JPH06125456A - Picture processor - Google Patents

Abstract

PURPOSE:To simplify adjustment of the gradation by switching between a threshold and a quantization value, which are determined at the time of quantization in the intermediate processing, in accordance with the classification of an original or the change of environments. CONSTITUTION:A high gradation read as picture data, for instance, picture data extending from white (0) to black (255) of A to D can be switched between a mode B (document) and a mode A (photograph) of lower gradation corresponding to the original, and for example, fixed thresholds t1, t2, and t3 and quantization values are determined in the mode B, and variable threshold values t1', t2', and t3' and quantization values A', B', C', and D' different from those in the mode B are determined in the A mode, and errors between adjacent quantization values and between digital data of high gradation are distributed to adjacent picture elements to make the density change gentle.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image recording device,
More specifically, the present invention relates to an image recording device having a pseudo intermediate processing device that changes a threshold value in accordance with a document image and environmental conditions, and is used, for example, in digital copying machines, facsimiles, printers and the like.

[0002]

2. Description of the Related Art An original image is optically scanned through an exposure optical system of an image reading device, reflected light from the original image is formed on a photoelectric conversion element (CCD), and converted into an analog voltage corresponding to the image density. It This analog voltage is digitally converted by an analog / digital (A / D) converter, and after various corrections are made, the image density is output as a digital signal of 256 gradations (8 bits), for example. The laser output section, which is ON / OFF-controlled by this digital signal, is converted into an electric light again and an image is recorded by the image recording device. However, reproducing a density image of 256 gradations for each pixel requires an enormous memory and is not realistic. Therefore, density information quantized to 256 gradations (8 bits) is converted into, for example, 4 gradations ( 2 bit) is quantized, and a fixed threshold value is set for the quantized density for simplification.

As described above, in the prior art, since the quantization value and the threshold value by the quantization are fixed and there is no switching mode for switching depending on the type of the original image, the gradation expression remains fixed. It was That is, since the halftone expression is constant regardless of the type of the original, the gradation of the recording is adjusted by changing the ON / OFF time of the laser by setting the laser gradation.

The above-mentioned prior art will be described in detail. First, the input data is read by the image reading device as digital information having a density of 256 gradations per pixel. Next, a quantized value for converting high value data of 256 gradations of one pixel into low value data and a threshold value of quantized density gradation are determined. FIG. 21 is a diagram showing an example of the relationship between the quantization and the threshold value, in which data of density levels 0 to 255 are A, B, C,
The four-valued data of D is quantized, and the threshold value between the concentrations A and B is defined as t ₁ . Similarly, the threshold values between the densities B and C and between the densities C and D are set to t ₂ and t ₃ , respectively. Next, a laser gradation is set in order to express a halftone due to a binary area change.

22 (a), (b), (c), and (d),
In the figure showing an example of the laser gradation, the vertical axis represents the laser output, the horizontal axis represents the laser ON-OFF time in one pixel (1 pixel), and the gradation is the laser output constant and the laser ON time is long. It is expressed as The laser gradation setting for quaternarization is
0 in the figure (a), 1 in the figure (b), 2 in the figure (c),
(D) shows 3 and 1 pixel is the time t (0 to tp) corresponding to 1 pixel. At gradation 0 in FIG. It is turned on, and for gradations 1 and 2, a predetermined time within 0 to tp time is defined.

FIG. 23 is a diagram showing an example of a four-valued laser gradation structure, in which the density levels correspond to 0 to 255 (white to black). FIG. 24 shows a density curve with respect to the gradation of FIG. 23. The gradation of the laser is, as shown in FIG. 23, a laser gradation and a laser step defined between the laser gradation and the threshold value. Tones are 0 (white), 0-1, 1, 1-
Seven gradation levels of 2, 2, 2 and 3 and 3 (black) are obtained. However, as shown in FIG. 24, this density curve has a saturated form in which the density change gradually decreases as the gradation increases between gradations B to A in the high density range.

As described above, in the prior art, when the quantization for halftone processing is performed, the quantization value and the threshold value are fixed. To change the tones, as shown in FIG. 22, it was necessary to change the ON time width of the laser between one pixel expressing each gradation of the laser gradation. However, there is a problem that it is difficult to set according to environmental changes such as changes over time and time.

The "image stabilizing device" in Japanese Patent Publication No. 61-29502 discloses a bright portion and a dark portion on a photoconductor obtained by irradiating with light of a predetermined intensity (when not irradiating with light). The density of the toner image is detected by an optical density detector, and the charging output voltage is controlled based on the toner image in the dark area. By controlling the formation of a latent image, electrophotographic process control is performed.

