.

The explanation of the circuit of each image reading system is given together with the explanation of the overall operation. The original to be copied is illuminated by an exposure lamp, and the reflected light is reflected by the RGB filter of the CCD 5401.
(R), Green (G), and Blue (B) are separated into respective colors, converted into analog electric signals, and amplified by an amplifier circuit 5.
The signal is amplified to a predetermined level by 402.

An image signal converted into an analog electric signal by the CCD 5401 based on the timing supplied by the pulse generator 5410 is sampled and held by an S / H circuit (sample and hold circuit) 5403, and is subjected to an A / D conversion circuit. It is converted into a digital electric signal by 5404. The image signal converted into the digital electric signal is reduced in the black correction circuit 5405 by a black level variation which is an electric signal when the amount of reflected light between chips and pixels of the CCD 5401 is small. This prevents the occurrence of streaks or unevenness in the black portion of the image.

The image signal from which black lines and unevenness have been eliminated by the black correction circuit 5405 is output to the shading correction circuit 54.
At 06, the white level, which is an electric signal when the amount of reflected light is large, is corrected. The white level is based on the white data obtained when the scanner 420 is moved to the position of the uniform white plate and is illuminated, and is read.
01 is corrected. FIG. 47 shows a conceptual diagram of the black correction and the white correction.

The image signal white-corrected by the shading correction circuit 5406 is processed by the above-described image processing unit 5407, printed and output by the printer 413, and a series of operations of the copying machine is completed.

The CCD driver 5409 includes a CCD 540
1 is supplied. A pulse source necessary for driving the CCD driver 5409 is generated by a pulse generator 5410, and the pulse generator 5410 is a reference signal generated by a clock generator 5411 including a crystal oscillator or the like. As described above, the pulse generator 5410 includes the S / H circuit 5
Reference numeral 403 supplies timing necessary for sampling and holding the image signal from the CCD 5401.

Hereinafter, each image reading circuit will be described in detail. The amplification amount of the amplification circuit 5402 is determined based on the A / D conversion circuit 5404 for a specific document density.
Is determined so that the output value of the above becomes a desired value. As an example, when an image signal with a document density of 0.05 (0.891 in reflectivity) is input during normal copying, assuming that the output value of the A / D conversion circuit 5404 is 8 bits, a 240 value is obtained. Amplify so that it is output.

On the other hand, when an image signal requiring white correction in the shading correction circuit 5406 is input, the amplification factor is reduced to increase the correction sensitivity. By doing so, when an image signal having a large amount of reflected light and exceeding the value of 255 as an 8-bit signal is input, even when the amplification factor at the time of normal copying saturates to a value of 255, a more accurate white signal can be obtained. Since the level can be detected, no error occurs in the white correction.

FIG. 48 is a schematic diagram in which an image read signal amplified by the amplifier circuit 5402 is sampled and held by the S / H circuit 5403. The horizontal axis represents the time during which the amplified analog image signal passes through the S / H circuit 5403, and the vertical axis represents the time of the image signal that is the analog electric signal that is amplified by the amplifier circuit 5402 and passes through the S / H circuit 5403. Indicates the size. The image signal of the analog electric signal input to the S / H circuit 5403 is sent to the A / D conversion circuit 5404 after being sampled and held for a predetermined sample / hold time 5501.

FIG. 52 shows an example in which an image signal requiring white correction is input, and an example in which an image signal not requiring white correction is input. As in the case described above, if an image signal having a document density of 0.05 is input, and white correction is required, the value after being converted to a digital electric signal is amplified to 180 values, When unnecessary, it is amplified to 240 values.

In the following, a description will be given of the toner adhesion characteristic of how much toner adheres to the photosensitive drum 102 with respect to the potential for forming a latent image. FIG. 18 shows a diagram of the toner adhesion characteristics with respect to the latent image potential. Although this characteristic changes depending on the change in the characteristic of the developer, it also changes when the AC (Alternative Current) developing bias condition changes.

Among the AC development bias conditions, the frequency Vf of the AC component and the voltage from peak to peak of the AC component.
If the peak-to-peak voltage, the duty ratio, or the like is changed, the characteristics of the developing unit when the toner adheres to the photosensitive drum 102 change.

In the toner adhesion characteristic indicated by the straight line a) in FIG. 18, the toner adhesion amount is not 0 when the developing potential is 0, but when an image is formed with this characteristic, the toner adheres to the non-image area. (A phenomenon called background fouling or fogging) or the development potential ΔV1
When the toner adhesion amount M1 is a desired adhesion amount,
In the case of the characteristic a), a problem such as a low toner adhesion amount and a low image density occurs, and a desired image cannot be obtained.

To avoid the problems mentioned above, b)
By changing the developing bias conditions so as to obtain the characteristics shown by the following equation, the generation of background stains can be prevented, and the amount of adhered toner required for image formation can be obtained.

FIGS. 19 and 20 are schematic diagrams of the latent image and toner adhesion. FIG. 19 is a schematic diagram of a latent image of a square patch formed on the photoreceptor, where each circle indicates a laser-irradiated latent image, and the figure below it indicates the intensity of the latent image.

FIG. 20 is an example in which the latent image shown in FIG. 19 has been developed. The developing bias frequency Vf and peak-to-pe
In a state where the ak voltage Vpp is higher than an appropriate value, such a toner attached state is obtained. The white area is the non-image area, the shaded area is the image area, and the latent image requires that the image area be in the uniform toner-adhered area. The toner is developed in a state where the toner adhesion is low.

In the following, the user selects the function so that automatic gradation correction for automatically correcting the image signal output from the scanner / IPU unit by detecting the gradation characteristic of the printer unit in FIG. 1 functions. The operation screen to be performed is described. In the copying machine according to the present embodiment, an example of what density is output for an image signal of a certain size is given as follows.
There is provided a means for creating a gradation pattern formed in a patch at a certain stage for each color and for each of the character mode and the photograph mode as shown in FIG.

On the liquid crystal screen of the operation unit shown in FIG. 22, automatic gradation correction (ACC: Auto Color Calibration) is performed.
When the menu is called, a screen as shown in FIG. 25 is displayed on the liquid crystal screen. When "execute" is selected on the screen shown in FIG. 25, the screen shown in FIG. 26 is displayed. When "when using copy" is selected, the tone correction table when using copy is used, and when "when using printer" is selected, the tone correction table when using printer is based on the reference data. Changed.

If the image formed by the YMCK gradation correction table after the change is not desirable for the user, in order to form an image with the YMCK gradation correction table before the processing, the above-mentioned respective cases in FIG. An "undo" key is attached.

Other selection items on the screen shown in FIG. 25 include “correction of background”, “correction of high density portion”, “update of target when using copy”, and “update of target when using printer”. On the other hand, there is an item in which “do” or “do not” can be selected. Since these selections are not always necessary, it may be set so that “execute” is always selected.

The operation of the copying machine according to the present embodiment for automatically correcting the image density will be described below with reference to the flowchart shown in FIG. FIG.
When the "Execute" of the automatic gradation correction for copy use or printer use is selected on the screen (step S1 /
YES), the screen of FIG. 26 is displayed. When the print start key in the screen of FIG. 26 is depressed, a plurality of density gradation patterns corresponding to each color of YMCK and each image quality mode of characters and photographs as shown in FIG. 30 are formed on the transfer medium.