[0009]

As described above, in the quantization in the halftone processing of the prior art, the quantization value and the threshold value are fixed, and the laser side is used to adjust the gradation. Since it was necessary to set the gradation of, the operation was complicated, and it was difficult to obtain the optimum gradation depending on the type of document. Further, when an environmental change such as a change in luminous intensity of the exposure lamp occurs, the gradation changes, but it is difficult to adjust the optimum gradation according to such environmental change. In the image stabilizer disclosed in Japanese Patent Publication No. 61-29502, the electrophotographic process is controlled on the basis of the signals of the bright and dark parts of the toner image, so that one point of halftone (gray level) density is set. However, it is not possible to independently control two or more halftone densities, and it is not possible to finely adjust the reproducibility of the copy with respect to the original density and obtain a high-quality copy. It was

[0010]

In order to solve the above-mentioned problems, the present invention (1) sets a low gradation fixed threshold value to high gradation digital data output from image data output means, Fixed mode setting means for setting a low gradation gradation value based on the fixed threshold value, and a variable threshold value based on the image quality of an image, and a quantization value based on the variable threshold value. An error between the variable mode setting means to be defined, the variable threshold and the quantized value of the low gradation set by the fixed mode or the variable mode setting means, and the digital data of the high gradation is processed for each target pixel. A halftone processing means for allocating to the pixels around the target pixel to be processed, and a mode switching means for switching between the fixed mode setting means and the variable mode setting means; and (2) in (1), Image reference day First reference data storage means for reading and storing the reference data, second reference data storage means for reading and storing the reference data read thereafter, first reference data and second storage stored in the first storage means Comparing means for comparing the second reference data stored in the means and outputting the comparison value as an error, the error output by the comparing means, the error and the reference value are compared, and the comparison value is obtained. Or (3) density detecting means for optically detecting the density of the reference toner image formed on the photoconductor, and the density detecting means. Based on the density of the reference toner image detected by the means, the charging output is controlled so that the density of the reference toner image formed by fixing the developing bias voltage to a predetermined value.
First electrophotographic process control means for determining the potential difference between the developing bias voltage and the charging output voltage, and second electrophotography for controlling the absolute value of the voltage when the potential difference between the developing bias voltage and the charging output voltage is constant. And a third electrophotographic process control means for determining a plurality of threshold values and a quantization value of the halftone density of the reference toner image.

[0011]

The analog voltage obtained by photoelectrically converting the original image transmitted through the exposure optical system is converted into one pixel density, for example, from 0 (white) to
The digital image data is weighted to a high bit value of 255 (black), and is further converted to digital image data weighted to a low value of 2 bits, for example. This transformation has a quantisation value and a threshold value that can be switched according to the image to keep the area gradation of the laser constant. In addition, for the image quality deterioration due to the density change between the quantum value obtained by quantizing at a low value and the threshold value, the difference between the data when quantizing at a low value and the original data is processed. The error is distributed to the pixels around the to make the image quality smooth with no error. Further, with respect to the change in the light intensity of the exposure lamp, the white reference level is stored, and the change with the white reference level obtained thereafter is calculated for this stored value, and the quantized value is determined based on the calculation result. Then, the threshold value was changed to eliminate the influence of the change in the light intensity of the exposure lamp depending on time and environment. Further, the density of the reference toner image formed on the photoconductor is optically detected, and first, on the low density side, the potential difference of the charging output voltage with respect to the developing fixed bias voltage is adjusted so that the background fog level is within the allowable range. After that, eliminate the occurrence of background fog, and then determine the absolute voltage of the developing fixed bias voltage and the charging output voltage while keeping the potential difference constant so that the high density is within the allowable range on the high density side. Further, two or more halftone density threshold values and quantization values are independently changed to stabilize the copy image quality and finely adjust the halftone.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, a digital copying machine according to the present invention will be described. FIG. 1 is a sectional view showing the overall construction of an embodiment of a digital copying machine according to the present invention. The digital copying machine 10 includes a scanner 11 and a laser printer section 1.
2, a multi-stage paper feeding unit 13 and a sorter 14 are provided. The scanner unit 11 is composed of a document placing table 15 made of transparent glass, a double-sided automatic document feeder (RDF) 16 and a scanner unit 20. The multi-stage paper feeding unit 13 includes a first cassette 31, a second cassette 32,
It has a third cassette 33 and a fifth cassette 35 that can be added by selection. In the multi-stage paper feeding unit 13, the papers are fed one by one out of the papers stored in the cassettes of the respective tiers, and are conveyed toward the laser printer unit 12.
RDF16 sets several originals at once,
The originals are automatically fed one by one to the scanner unit 20, and the scanner unit 20 is made to read one side or both sides of the original according to the selection of the operator. The scanner unit 20 is a lamp reflector assembly 2 that exposes a document.
1. Photoelectric conversion element (CCD) 22 for reflecting light image from the original
It includes a plurality of reflecting mirrors 23 for guiding the reflected light from the document and a lens 24 for forming a reflected light image from the document on the CCD 22.