The density gradation pattern as shown in FIG. 30 is stored and set in advance in the ROM of the scanner / IPU. The write value, which is the input image signal of the pattern,
00h, 11h, 22h, ..., EE in hexadecimal notation
h and FFh. In FIG. 30, patches for five gradations are displayed except for the background portion, but any value can be selected within the range of 8-bit signals from 00h to FFh. In the case of the character mode (step S3 / NO), dither processing such as pattern processing is not performed, and a pattern is formed with 256 gradations for one dot. In the case of the photo mode (step S3 / YE
S), dither processing is performed (step S5).

After the pattern is output on the transfer medium (step S6), a screen as shown in FIG. 28 for instructing to place the transfer medium on the contact glass 118 is displayed (step S7). In accordance with the instructions on the screen, the user places the output transfer medium on the contact glass 118, and selects “Start reading” or “Cancel” on the screen of FIG. 28 (Step S8).

If "cancel" is selected, the process ends. If "read" is selected, RGB data of the YMCK density pattern is read (step S9). At this time, data of a pattern portion on which a pattern is written and data of a background portion on which no pattern is written are read. Next, it is determined whether or not the data of the pattern portion has been normally read (step S10). If it is not read normally,
The screen 8 is displayed, the operation from step S9 is repeated, and if the image is not normally read a second time, the process ends (step S11).

When the data of the pattern portion is normally read, the read data is corrected for machine difference, and it is determined whether or not the background correction is performed based on the result selected on the screen of FIG. 25 ( Step S12). When "Perform" for background correction has been selected (step S13 /
YES), a background correction process is performed on the read data (step S14). If “do not perform” is selected, the process proceeds to the next step without performing the background correction process.

Based on the result selected on the screen of FIG. 25, it is determined whether or not correction is to be made in the high density portion (step S15). When “Perform” for the correction of the high density portion has been selected (step S15 / YE)
S), the high density portion is corrected (step S16).
When “Do Not Perform” is Selected (Step S15 /
NO), the process of the high density portion correction is not performed, and the process proceeds to the next step.

A YMCK gradation correction table is created and its selection is performed (step S17). Based on the result of the selection, gradation correction processing is performed on each of the YMCK colors, and if the processing is not performed (step S18 / NO).
The operation of the flowchart returns to Step S10. Similarly, the gradation correction process is performed for each image quality mode such as a photograph and a character, and if the same process is not performed (step S
(19 / NO) Return to step S4.

During the processing from step S9, the screen shown in FIG. 29 is displayed on the operation screen. YM after processing
If the image formed by the CK gradation correction table is not desirable for the user, a “Restore” key is displayed on the screen of FIG. 25 so that the user can select the YMCK gradation correction table before processing. Is displayed.

The purpose of the background correction performed in step S14 of FIG. 31 will be described below. The purpose is twofold. One is to correct the whiteness of the transfer medium itself used in executing the automatic gradation correction.
The other is that the thickness of the transfer medium (hereinafter, referred to as paper thickness) used in executing the automatic gradation correction is thin, and the color of a pressure plate or the like that presses the transfer medium is read by the scanner. This is to correct the whiteness of the transfer medium at the time.

The reason why the whiteness of the first transfer medium itself must be corrected is that even if the same gradation pattern of FIG. 30 is formed by the same machine at almost the same time, This is because the image signal from which the scanner reads the gradation pattern may have a value different from the actual value depending on the whiteness of the image.

A disadvantage in the case where this correction is not performed is that, when recycled paper having a low whiteness is used, since the recycled paper generally has a large amount of yellow component, the gradation correction table is set so that the yellow component is reduced. Will be corrected. When the image is copied on an art paper or the like having a high whiteness in the gradation correction table, an image having a small yellow component is formed.

When the second paper thickness is thin and the color of the pressure plate is read through, the reason why the whiteness of the transfer medium must be corrected is that the image signal read by the scanner depends on the color of the transparent platen. This is because the value may be different from the one described above.

For example, an ADF (Auto Doc)
ument Feeder), a rubber belt is sometimes used to transport the originals.However, the rubber material may have low whiteness and a slight gray tint. is there. In this case, since the image signal to be read is also read as an image signal that has been increased as a whole, when the YMCK gradation correction table is created, the YMCK tone correction table is created so as to be thinner by the increase. When a transfer medium having a large paper thickness and poor light transmittance is used in the gradation correction table, the image is reproduced as a low density image as a whole.

In order to eliminate the problems represented by the above two cases, the image signal read from the pattern portion is corrected based on the image signal read from the background portion of the transfer medium. However, there is a merit even when the above correction is not performed. When transfer media having the same properties are always used, a gradation correction table is created according to the properties of the transfer medium. If only a medium is used, there is an advantage that color reproduction is improved.

Considering a specific example, when a transfer medium having a large amount of yellow component such as recycled paper is always used, the background correction is not performed for the color containing the yellow component. Says that a tone correction table that creates good color reproduction is created, and if a transfer medium with a small paper thickness is always used, the tone correction table is created in a state that matches the transfer medium with a thin paper. There are benefits. Turn on the background correction according to the user's situation and preference as described above
/ OFF is performed.

In the following, there will be described a procedure for generating a gradation conversion table in the image processing printer γ correction circuit 409 during execution of the automatic gradation correction shown in the flowchart of FIG.

The magnitude of each image signal when forming the gradation pattern formed on the transfer paper shown in FIG.
D [i] (i = 0, 1,... 9), and the value obtained by reading the formed pattern with a scanner is represented by v [t] in a vector format.
[I] = (r [t] [i], g [t] [i], b [t]
[I]) (t = Y, M, C, orK, i = 0, 1,...)
・, 9).

Instead of (r, g, b), brightness, saturation,
Hue angles (L *, c *, h *) or lightness, redness, bluishness (L *, a *, b *) may be used. RO beforehand
The reference white reading value stored in M414 or RAM 416 is represented by (r [W], g [W], b [W]).
And

The read value of the pattern v [t] [i] =
(R [t] [i], g [t] [i], b [t] [i])
, The image signals of the respective complementary colors of the YMC toner are b [t] [i], g [t] [i], and r [t] [i], therefore, only the image signals of the respective complementary colors are used. .
In this embodiment, in order to simplify the following description, a [t]
[I] (i = 0, 1, 2,..., 9; t = C, M,
Y, or K) are used. Once the gradation conversion table is created, the processing is simple. Regarding the black toner, sufficient accuracy can be obtained regardless of which of the RGB image signals is used, but the G component is used here.

The reference data is the reading v0 of the scanner.
[T] [i] = (r0 [t] [i], g0 [t]
[I], b0 [t] [i]) and the corresponding laser writing value LD [i] (i = 1, 2,..., M). Similarly, A [t] is used only for the complementary color image signals of YMCK to simplify the following description.
[N [i]] (0 ≦ n [i] ≦ 255; i = 1, 2,
.., M; t = Y, M, C, or, K).
m is the number of reference data.

Table 4 below shows an example of the image difference correction value of the image signal. Black (G), Cyan (R), Magent
a (G) and Yellow (B) are correction values corresponding to the respective toners, and RGB in parentheses are Red (R), Green (G), and Blu read by a scanner used for automatic gradation correction.
represents the signal of e (B). For each color toner,
k (0) and k (1023) indicate correction values for the reference data value 0 and the reference data value 1023.