When scanning a document placed on the document placing table 15, the scanner section 11 is configured to read the document image while the scanner unit 20 moves along the lower surface of the document placing table 15. When the RDF 16 is used, the original image is read while the original is conveyed while the scanner unit 20 is stopped at a predetermined position below the RDF 16. The image data obtained by reading the original image with the scanner unit 20 is sent to the image processing unit and subjected to various kinds of processing, and then temporarily stored in the memory of the image processing unit. Data is supplied to the laser printer unit 12 to form an image on a sheet.

The laser printer unit 12 includes a manual document tray 25, a laser writing unit 26, and an electrophotographic process unit 27 for forming an image. The laser writing unit 26 is a semiconductor laser that emits a laser beam according to the image data from the above-mentioned memory, a polygon mirror that deflects the laser beam at an equal angular velocity, and a laser beam that is deflected at an equal angular velocity is exposed to the electrophotographic process section 27. Body drum 28
It has an f-θ lens and the like for performing correction so as to have a uniform velocity tendency. The electrophotographic process unit 27 includes a charging device, a developing device, a transfer device, a peeling device, a cleaning device, a static eliminator, and a fixing device 29, which are arranged around the photoconductor drum 28 according to a known manner. An optical reading unit 100 that optically detects the reflected light of the reference toner image formed on the photosensitive drum 28 is disposed upstream of the cleaning device. A transport path 30 is provided downstream of the fixing device 29 in the transport direction of the sheet on which the image is to be formed. The transport path 30 communicates with the transport path 37 leading to the sorter 14 and the multi-stage paper feeding unit 13. It branches into a transport path 38.

The conveying path 38 is branched in the multi-stage sheet feeding unit 23, and the reversing conveying path 30 is a conveying path after branching.
a and a double-sided / composite transport path 30b are provided. The reverse conveyance path 30a is a conveyance path for reversing the front and back sides of a sheet in a double-sided copy mode for copying both sides of a document. The double-sided / composite conveyance path 30b conveys a sheet from the reverse conveyance path 30a to the image forming position of the photosensitive drum 28 in the double-sided copy mode, or forms an image of a different original or a toner of different colors on one side of the sheet. This is a conveyance path for conveying the sheet to the image forming position of the photosensitive drum 28 without reversing the sheet in the single-sided composite copying mode for copying.

The multi-stage paper feeding unit 13 includes a common conveyance path 36, and the common conveyance path 36 includes a first cassette 31 and a second cassette 31.
The sheets from the cassette 32 and the third cassette 33 are conveyed toward the electrophotographic process unit 27. The common transport path 36 joins the transport path 39 from the fifth cassette 35 on the way to the electrophotographic process section 27 and communicates with the transport path 40. The conveyance path 40 merges with the conveyance path 41 from the double-sided / composite conveyance path 40b and the manual feed document tray 25 at a confluence point 42 to reach an image forming position between the photoconductor drum 28 of the electrostatic photography process unit 27 and the transfer device. The confluence point 42 of these three conveyance paths is provided at a position close to the image forming position. Therefore, in the laser writing unit 26 and the electrophotographic process unit 27, the image data read from the above memory is
The toner image formed as an electrostatic latent image on the surface of the photoconductor drum 28 by scanning the laser beam by the laser writing unit 26 and visualized by the toner is the toner image of the sheet conveyed from the multi-stage sheet feeding unit 13. It is electrostatically transferred and fixed on the surface. The sheet on which the image is formed in this manner is sent from the fixing device 29 to the sorter 14 via the transport paths 30 and 17, or is transported to the reverse transport path 30a via the transport paths 30 and 38. Next, the operation / display panel provided on the upper portion of the digital copying machine 10 in FIG. 1 will be described.