[0098]

[Table 4]

The value of the corrected reference data is represented by A1 [t] [n
[I]], the reference data A
[T] [n [i]] is corrected as follows. A1 [t] [n [i]] = A [t] [n [i]] + (k (1023) −k (0)) * n [i] / 1023 + k (0) (3)

FIG. 32 shows an example in which the function shown in Equation 3 is represented by a graph. The correction values in Table 4 can be set at the time of manufacture and held in the machine, or can be set by the user's hand on the liquid crystal screen of the operation unit shown in FIG. In the following, A1 [t] [n [i]] obtained by Expression 3 is newly added to A [t] [n
[I]].

The YMCK gradation conversion table is a [LD]
And the reference data A [n] stored in the ROM 414. Here, n is an input value to the YMCK gradation conversion table, and the reference data A [n]
Is to convert the input value n into gradations and write the laser value LD [i]
Is a target value of a read image signal obtained by reading the YMCK gradation pattern output by the scanner.

Here, the reference data includes two types of values: a reference value A [n] for performing correction in accordance with the image density that can be output by the printer, and an uncorrected value A [n]. The determination as to whether or not the correction is made is made based on the determination data stored in the ROM 414 or the RAM 416 in advance.

By determining the LD corresponding to A [n] from the aforementioned a [LD], the laser output value LD [n] corresponding to the input value n to the YMCK gradation conversion table is determined. This is obtained for the input values i = 0, 1,..., 255, so that a gradation conversion table is obtained.

At this time, instead of performing the above-described processing on all the values for the input values n = 00h, 01h,..., FFh (expressed in hexadecimal) in the YMCK gradation conversion table. ni = 0, 11h, 22h,
.., The above processing is performed on discrete values such as FFh, and the other points are interpolated by a spline function or the like, or in the YMCKγ correction table stored in the ROM 414 in advance. (0, LD [0]), (11h, LD [11h]) obtained by the processing,
(22h, LD [22h]), ..., (FFh, LD
[FFh]) is selected.

FIG. 34 is a quaternary chart for explaining the calculation procedure of the automatic gradation correction. In FIG. 34, the horizontal axis of the first quadrant (a) represents the input value n to the YMCK gradation conversion table. The vertical axis is the read value of the scanner after the processing, and represents the above-described reference data A [i]. The read value of the scanner after processing in this graph is obtained after the scanner reads the gradation pattern, performs RGBγ conversion on the read value, and performs average processing and addition processing on several read data in the gradation pattern. In FIG. 34, it is processed as a 12-bit data signal to improve the calculation accuracy.

In FIG. 34, the horizontal axis of the second quadrant (b) indicates the read value of the scanner after the processing similarly to the vertical axis. The vertical axis of the third quadrant (c) represents the writing value of the laser beam. This data a [LD] represents the output characteristics of the printer unit. Also, the write value of the laser light of the pattern actually formed is 00h (background), 11h, 22h,.
Although FFh is 16 points and indicates discrete values, in FIG. 34, the detected points are interpolated to form a continuous graph.

The graph in the fourth quadrant (d) represents the YMCK gradation conversion table LD [i]. The purpose is to find this table. The vertical and horizontal axes of the graph (f) are the same, including the positional relationship between the vertical and horizontal axes of the graph (d).
When a gradation pattern for detection is formed, a YMCK gradation conversion table (g) shown in a graph (f) is used.

The horizontal axis of the graph (e) is the same as that of the third quadrant (c), and the relationship between the write value of the laser beam when the gradation pattern is created and the value read by the scanner after the gradation pattern is processed is shown. For convenience, a linear transformation for convenience is shown. Reference data A for a certain input value n
[N] is obtained, and LD output LD for obtaining A [n]
[N] is obtained along the arrow (l) shown in FIG. 34 using the read value a [LD] of the gradation pattern.

The calculation procedure will be described below with reference to the flowchart of FIG. An input value required for obtaining a YMCK gradation conversion table is determined (step S101). Here, n [i] = 11h *
i (i = 0, 1,..., imax = 15).

The machine difference correction is performed (step S10).
2). The processing of the machine difference correction is as described above. Next, the reference data A [n] is corrected according to the image density that can be output by the printer (step S103).

A calculation method when the reference data A [n] is corrected to a value corresponding to the density that can be output by the printer is described below. The scanner γ correction circuit 402
This is a calculation method on the assumption that an image signal is not GBγ-converted but is proportional to the document reflectance.

It is assumed that the writing value of the laser beam that gives the maximum image density that can be created by the printer unit is FFh, and the read value m [FFh] of the pattern at this time is mmax. Reference data A [i] (i = 0, 1,..., I) not corrected from the low image density side to the intermediate image density side
1) Reference data A that is not corrected on the high image density side
[I] (i = i2 + 1,..., Imax−1) (i1
.Ltoreq.i2, i2.ltoreq.imax-1), and reference data A [i] to be corrected (i = i1 + 1,..., I2).

Of the uncorrected reference data, reference data A [i2 + 1] having the lowest image density in the high image density portion and reference data A [i1] having the lowest image density in the low image density portion.
Then, the difference Δref of the data is obtained. That is, Δref = A [i1] −A [i2 + 1] (4) RGB by the scanner γ correction circuit 402 which is an inversion process
In the case of linear reflectance or lightness where γ conversion is not performed, Δref> 0.

On the other hand, Δdet is similarly obtained from the read value mmax of the pattern that provides the maximum image density that can be created by the printer unit. That is, Δdet = A [i1] −mmax (5)

Thus, the reference data A [i] (i = i1 + 1,..., I2) in which the high image density portion has been corrected is obtained as follows: A [i] = A [i1] + (A [i] -A [I1]) * (Δdet / Δref) (i = i1 + 1, i1 + 2,..., I2-1, i2) (6)

Next, an image signal m [i] read by the scanner corresponding to n [i] is obtained from the reference data A [n] (step S104). The calculation procedure is shown below. Actually, the reference data A [n [j]] (0 ≦ n [j] ≦ 25) corresponding to the discrete value n [j]
5, j = 0, 1,..., Jmax, n [j] ≦ n
[K] forj ≦ k) is set as follows.

(0 ≦ j ≦ jmax) that satisfies n [j] ≦ n [i] <n [j + 1] is obtained. In the case of an 8-bit image signal, n [0] = 0, n [j] = 255, n [jmax +
1] = n [jmax] +1, A [jmax + 1] = A
If the reference data is obtained as [jmax], the calculation is simplified. In addition, the accuracy of the γ correction table finally obtained becomes higher when the interval of the reference data is as small as possible at intervals of n [j].

From j obtained as described above, m [i]
Is obtained by the following equation (step S105). m [i] = A [j] + (A [j + 1] −A [i]) · (n [i] −n [j]) / (n [j + 1] −n [j]) (7) )

In the present embodiment, the interpolation is performed by using a linear expression. However, a desired result can be obtained even when interpolation is performed using a higher-order function, a spline function, or the like. If it is desired to use such an expression, m [i] = f (n [i]). In the case of a k-th order function, f (x) may be represented by the following Expression 8.

[0120]

(Equation 2)

The LD write value LD [i] and the input value n
From [i], a gradation conversion table is obtained (step S106). LD [i] for that is determined in step S105.
Is determined by the same procedure as described above.

When image signal data that has not been subjected to RGBγ conversion is processed, a [LD] decreases as the value of LD increases. That is, LD [k] <LD
For [k + 1], a [LD [k]] ≧ a [LD [k
+1]].

The value at the time of pattern formation is LD [k] = 00
h, 11h, 22h, 33h, 44h55h, 66h,
88h, AAh, FFh, (k = 0, 1,..., 9)
Of 10 points. At the image density where the toner adhesion amount is small,
Since the change in the reading value of the scanner with respect to the toner adhesion amount is large, the interval between the pattern writing values LD [k] is read at a close interval.
Since the change in the reading value of the scanner with respect to the toner adhesion amount is small, the reading is performed with the interval widened.

The advantages set in this way include:
LD [k] = 00h, 11h, 22h, 33h, 44
h, 55h, 66h, 77h, 88h, 99h, AA
There is a merit that the amount of consumed toner can be suppressed as compared with the case where the number of patterns increases as in h, BBh, CCh, DDh, EEh, and FFh. As a disadvantage of narrowing the interval, the read value is often reversed due to the influence of uneven potential on the photoconductor, uneven toner adhesion, uneven fixing, uneven potential, and the like. It may not be effective for improvement.
In consideration of these merits and demerits, a pattern is formed with the above-mentioned ten LD write values.

A [LD [k]] ≧ m [i]> a [LD
For LD [k] that is [k + 1]], LD [i] = LD [k] + (LD [k + 1] -LD
[K]) · (m [i] −a [LD [k]]) / (a [L
D [k + 1]]-a [LD [k]]).

When 0 ≦ k ≦ kmax, a [LD
When [kmax]]> m [i] (when the image density of the target value obtained from the reference data is high), LD [i]
= LD [k] + (LD [kmax] −LD [kmax−
1]) · (m [i] −a [LD [kmax−1]]) /
(A [LD [kmax]]-a [LD [kmax-
1]]), and is predicted by extrapolation using a linear expression.

As described above, the set of the input value n [i] and the output value LD [i] to the YMCK gradation correction table is (n)
[I], LD [i]) (i = 0, 1,..., 15) are obtained. (N [i], LD [i]) (i
= 0, 1,..., 15) to create a tone conversion table by interpolation with a spline function or the like,
The gradation conversion table stored in the ROM 414 is selected.

FIG. 3 shows a method for detecting the development characteristics.
This is described based on the flowchart of FIG. The purpose of detecting the development characteristics is to detect when the background stain occurs, or to detect whether or not the density is secured.

In a pattern as shown in FIG. 37, np pieces (here, np = 12) on the photosensitive drum 102
Is formed (step S23).
0). The surface potential VSi (i = 1, 2,..., N) of each patch of the gradation pattern formed on the photosensitive drum 102 is detected by the photosensitive drum surface potential sensor 139.
p) is detected (step S231), and the developing device 10
The image is developed by being developed by any one of 5, 106, 107 and 108 (step S23).
2).

The detection output VPi (i = 1, 2,...) Of the toner image formed on the photosensitive drum 102 is detected by the optical sensor 136 located on the downstream side in the rotation direction of the photosensitive drum 102.
.., Np) are obtained (step S233). As an example, the output of the laser used for detecting the development characteristics is 00h, 10h, 20h, 30h,
40h, 50h, 60h, 70h, 90h, B0h, E
Values 0h and FFh are used. The development characteristics of the formed gradation pattern are predicted (step S234), and a gradation conversion table is created (step S235).

A method for correcting an image signal in consideration of the development characteristics of a copying machine will be described below with reference to FIG. The following describes the graph in each coordinate shown in FIG.

In the coordinates a), the vertical axis represents the amount of laser writing light or the image output signal when an image is formed on the photosensitive drum 102, and the horizontal axis represents the output of the optical sensor 136. The curve of the coordinate a) is obtained by the optical sensor 136 detecting the reflected light amount of the developed toner image after np density gradation patterns are developed on the photosensitive drum 102.

In the coordinate b), the vertical axis represents the laser writing light amount or the image output signal, similarly to the coordinate a).
The horizontal axis represents the surface potential of the photosensitive drum 102. Coordinates b)
2 represents the light attenuation characteristic of the photosensitive drum 102. This linear graph indicates the surface potential of the photosensitive drum surface potential sensor 139 when np density gradation pattern latent images are formed on the photosensitive drum 102.
Is obtained by the measured results.

In the coordinate c), the ordinate represents the laser writing light amount or the image output signal as in the case of the coordinates a) and b), and the abscissa represents the image input signal (an amount proportional to the density of the original image). . Note that a graph a and a graph b in the coordinates represent a gradation correction table used in the printer unit of FIG. At coordinate c), the image input signal is 8 bits (2
56 values), and the writing light amount of the laser also has a resolution of about 8 bits between the minimum value and the maximum value. “A” shown at the coordinate c) indicates the relationship between the laser writing light amount used at the time of detection and the image input signal.

In the coordinate d), the vertical axis represents the photosensitive drum 1
02 represents the amount of toner adhered to the image sensor 02, and the horizontal axis represents the output of the optical sensor 136. The graph drawn at the coordinate d) represents the output characteristic of the optical sensor 136. This characteristic varies depending on the type of sensor used, the mounting angle, the distance from the photosensitive drum 102, and the like, but is substantially constant in a general copying machine.

In the coordinate e), the vertical axis represents the amount of toner adhered similarly to the coordinate d), and the horizontal axis represents the surface potential of the photosensitive drum 102. The graph shown at the coordinate e) indicates the toner adhesion amount with respect to the surface potential of the photosensitive drum 102, and indicates so-called development characteristics. H at coordinates e)
Represents a DC (Direct Current) component of the developing bias. In the coordinate f), the vertical axis represents the amount of toner adhered similarly to the coordinates d) and e), and the horizontal axis represents the image input signal. The graph shown at the coordinate f) indicates the toner adhesion amount on the photosensitive drum 102 with respect to the image input signal.

In the following, the toner adhesion amount (M / A) i [mg / cm2] on the photosensitive drum 102 is calculated from the detection output VPi of the optical sensor 136 using the relationship shown in the coordinate d).
(I = 1, 2,..., Np) is described.

The reflected light from the toner adhering area formed on the photosensitive drum 102 is detected by the optical sensor 136 and transmitted to the main control unit 130 as a detection signal.
VSi is defined as the output of the optical sensor 136 which detects the reflected light from the toner adhering area of the formed pattern, and VSG
Is the output of the optical sensor 136 that has detected the reflected light from the background.

By using VSP, VSG and the relationship shown in Expression 9, the toner adhesion amount is converted from the output of the optical sensor 136. m1 = -ln (VSp / VSG) / β (9) β = −6.0 * 10 3 [cm / g] In Expression 8, β is a constant determined by the optical sensor 136 and the toner component. The value indicated by 8 is the value of the black toner. Yellow, cyan, and magenta can be similarly converted. Here, it is obtained by calculation, but may be converted by a look-up table created in advance.

By the above-described method, the toner adhesion amount on the photosensitive drum 102 is converted from the detection output VPi of the optical sensor 136, and the surface potential VSi of the photosensitive drum 102 and the amount of toner on the photosensitive drum 102 are calculated based on the obtained relationship. Of the toner (M / A) i is required. The determined result is given by a graph f shown in coordinates e).