FIG. 2 is a plan view showing an example of the operation panel of the digital copying machine 10 shown in FIG.
On the operation panel 50 of the
0a, a print switch 50b and a mode selection key 50c are arranged at the right end. The display unit 50a may be configured by a transparent touch panel on the display surface of the dot matrix liquid crystal display unit, for example.

FIG. 3 shows a display section 50a of the operation panel shown in FIG.
FIG. 8 is an explanatory diagram showing an example of a setting screen for copy mode, etc. The copy mode setting screen shown in FIG. 3 is provided with touch panels for a magnification 51, a paper size area 52, a copy density 53, a copy number 54, a sorter 55, and a plus function 56. When the plus function 56 is touched, Mode setting permission 5
8. A mode setting 57a or B mode setting 57b is selected. The mode setting of the present invention is performed by operating the touch panel 50a as the processing mode on the display screen of the display unit 50a of the operation panel shown in the figure. That is, the mode can be set by the mode setting permission 58 of the touch panel.
Mode (photo) setting 57a or B mode (text) setting 5
A desired image is selected and set by 7b.

Next, the configuration and function of the image processing unit and each control system included in the digital copying machine 10 will be described. FIG. 4 is a block diagram of the image processing section and each control system included in the digital copying machine 10 shown in FIG. The image processing unit included in the digital copying machine 10 includes an image data input unit 60, an image processing unit 61, an image data output unit 62, a memory 63 including a RAM (random access memory), and an image processing central processing operation. A device (CPU) 64 is provided.

The image data input section 60 is a CCD section 60a,
It includes a histogram processing unit 60b and an error diffusion processing unit 60c. The image data input unit 60 is the CCD 22 of FIG.
Binary conversion of the image data of the original read from
While taking a histogram as a binary digital amount, the image data is processed by the error diffusion method and temporarily stored in the memory 63. That is, the CCD unit 60
In a, the analog electrical signal corresponding to each image density of the image data is A / D converted, and then MTF (Modulation Trans
fer function) correction, black-and-white correction, or gamma correction, and output to the histogram processing unit 60b as a digital signal of 256 gradations (8 bits). In the histogram processing unit 60b, the digital signal output from the CCD unit 60a is added for each pixel density of 256 gradations to obtain density information (histogram data), and if necessary, the obtained histogram data is subjected to image processing. It is sent to the CPU 64, or sent to the error diffusion processing unit 60c as pixel data. The error diffusion processing unit 60c outputs the 8-bit data output from the CCD unit 60a by an error diffusion method, which is a kind of pseudo-halftone processing unit, that is, a method of reflecting a quaternization error in quaternization determination of an adjacent pixel. The digital signal of the pixel is converted into 2 bits (8 values), and the redistribution operation for faithfully reproducing the local area density in the original is performed.

The image processing unit 61 includes multi-valued processing units 61a and 61b, a density conversion processing unit 61c, and a 1/8 scaling processing unit 6.
1d, an image processing unit 61e, a scaling processing unit 61f, a density conversion processing unit 61g, an error diffusion processing unit 61h, and a compression processing unit 61i. The image processing unit 61 is a processing unit that finally converts the input image data into image data desired by the operator, and this processing unit until the image data is finally stored in the memory 63 as the converted output image data. Are configured to process. However, each of the above-described processing units included in the image processing unit 61 functions as necessary and may not function.

That is, in the multilevel halftoning processing units 61a and 61b, the data quaternized by the error diffusion processing unit 61h is re-generated.
Converted to 256 gradations. The image processing unit 61e performs various image processes on the input pixel data,
In addition, information collection such as feature extraction may be performed on the data string. In the scaling processing unit 61f, the pixel data (density value) for the target pixel after scaling is obtained by performing interpolation processing based on the input known data according to the instructed scaling ratio, and the sub-scanning is varied. After being multiplied, the main scanning is subjected to a scaling process. In the density conversion processing unit 61g, the relationship between the input density and the output density is arbitrarily set for the 256 gradation digital signal based on a predetermined gradation conversion table. In the error diffusion processing unit 61h, the image data input unit 60
Processing similar to that of the error diffusion processing unit 60c is performed. In the compression processing unit 61h, 2
Value data is compressed. Regarding the compression of image data, the compression functions in the final processing loop when the final output image data is completed. Image data output unit 6
Reference numeral 2 includes a restoration unit 62a and a laser output unit 62b.