However, as shown in the graph drawn at the coordinate d), the output of the optical sensor 136 is (M / A in the range of coordinates) in the adhesion amount region higher than a certain toner adhesion amount (M / A) c. ) I ≧ (M / A) c), constant value of V
Only PMIN can be taken. The reason for this is that as the amount of adhered toner increases, the amount of reflected light from the irradiated location decreases, and eventually the reflected light cannot be detected at the point of VP = VPc.

Therefore, even if the amount of laser writing light or the image input signal rises, the surface potential of the photosensitive drum 102 decreases, and the amount of toner adhering increases, the toner obtained from the detection output of the optical sensor 136 can be used. The amount of adhesion does not increase after a certain value (M / A) c.

The coordinates e) show a graph f indicating the amount of toner adhering from the detection output of the optical sensor 136, and a graph e indicating the amount of toner actually adhering to the photosensitive drum 102.
Are shown. As can be seen from these graphs, in the range where the toner adhesion amount is larger than (M / A) c, there is a deviation between the actual toner adhesion amount and the toner adhesion amount obtained from the detection output of the optical sensor 136. Will occur.

Hereinafter, a method for correcting the above-described deviation will be described. When the detected value VPi of the optical sensor 136 with respect to the image input signal is equal to or greater than VPmin = VPC, the detected value VPi is converted into a toner adhesion amount on the photosensitive drum 102 or an amount (M / A) i proportional to the toner adhesion amount. You. Based on these values, a relational expression between the output value VSi of the photosensitive drum surface potential sensor 139 and (M / A) i is obtained.

The linear equation of 1 is used. (M / A) i = a * VSi + b (VPi≥VPc) (10) Assuming that the DC component of the developing bias is VDC, (M / A) i = a * (VSi-VDC) + b (VPi≥VPc) (11) Here, a and b represent coefficients, and some VSi and (M /
A) It is determined from a set of values with i by a method such as the least squares method. Here, in Expressions 10 and 11, the range in which the value is given is considered in terms of the toner adhesion amount, and (M / A) i ≦ (M / A
A) Similar results are obtained for c.

The deviation from the above linear relationship with the surface potential may be large. In order to prevent such a situation, (M / A) min ≦ (M / A) ≦ (M / A) c
The coefficients a and b in Equations 9 and 10 are determined for the detection result of the toner adhesion amount on the photoconductor drum 102 that satisfies the condition. Here, the toner adhesion amount was used, but (M /
A) The detection output of the optical sensor 136 corresponding to min is VP
As max, the coefficients a and b of Equation 10 or Equation 11 may be determined from the toner attachment area corresponding to the toner attachment area satisfying VPc ≦ VP ≦ VPmax.

<Change of Gradation Processing Table> An example of a gradation processing table that is changed in accordance with the detected development characteristic or the second gradation correction characteristic will be described below with reference to FIGS. 57 and 58. I do. FIG. 57 shows a table for gradation processing.
The write value at which the gradation correction table is saturated is 4 × ni,
Or a table in the case of 4 × nj. In addition,
i and j are both positive integers, and ni and nj mean the i-th and j-th write values, respectively, and are arbitrary integers from 0 to 64. If ni or nj is 64, 4 × n
i or 4 × nj is 255. In this figure, 4
3 tables of T3i + 1, T3i + 2, and T3i + 3 corresponding to × ni, and T3j + 1, T3j + 2, and T3j corresponding to 4 × nj.
The three tables +3 are illustrated.

FIG. 58B illustrates a 2 × 2 index table, and illustrates the pixel numbers. Fig. 58
(C) shows the relationship between the gradation processing table and the pixel position when the saturated write value is 4 × ni. FIG. 58 (d) shows that the saturated write value is 4 × n
The relationship between the gradation processing table and the position of the pixel when j is shown is shown.

In this embodiment, it is assumed that the number of gradation processing tables can store a total of 36 pixels. Using a gradation processing table common to the four colors of YMCK, three sets of gradation processing tables T3i + 1 to T3i + 1, T3 each corresponding to the write values ni and nj that are saturated as described above.
When 3j + 1 to T3j + 3 are used, a selection is made from 12 combinations according to the saturated write value. The index table can set different parameters for each color of YMCK, but the gradation processing table for 36 pixels is used in common for each color of YMCK, and the register storing the same is not rewritten. When this rewriting is performed, it becomes necessary to rewrite a parameter of 36 pixels × 256 bytes (or a word), and the CPU must be occupied for a long time for this rewriting. As described above, even if the gradation processing table for 36 pixels is common, different gradation processing can be performed for each color because the index table is different for each YMCK color. Note that a different gradation processing table may be used for each YMCK color. In this case, for example, eight pixels in the gradation processing table for 36 pixels may be assigned to each of the YMCK colors.

<Conversion of Dither Table> A method of changing the gradation conversion table for image formation (index table, dither table) according to the detected gradation characteristic will be described below with reference to FIG. Image processing printer gamma correction circuit 409 according to the embodiment of the present invention
Changes the set value of the index table according to the conversion characteristics of the gradation conversion means. In other words, for each detected gradation characteristic, a corresponding index table is changed. For example, if the gradation characteristic of the image forming unit is C0h to FF
h, when the laser writing value (emission energy) is saturated, the output value of the dither table is C0h to FF
The image processing printer γ correction circuit 409 reproduces a smooth image without gradation jump by changing to an index table having a dither processing characteristic using another pixel without using the write value of h.

More specifically, the image processing printer γ correction circuit 409 stores a plurality of index tables. Then, the gradation conversion characteristic is changed based on the gradation characteristic detected from the image signal, and according to the gradation conversion characteristic,
Pixels of a dither matrix of m ′ × n ′ pixels (m ′,
(n 'is a positive integer). This index table shows pixels in a dither matrix of m × n pixels (m and n are positive integers, and the product of m ′ and n ′ is a value equal to or less than the product of m and n). The correspondence relationship with the gradation conversion table characteristics of m ′ × n ′ pixels that can be overlapped and corresponded is shown. The operation of the image processing printer γ correction circuit 409 for changing the index table to a predetermined one according to the gradation conversion characteristic will be described below with reference to FIG.

The latent image of the reference pattern created on the photoreceptor (step S240) is detected by the surface potential sensor (step S241) and visualized (step S24).
2). When this reference pattern is detected by the optical sensor (step S243), the image processing printer γ correction circuit 409 determines the development characteristics from the detection result and the output value (image signal) of the laser used at the time of pattern formation. Predict. Further, the upper limit value N1 of the write value is obtained from the detected gradation characteristic (step S245). A second tone conversion table is created from N1 and the development characteristics obtained in this way (step S246), and based on this, the parameters of the index table of the tone processing (dither processing) are changed, whereby Perform the operation. For example, the write value before detection is ni, and the write value after detection is nj
, The image processing printer γ correction circuit 409
Represents ni and the gradation processing table T selected before detection.
3i + 1 to T3i + 3, and 4 × ni and 4 closest to nj, respectively.
For ni and nj, which are xnj, change from FIG. 58 (c) to (d).

<Change of Gradation Processing Parameter Based on Tone Conversion Characteristics Performed in Subsequent Stage> The gradation characteristics of the image conversion process performed in a stage subsequent to the image processing printer γ correction circuit 409 of the image forming apparatus will be described below. Accordingly, the operation of the image processing printer γ correction circuit 409 changing the index table (changing the parameters of the index table) will be described.