The image data output unit 62 restores the original image data stored in the memory 63 in a compressed state to the original 25
The laser output unit 6 converts the data into 6 gradations again and performs error diffusion of the 4-valued data that is a smoother halftone expression than the 2-valued data.
2 is configured to transfer data. That is, the decompression unit 62a decompresses the image data compressed by the compression processing unit 61i. In the laser output unit 62b, the digital image data is converted into a laser on / off signal based on a control signal from the print unit control CPU, and the laser is turned on / off. Next, the halftone process used in the image forming apparatus of the present invention will be described.

The reflected light of the original image is passed through the exposure optical system to C
The analog signal that is imaged on the CD and photoelectrically converted is A / D
The image is digitally converted by the conversion circuit, and the original image data is extracted using this as digital information. It is assumed that one pixel is read from the extracted image data with a gradation of 0 (white) to 255 (black), for example. In this state,
Since one pixel is represented by any gradation from 0 to 255, 8
Bit information is required, and if this is to be stored as an entire image, a huge memory is required. Further, in this state, the read image cannot be printed unless there is an image recording device capable of printing one pixel with a gradation of 0 to 255. Therefore, in the present invention, the data amount of one pixel is reduced to reduce the gradation of one pixel at the time of printing. Use a recording circuit.

The gradation of pixels at the time of image input is 0
Assuming that the level was read from (white) to 255 (black),
First, low-value quantization corresponding to 0 to 255 is performed on the read data of pixel gradations 0 to 255. Less than,
The method of setting the quantization will be described.

FIGS. 5A and 5B are diagrams showing examples of quantized values of image density and threshold values. FIG. 5A is a B mode (character image, etc.), and FIG. It is for A mode (photo image, etc.). First, certain fixed threshold values t ₁ , t ₂ and t ₃ are set in the B mode of FIG. Ask for. Here, the input data is f. When 255 ≥ f> t ₁ ... At t ₁ ≥ f> t ₂ ... B t ₂ ≥ f> t ₃ ... C (1) When t ₃ ≥ f ≥ 0 ... D Similarly (b) In the A mode in the figure, the threshold value t ₁ ′,
By defining t ₂ ′ and t ₃ ′ and using the above-mentioned judgment formula,
Find A ', B', C ', D'.

The B mode shown in FIG. 3A is a mode selected in the case of a binary original such as characters, and the quantized values A, B, C, D and the threshold value t _{1 are} calculated from the pixel gradation density curve. , T ₂ , t ₃
Are selected at approximately equal intervals. On the other hand, in the A mode shown in FIG. 9B, the quantized values A ′, B ′ and
C ', D' and the threshold _{t 1 ', t 2',} t 3 ' is selected. For example, the threshold value in the A mode is (t ₁ ′: 224, t ₂ ′: 161, t ₃ ′:
64) in B-mode _{(t 1: 212, t 2} : 128, t 3: 42) quantization amount in A mode (A ': 255, B' : 192, C ': 12
8: D ': 0) In B mode (A: 255, B: 170, C: 8
5, D: 0).

FIG. 6 is a diagram showing a mode gradation structure.
FIG. 7A shows a gradation mode in B mode, and FIG. 8B shows a gradation structure in A mode. In the figure, the quantized values D and D'correspond to the white image (0) and A and A'corresponding to the black image (255).

FIGS. 7A and 7B are diagrams showing the gradation change and the density curve of FIG. 6, where FIG. 7A is for the B mode and FIG. 7B is for the A mode, with the vertical axis representing the density (ID) and the horizontal axis. Has a quantum value.
In the mode A of FIG. 9B, the density changes linearly with respect to the gradation change, whereas in the mode B of FIG. 9A, the curve becomes saturated as the density becomes higher. These curves are obtained by measuring the print density after intermediate processing with a Macbeth measuring device for input data 0 to 255. For example, when the input data is 100, the vertical axis corresponds to the horizontal axis 100. Shows the ID concentration of the.

FIG. 8 is a diagram for explaining an example of halftone processing when the input density values in the A mode and the B mode according to the present invention are changed. FIG. 8 shows the same image in 10 × 10 dots. It is obtained by applying the quantized values and threshold values of the A mode of FIG. 5B and the B mode of FIG. 5A to the input densities 100 and 200 of the respective dots. is there.
When the input density is low at 100, in the A mode and the B mode, the gradation of the central dot is composed of 0, 1 and 2, and the gradation change is large. The gradation of the dots is composed of a few and the gradation change is small.

However, as described above, the density cannot be preserved with the original data in the small area of the image only by performing the quantization, so that the image quality in the halftone part is not smooth. In order to eliminate this, the difference from the original data generated at the time of quantization is set as an error amount, and by performing a process that affects the pixel density around the pixel of interest to be processed, Store the concentration. The principle of this processing will be described below. The high processing is divided into Step 1 and Step 2.

FIG. 9 is a diagram showing the distribution method of Step 1 in which the quantization error ε of the pixel of interest to be processed is distributed to surrounding pixels.
Error ε from the original data generated in the processing pixel E of the first row
The minute in the previous row (i-th row) is processed.
Each pixel of A, right above: B, upper right: C and the pixel of interest E on the same line i + 1 row as the pixel of interest E to be processed is distributed at a certain distribution ratio to the pixel D on the left side scanned immediately before the pixel of interest E to be processed. When the error distribution is completed from all the processed target pixels in the i + 1th row to all pixels in the ith row in this way, the processed target pixel is moved to the ith row, and the error is distributed to each pixel in the i + 1th row.
Is processed. The process of Step 2 will be described below.

FIG. 10 is a diagram showing the distribution method of Step 2 for distributing the pixel-of-interest quantization error ε'to surrounding pixels.
Process i row Left of i + 1 row from pixel B of interest: D, directly below:
The error ε ′ is distributed at a predetermined ratio to the right side: C read next to the processing target pixel B of the E, right side F and i rows. In this way, the pixel to be processed is moved and quantized again, and the quantized value is used as the result. At this time, the residual error obtained in the calculation result is C, D,
Distribute by designating one of four points, E and F. Next, the line of interest is moved to the (i + 2) th line, the same processing as in Step 1 above is performed, and then the processing in Step 2 is performed. This processing operation is performed until the end of the input image data. Next, the above Step 1, Ste
Processing of p2 The halftone processing means for distributing the error of the pixel of interest will be described.

FIG. 11 is a block diagram of a halftone processing circuit of the image recording apparatus according to the present invention. In the figure, 1 is an intermediate processing circuit, 2 is a Step 1 processing unit, 3 is a Step 2 processing unit, 5,
Reference numerals 6, 7, and 8 denote addition units, 9a denotes an input unit, and 9b denotes an output unit. A mode 57a or B mode 5 of the operation panel 50a
Based on the switching signal of 7b, the CPU 64 sets the quantization value and the threshold value according to the mode in the Step 1 processing unit 2. The input data includes, for example, density information of 0 to 255 (8 bits) via the input unit 9a and the Step 1 processing unit 2
The density value is quantized for each pixel and the error ε between the input data and the quantized data is obtained at the same time. The error ε is distributed according to the operation of Step 1.

12A and 12B show the Step 1 processing unit 2
FIG. 5A is a diagram for explaining error distribution in FIG.
Input / output signals of the Step 1 processing unit 2 portion, (b) is a diagram showing a method of error distribution. In FIG. 11, the input unit 9a
The input data from is processed by the Step 1 processing unit 2 shown in FIG. 12 from the left and quantized. As shown in FIG. 12B, the quantization error ε of the processing target pixel in the i + 1-th row is as shown in FIG. 12B.
And i + 1 pixel d. The errors distributed to the pixels a, b, c in the i + 1 row are sequentially added pixel by pixel in the a, b, c directions for the processing of the Step 2 processing unit 3 for each pixel 5, 6, 7.
Is added by. Also, the pixel d immediately before the pixel of interest to be processed
Error distributed to the error buffer 4 via the adder circuit 8.
Is added to.

FIGS. 13A and 13B are diagrams for explaining the error distribution in the Step 2 processing section, and FIG.
(2) Input / output signals of the two processing units, and FIG. 7B is a diagram showing a method of error distribution. In Step 2 processing unit 3 of FIG. 11, the image data in which the quantization error is corrected is output from the output unit 9b.
As shown in FIG. 13B, the Step 2 processing unit 3 scans the error ε ′ of the pixel of interest to be processed in the i-th row, the lower left pixel e, the lower right pixel f, the lower right pixel g of the i + 1 row, and the next scanning. Are assigned to the corresponding pixel h. The distribution error of the i + 1 row in the e, f, g directions is
The error is sequentially added to the error buffer 4 via the adder 8 and i
The distribution error of the row in the h direction is calculated in the Step 2 processing unit 3 at the next time.
Because of the processing in step 2, the data is added to the Step 2 processing unit via the addition unit 6.