FIG. 39 is a diagram showing an example of the relationship between the input value to the above-mentioned second gradation conversion table (index table) and the toner adhesion amount converted by the second gradation conversion table. It is. FIG. 39A shows an example in which the input value is output without conversion. In this case, when the toner is transferred from the photoconductor to the intermediate transfer belt or from the intermediate transfer belt to the transfer medium, the toner adheres to the photoconductor more than necessary. As a result, the toner carried on the photoreceptor or the transfer body is not sufficiently transferred to the transfer medium, and the desired color reproduction is not obtained. And other problems arise.

Accordingly, as shown in FIG. 39B, the toner adhesion amount (M / A) is limited so as not to be larger than the upper limit value (limit value) (M / A) 1 of the toner adhesion amount.
This is because the toner adhesion amount on the photoreceptor does not use the writing value N having the exposure energy larger than the writing value N1 which is the upper limit value (M / A) 1 of the toner adhesion amount. This is done by changing (creating).

As described above, in the gradation conversion processing on the downstream side of the image processing printer γ correction circuit 409,
If the gradation conversion characteristic has a saturation region or the output value does not change in accordance with the input signal, the image processing printer γ correction circuit 409 performs the conversion based on the gradation conversion characteristic of the gradation conversion processing. By changing the index table or changing the gradation processing parameter, it is possible to prevent the gradation from skipping and reproduce an image in which the gradation is smoothed.

<Conversion of Index Table According to Write Value or Irradiation Energy> Hereinafter, the case where the gradation conversion characteristic of the gradation conversion processing performed downstream of the image processing printer γ correction circuit 409 has a saturation region will be described. ,
The operation of changing the index table (second gradation correction table) of the image processing printer γ correction circuit 409 in accordance with the writing value serving as the saturation region or the irradiation energy for forming the latent image will be described in detail. I do. When the gradation conversion characteristic has a saturated region, a value (parameter) of an index table for dither processing is set in accordance with a writing value which becomes a saturated region or an irradiation energy value for forming a latent image. This prevents a gradation jump that occurs when the gradation conversion characteristic of the gradation processing existing downstream of the image processing printer γ correction circuit 409 is such that the input signal and the output signal are not linked. Can be smoothed. This example will be described with reference to FIG. FIG. 40 shows the relationship between the input value to the second gradation conversion table and the toner adhesion amount after the second gradation conversion.

FIG. 40 is a view for explaining a method of obtaining a deviation amount from the ideal gradation characteristic with respect to the gradation conversion characteristic of the second gradation conversion table for the image processing printer γ correction circuit 409. FIG. First, the image processing printer γ correction circuit 409 calculates Δ (M / A) which is the difference between the ideal characteristic (a) and the gradation characteristic created for correcting the gradation correction characteristic of the image forming unit. Find out. N in Expression 12 and FIG. 40 is an input value to the second gradation conversion table. For example, in the case of 8 bits, 0 to FFh is applicable. Equation 11 shows the relational expressions of (a), (b), and Δ (M / A). (A) = (M / A) a (n) (b) = (M / A) b (n) n = 00h to FFh Δ (M / A) = (M / A) b (n) − (M / A) a (n) (12)

As shown in FIG. 40, Δ (M / A) is 00
In the range from h to N2, (a) and (b) are almost the same, but as N2 approaches FFh, the difference between the two increases. Δ (M / A) 0 is a function indicating the allowable width of the shift.
(N), if this function has the same allowable width on the plus side and the minus side with respect to the ideal characteristic (a), the area surrounded by the two-dot chain line shown in FIG. (A)
From the allowable deviation. In the example shown in FIG. 40, the intersection of the detection characteristics (b) and (d) is such that n is E0
Since the point is h, the region where n is E0h to FFh is
It is determined that the region is a saturated region.

According to FIG. 40, n ranges from 10h to C0.
Since Δ (M / A) is almost 0 in the range of h, the image processing printer γ correction circuit 409 determines that n is C0h
The region from FFh to FFh may be determined as a saturation region. In this case, the tolerance Δ (M / A) 0 is substantially zero. However,
When the tolerance Δ (M / A) 0 is set to almost 0, the gradation is improved. However, unevenness may be present at the time of toner adhesion, and the detection result may be varied. It may be difficult to make an accurate determination of
In the following description, the tolerance Δ (M / A) 0 is set to approximately 0, and the saturation region is a region where n is C0h to FFh.

FIG. 44 shows an image processing printer γ correction circuit 409 which converts the detection result of the gradation characteristic (development characteristic) of the image forming section into an image.
9 is a flowchart for explaining an operation of changing a dither processing parameter of the gradation processing unit 410 by reflecting the dither processing parameter on the dither processing parameter. First, a reference pattern is created, and when the reference pattern is visualized (step S280), a gradation characteristic (development characteristic) is predicted by detecting the formed internal pattern (step S281). When the upper limit value N1 of the write value is obtained from the detected gradation characteristic, the gradation conversion characteristic of the first gradation processing unit is calculated from the gradation characteristic and the upper limit N1 (step S283). Then, the index table is changed according to the upper limit value N1.

<Parameter setting method> Hereinafter, the write value and the latent image formation energy on the latent image carrier are prioritized over the write value that does not reach the saturation region in the gradation characteristic or the latent image formation energy (emission energy). A method for changing to an index table that increases the number of times will be described. Here, a description will be given of the gradation processing using a total of four pixels of two main scanning pixels and two sub-scanning pixels. This is performed by finding the maximum integer n0 that satisfies the upper limit N1 ≧ 4 · n0 when the laser has a linear developing characteristic up to the laser writing value N1. In the following description using FIG. 39, it is assumed that the saturation region is a region where n is C0h to FFh, N1 is C0h, and n0 is 40h. When the input value to the dither table is n and the write value to start increasing the number of adjacent pixels is 4 · n0, as shown in Expression (13), a00 = 4n (0 ≦ n ≦ n0-1) a00 = 4・ N0 (n0 ≦ n ≦ 4 ・ n0-1) a00 = n (4 ・ n0 ≦ n ≦ FFh) a10 = 00h (00h ≦ n ≦ n0-1) a10 = 4 (n−n0) (n0 ≦ n ≦ 2 · n0-1) a10 = 4 · n0 (2 · n0 ≦ n ≦ 4 · n0-1) a10 = n (4 · n0 ≦ n ≦ FFh) a01 = 00h (00h ≦ n ≦ 2 · n0-1) a01 = 4 (n−2 · n0) (2 · n0 ≦ n ≦ 3 · n0-1) a01 = 4 · n0 (2 · n0 ≦ n ≦ 4 · n0-1) a01 = n (4 · n0 ≦ n) ≦ FFh) a11 = 00h (00h ≦ n ≦ 3 · n0-1) a11 = 4 (n−3 · n0) (3 · n0 ≦ n ≦ · N0-1) a11 = a n (4 · n0 ≦ n ≦ FFh) ··· (13).