FIG. 14 is a diagram for explaining the halftone processing. In the processing of Step 1 processing unit 2 and Step 2 processing unit 3 described above, the pixel of interest is selected in numerical order and moved to the final target pixel. It shows how it goes, P in i + 1 row
₂ processing target pixel upper left pixel 1 of row i as the error arrow n shown in, directly above the pixel 2, the upper right of the pixel 3, Step2
The pixel n−1 immediately before being added to the processing unit 3 is added to the error buffer 4 via the adder circuit 8, and the error buffer 4 is added.
The error corresponding to the pixel 3 on the upper right is output from and is added to the pixel 3 via the adder circuit 7 and corrected. Similarly, Step
The error of the pixel n−2 is input to the error buffer 4 also in the processing target element 1 indicated by P ₁ in the 2 processing unit 3. In this way, in order of number, Step1, Step2 processing units 2, 3
The final target pixel moves via the error buffer 4, and halftone processing of the entire image is performed.

Further, a method of changing the threshold when the environmental condition changes will be described. A specific example in which the environmental conditions change is a change in the light intensity of the exposure lamp. Since the brightness of the exposure lamp changes with time, the image data at the time of reading the original also changes. Therefore, by confirming that the image data fluctuates and changing the quantized value and threshold value of the halftone image processing circuit, it is possible to prevent the gradation from changing each time the brightness during reading changes. be able to. FIG. 15 is a diagram showing an exposure operation of the copying machine. In image scanning, a lamp reflector assembly 21 having an exposure lamp of an exposure optical system is provided from below the document table 15 from below.
Is scanned to read the original image on the CCD 22, but a white reference original 70 is arranged at the end of the original placing table 15. The reference original 70 is read by the CCD 22 for each scanning and the white data read is white. It is used as a data standard. However, when the light intensity of the exposure lamp changes, the white reference data also changes.

FIG. 16 shows an example of the change over time of the reference data (white). The same reference data (white) read every time changes over time. In the present invention, in order to eliminate the influence of the change in the reference data, such a data change is detected and the quantized value and threshold value of the halftone image processing circuit are changed. That is, the original is first scanned, the reference original is read before reading, and the reference data is a
Suppose it was 1. The CPU 64 of the image processing unit in FIG. 4 determines how much the difference is compared with the reference value a, thereby obtaining the difference g between a1 and a.
The quantized value and the threshold value are set according to the difference g.

FIG. 17 is a diagram for explaining the resetting of the quantized value and the threshold value, and the calculation of g is performed by the following equation (2). a1-a = g (2) If g is larger than the constant value K ₁ determined by the size of the marginal error, the quantized values B and C are changed in the + direction by the difference g. Next, the threshold values t ₁ , t ₂ , and t ₃ are g in the + direction.
/ 2 change. Conversely, if g is smaller than -K ₁ , then
-Change in the direction.

FIG. 18 is a flow chart for changing the quantized value and the threshold value when the reference value changes due to the environment according to the present invention. The operation will be described below with reference to the drawings.

Step 1: When performing image reading scanning, a reference (white) original is read and reference data a1 is stored. step2: The reference data a that was initially read and stored
The reference data a1 read and stored in step 1 is compared. step 3: Difference g between the reference data a1 obtained by comparison and a
The absolute value of is compared with the limit error value K ₁ to determine its magnitude. step4: If the difference g is larger than K ₁ , the quantized value is g,
Correcting the threshold 1/2 g plus direction, g is smaller than -K ₁ - to correct the direction. step5: When the absolute value of the error g is smaller than K ₁ and st
When the correction is completed in ep4, halftone image processing is performed based on the correction value. When the above operations are completed, the same s
The tep operation is performed.

As described above, it is possible to set the quantization threshold value and the mode for changing the quantization value according to the image in order to perform the halftone processing. Further, even when the reference data changes due to the environment change, Although it has been stated that changing the quantization threshold and the quantization value can provide a constant image quality without being affected by the environment, even if the environmental conditions are constant, the copier can be used for a long period of time and When a change occurs, the characteristic changes. For example, fog occurs when the environment changes, such as temperature change and developer deterioration.Even under these environmental conditions, the charge potential decreases due to the film loss of the photoconductor, and the development bias voltage and charge potential are reduced. The potential difference between the two becomes small and fogging occurs at low density. In the present invention, even in such a case, a copy with stable image quality is obtained.