If the dither table is changed and the color of the image changes, the above-mentioned automatic gradation correction is executed. As an example of this, when the image processing printer γ correction circuit 409 passes through the input value, there is a dither table shown in Expression 13 as an example of a dither table when only values up to B0h are used. a00 = 4n (0 ≦ n ≦ 3Fh) a00 = FFh (40h ≦ n ≦ FFh) a10 = 00h (00h ≦ n ≦ 3Fh) a10 = 4 (n−40h) +1 (40h ≦ n ≦ 7Fh) a10 = FFh ( 80h ≦ n ≦ FFh) a01 = 00h (00h ≦ n ≦ 7Fh) a01 = 4 (n−80h) +2 (80h ≦ n ≦ BFh) a01 = FFh (C0h ≦ n ≦ FFh) a11 = 00h (00h ≦ n ≦ BFh) a11 = 4 (nC0h) +3 (C0h ≦ n ≦ FFh) (14)

[0164] Further, equation 15 may be used. a00 = 4n (0≤n≤n0-1) a00 = 4 * n0 (n0≤n≤4 * n0-1) a00 = n (4 * n0≤n≤FFh) a10 = 00h (00h≤n≤n0- 1) a10 = 4 (n−n0) (n0 ≦ n ≦ 2 · n0-1) a10 = 4 · n0 (2 · n0 ≦ n ≦ 4 · n0-1) a10 = n (4 · n0 ≦ n ≦ FFh) ) A01 = a11 = 00h (00h ≦ n ≦ 2 · n0-1) a01 = a11 = 2 (n−2 · n0) (2 · n0 ≦ n ≦ 4 · n0-1) a01 = a11 = n (4 ·) n0 ≦ n ≦ FFh) (15)

<Change of Index Table Before Performing Calibration> The operation of performing automatic gradation correction after changing the dither parameter will be described below. Changing the index table as described above may change the color of the image. In this case, when performing the automatic gradation correction, the index table in which the parameters of the dither processing are described based on the automatic gradation pattern (ACC) is changed. Using this changed index table, an image of an automatic gradation pattern is further formed, and automatic gradation correction is performed. As a result, it is possible to prevent a problem that the color of an image is changed, which may occur when the index table is changed.

That is, when performing automatic gradation correction, a gradation pattern is output on transfer paper, dither processing is changed by this gradation pattern, and the changed dither processing is used.
Output a pattern for gradation processing. This makes it possible to apply dither processing to the automatic gradation pattern output for gradation processing when only the index table of dither processing is changed, and even if the color is changed, to obtain a gradation corresponding to this. The table can be corrected, and a change in color due to a change in dither processing can be suppressed. For example, when the dither parameter is changed, the gradation characteristic when copying the original may be changed. In this case, automatic gradation correction is performed, and it is possible to suppress a change in tint due to a change in dither processing.

This operation will be described with reference to FIG. First, when a reference (automatic gradation) pattern is formed and visualized (step S250), a gradation characteristic (development characteristic) is detected by detecting the formed internal pattern (step S251). From the detected gradation characteristics, the upper limit value N1 of the write value (exposure energy to the photoconductor) according to the allowable toner adhesion amount is obtained (step S252). The detected gradation characteristic and the upper limit N1
, The second gradation conversion characteristic is calculated (step S25).
3) Changing (rewriting) the index table of the gradation processing (dither processing) according to N1; step S25
4). After changing the index table like this,
It is determined whether or not to display an inquiry as to whether or not to execute automatic gradation correction (step S255). If not, the process is terminated (step S255 / NO).
If it is to be displayed (step S255 / YES), a screen as shown in FIG. 24 is displayed on the liquid crystal screen of the operation unit whose example is shown in FIG. 22 (step S256). Based on the screen as shown in FIG. 25 displayed on the liquid crystal screen, the user selects whether to execute automatic gradation correction (step S2).
57). When not performing automatic gradation correction (step S2
58 / NO), when the process is terminated and the automatic gradation correction is executed (step S258 / YES), the process proceeds to step S6 shown in FIG. 31 and the automatic gradation correction is performed.

As shown in FIG. 42, when execution of automatic gradation correction is selected, detection of gradation characteristics (development characteristics) of the image forming section, creation of a second gradation conversion table, and automatic gradation correction Tone correction may be performed. First, a "print start" screen for automatic gradation correction (when using copy or using a printer) as shown in FIG. 27 is displayed on the liquid crystal screen (step S270). Next, the user, from this screen,
"High precision adjustment", "Normal correction", or "Cancel"
Is selected (step S271). When "high precision adjustment" is not selected (step S272 / N
O), automatic gradation correction is performed without changing the image forming conditions (step S278). When "high precision adjustment" is selected, a reference pattern is formed and visualized (step S273). Next, gradation characteristics (development characteristics) are detected (step S274), and from the detected gradation characteristics,
The upper limit value N1 of the writing value (exposure energy to the photoconductor) according to the allowable toner adhesion amount is obtained (step S).
275). A second gradation conversion characteristic is calculated from N1 and the gradation characteristic (step S276), and an index table for gradation processing (dither processing) is changed according to the upper limit value N1 of the write value (step S277). Thereafter, by executing the automatic gradation correction (step S273),
It is possible to suppress a change in tint due to a change in dither processing.

[0169]

As is apparent from the above description, according to the present invention, there is provided an image forming apparatus which forms an image in which gradation skip does not occur even if the gradation characteristic has a saturation region. It is possible to do. Further, in this image forming apparatus, even if the gradation characteristic in the gradation conversion processing performed on the downstream side of the dither processing has a saturation region, the gradation skip does not occur.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating an internal configuration of an image forming apparatus according to the present invention.

FIG. 2 is a configuration diagram showing the configuration of the entire copying machine according to the present invention.

FIG. 3 is a block diagram for explaining a control system of the copying machine according to the present invention.

FIG. 4 is a block diagram of a laser modulation circuit according to the present invention.

FIG. 5 is a block diagram showing an internal configuration of an MTF filter according to the present invention.

FIG. 6 is an example of a filter for detecting an edge in the sub-scanning direction according to the present invention.

FIG. 7 is an example of a filter for detecting an edge in the main scanning direction according to the present invention.

FIG. 8 is an example of a first filter for detecting oblique edges according to the present invention.

FIG. 9 is an example of a second filter for detecting oblique edges according to the present invention.

FIG. 10 is a conceptual diagram for explaining area processing according to the present invention.

FIG. 11 shows a first example of a 2 × 2 dither table according to the present invention;
This is an example.

FIG. 12 is a diagram illustrating an example of a positional relationship between pixels of a 2 × 2 dither table according to the present invention.

FIG. 13 is an example of development characteristics of a copying machine.

FIG. 14 is an example of a gradation jump of a copying machine.

FIG. 15 shows a second example of a 2 × 2 dither table according to the present invention;
This is an example.

FIG. 16 shows a third example of the 2 × 2 dither table according to the present invention;
This is an example.

FIG. 17 shows a fourth example of the 2 × 2 dither table according to the present invention;
This is an example.

FIG. 18 is a diagram for explaining a relationship between a toner adhesion amount and a development potential according to the present invention.

FIG. 19 is a schematic diagram of an electrostatic latent image on a photoconductor according to the present invention.

FIG. 20 is a diagram for explaining a visible image by a toner on a photoreceptor according to the present invention.

FIG. 21 is a diagram showing an example of an adaptive edge enhancement filter table according to the present invention.

FIG. 22 is a diagram showing an example of an operation unit of the copying machine according to the present invention.

FIG. 23 is a diagram showing an example of a liquid crystal screen of the operation unit of the copying machine according to the present invention.

FIG. 24 is an example of a screen inquiring about execution of automatic gradation correction according to the present invention.

FIG. 25 is an example of a screen of an operation unit displayed to select execution of automatic gradation correction according to the present invention.

FIG. 26 is a first diagram illustrating an example of a screen displayed on the operation unit during execution of automatic gradation correction according to the present invention.

FIG. 27 is a second diagram illustrating an example of a screen displayed on the operation unit during execution of automatic gradation correction according to the present invention.

FIG. 28 is a third diagram illustrating an example of a screen displayed on the operation unit during execution of automatic gradation correction according to the present invention.

FIG. 29 is a fourth diagram illustrating an example of a screen displayed on the operation unit during execution of automatic gradation correction according to the present invention.

FIG. 30 is an example of a gradation pattern transferred onto a transfer medium by automatic gradation correction according to the present invention.

FIG. 31 is a flowchart illustrating an operation of the image forming apparatus when executing automatic gradation correction according to the present invention.

FIG. 32 is an example of a machine difference correction value of the automatic gradation correction according to the present invention.

FIG. 33 is an example of a liquid crystal screen displayed on an operation unit for inputting a machine difference correction value of a childhood association correction according to the present invention.

FIG. 34 is a quaternary chart for explaining a calculation method in the automatic gradation processing according to the present invention.

FIG. 35 is a flowchart for explaining a calculation procedure of an automatic gradation process according to the present invention.

FIG. 36 is a flowchart illustrating an operation of detecting a development characteristic according to the present invention.

FIG. 37 is a diagram for explaining a detection pattern on a photoconductor according to the present invention.

FIG. 38 is a diagram illustrating a method of correcting an image signal according to the present invention.

FIG. 39 is a first diagram illustrating a relationship between a second gradation conversion characteristic and a toner adhesion amount according to the present invention.

FIG. 40 is a second diagram illustrating the relationship between the second gradation conversion characteristic and the toner adhesion amount according to the present invention.

FIG. 41 is a first flowchart illustrating the operation of correcting a dither table according to the present invention.

FIG. 42 is a second flowchart illustrating the operation of correcting a dither table according to the present invention.

FIG. 43 is a flowchart for explaining an operation when executing automatic gradation correction according to the present invention.

FIG. 44 is a third flowchart illustrating the operation of correcting a dither table according to the present invention.

FIG. 45 is a block diagram showing an example of a reading system according to the present invention.

FIG. 46 is a schematic view of a scanner optical system according to the present invention.

FIG. 47 is a conceptual diagram for describing white correction and black correction of the scanner according to the present invention.

FIG. 48 is a diagram illustrating a sample hold of the scanner according to the present invention.

FIG. 49 shows a total of 6 pixels in the main scanning direction and 6 pixels in the sub-scanning direction.
This is a first example when six pixels are used for dither processing.

FIG. 50 is an example of an index table.

FIG. 51 is a first example of a gradation processing table for two pixels in the main scanning direction and two pixels in the sub-scanning direction.

FIG. 52 is a second example of the gradation processing table for two pixels in the main scanning direction and two pixels in the sub-scanning direction.

FIG. 53 is a third example of a gradation processing table for two pixels in the main scanning direction and two pixels in the sub-scanning direction.

FIG. 54 shows a total of 6 pixels in the main scanning direction and 6 pixels in the sub-scanning direction.
This is a second example in which six pixels are used for dither processing.

FIG. 55 is an example of an index table according to the present invention.

FIG. 56 is a first example of an index table of two pixels in the main scanning direction and two pixels in the sub-scanning direction according to the present invention.

FIG. 57 is a second example of an index table for two pixels in the main scanning direction and two pixels in the sub-scanning direction according to the present invention.

FIG. 58 is an example of an index table according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 101 Copier main body 102 Organic photoreceptor (OPC) drum (photoreceptor drum) 103 Charge 104 Laser optical system 105 Black developing device 106, 107, 108 Color developing device 109 Intermediate transfer belt 110 Bias roller 111 Cleaning device 112 Static elimination unit 113 Transfer bias roller 114 Belt cleaning device 115 Conveyor belt 116 Fixing device 117 Discharge tray 118 Contact glass 119 Exposure lamp 121 Reflection mirror 122 Imaging lens 123 Image sensor array 130 Main control unit (CPU) 131 ROM 132 RAM 133 Interface I / O 134 Laser optical system control unit 135 Power supply circuit 136 Optical sensor 137 Toner density sensor 138 Environment sensor 139 Photoconductor drum surface potential sensor 1 0 Toner replenishment circuit 141 Intermediate transfer belt drive unit 142 Operation unit 201 Developing sleeve 202 Developer stirring control member 401 Shading correction circuit 402 Scanner gamma correction circuit 403 Image memory 404 Image processing circuit 405 MTF filter 406 Color conversion UCR processing circuit 407 Zooming Circuit 408 Image processing circuit 409 Image processing printer γ correction circuit 410 Tone processing circuit 411 I / F / selector 412 Image formation printer γ correction circuit 413 Printer 414 ROM 415 CPU 416 RAM 417 System controller 418 External computer 419 Printer controller 420 Scanner 421 Pattern generation circuit 422 Pattern generation circuit 423 Area processing circuit 424 Serial I / F 425 Serial I / F 426 BUS 11 01 Smoothing filter 1102 Laplacian filter 1103 Edge amount detection filter 1104 Smoothing filter 1105 Table conversion circuit 5401 CCD 5402 Amplification circuit 5403 S / H circuit 5404 A / D conversion circuit 5405 Black correction circuit 5406 Shading correction circuit 5407 Image processing unit 5408 BUS 5409 CCD driver 5410 Pulse generator 5411 Clock generator

Continued on front page F term (reference) 2C262 AA24 AA26 AA27 AB07 BA09 BB06 BB27 BC10 BC17 EA04 GA02 5B057 CA01 CA08 CA12 CB01 CB07 CB12 CC01 CE11 CE13 CE16 CH07 CH18 DB02 DB06 DB08 5C077 MM27 MP01 MP08 NN09 PP23 PP03 PP03 PP03 TT03

Claims (5)

[Claims] 1. A storage means for storing a plurality of dither tables used for dither processing, a detection means for detecting a gradation characteristic, and a selection means for selecting the dither table corresponding to the gradation characteristic detected by the detection means An image forming apparatus that performs dither processing using the selected dither table. 2. The image forming apparatus according to claim 1, wherein the image forming apparatus performs the gradation conversion processing after performing the dither processing, and the detecting unit detects a gradation characteristic in the gradation conversion processing. The image forming apparatus according to claim 1, wherein: 3. The image processing apparatus according to claim 2, wherein the selecting unit is configured to, when the gradation characteristic has a saturation region in which an output signal does not change according to an input signal, write a region in a saturation region and a latent image in the saturation region. Any one of the formation energy values
The image forming apparatus according to claim 2, wherein a dither table corresponding to the area of the dither table and converting the dither table to one having no saturation area is selected. 4. The apparatus according to claim 1, wherein said selecting means selects a dither table for increasing a write value not included in said saturated region and a value of a latent image forming energy not included in said saturated region. Item 4. The image forming apparatus according to Item 3. 5. The image forming apparatus according to claim 1, wherein when forming a gradation pattern image after performing dither processing, the image forming apparatus performs dither processing on the gradation pattern. The image forming apparatus according to claim 1.