19 and 20 are flow charts for explaining another embodiment of the image processing apparatus according to the present invention, and the flow charts of FIG. 19 and FIG. 20 are connected by A. The operation will be described below with reference to the flow chart shown. The copying machine is a reversal development digital copying machine in which the charging polarity of the photoconductor and the charging characteristic of the toner are the same, and the operation after the process control of the copying machine is started is shown.

Here, the image processing apparatus of the present invention determines the voltage difference between the developing bias voltage and the grid voltage so as not to cause fog at a low density for image processing.
Control means, second control means for increasing the absolute value while maintaining the potential difference in order to determine the black density, and at least two or more threshold values and quantized values at the halftone level. 3 control means.

In the control means of the first stage, step 1 : In order to control the fog level of low density, the toner image a is formed on the photoconductor 27a, and the toner image a having the density data as a reference is formed. . step 2 : The optical reading means 100 measures the reflectance which is a function of the density of the toner image a. step3 : It is judged whether the density is low, that is, the background fog level is within the allowable range. step4 : If it is not within the allowable range, increase the charging output.
At this time, since the developing bias voltage is fixed, the background fog decreases as the charging output increases. step5 : When the low-density background fog level is within the allowable range, the potential difference between the developing bias and the charging output (grid voltage) is determined in this step.

In the second control stage, the following control is performed following step 5 above. step6 : The toner image b corresponding to the black solid level is printed on the photoconductor 27a. step7 : The density of the toner image b is measured by the optical reading means 100. step8 : It is judged whether the high density, that is, the black density is within the allowable range. step9 : When the density of the toner image b is not within the allowable range (that is, when black is not reproduced as black), the developing bias and the charging output (grid voltage) obtained by the first control means
The absolute value of both voltages is increased while the potential difference of is fixed. As for the black density, the larger the potential difference between the developing bias and the potential of the bright portion where the photosensitive member is irradiated with laser, the easier the toner is electrostatically attracted, and the toner image becomes darker. At this time, if only the developing bias is increased, the background fog corrected in step 1 occurs again, so that the absolute value of both voltages is increased without changing the potential difference between the developing bias and the charging output. Repeat this. step10 : When the density of the toner image b reaches the allowable level, the absolute values of the developing bias voltage and the charging voltage are determined.

Third Control Means The ground fog and solid black density are corrected by the first and second control means, but it is necessary to set an intermediate adjustment degree (gray level). In order to control the halftone density with a conventional analog machine, there is a method of changing the light amount of a copy lamp or the like. In this method, only one point of the halftone density can be adjusted. Then, it is possible to adjust at least two or more densities in the halftone. here,
The case of three density adjustments is shown. step 11 : The toner images c ₁ , c ₂ and c ₃ having a preset halftone density are printed on the photoconductor 27 a. step12 : The optical reading means 100 reads the three-stage densities of the toner images c ₁ , c ₂ and c ₃ . step13 : First, it is determined whether the density of the toner image c ₁ is at an allowable level. Step 14 : If the density of the toner image c ₁ is not at the allowable level, the threshold value t ₁ is controlled, and this is repeated until the density of the toner image c ₁ reaches the allowable level. Step 15 : After the threshold value t ₁ is controlled so that the density of the toner image c ₁ reaches the allowable level, it is determined whether the density of the toner image c ₂ is the allowable level.

Step 16 : If the density of the toner image c ₂ is not the allowable level, the threshold value t ₂ is controlled until the density reaches the allowable level. step17 : Finally, it is checked whether or not the density of the toner image c ₁ is at an allowable level. step 18: performs control of the threshold t ₃ to a concentration of the toner image c ₁ reaches the acceptable level, the process returns to step 11, and repeats the above operation. step 19 : It is judged whether or not all the densities of the toner images c ₁ , c ₂ and c ₃ are at the allowable level. If it does not reach the allowable level, return to step 11 and repeat the above operation. step 20: When the toner image c ₁ halftone, c ₂ and c ₃ has reached the acceptable level, the process control of the copying machine is completed.

As shown in the above description, the quantization threshold value and the change of the quantization value are read from the reference toner images a, b, c ₁ , c ₂ and c ₃ formed on the photoconductor 27a. It is performed based on the difference between the obtained density data and the predetermined density data that is determined in advance. In the halftone processing block diagram of FIG. 11, the difference between the reference toner image and the predetermined density is C
The threshold value and the quantized value of step 1 and step 2 are changed via the PU 64.

[0051]

As is apparent from the above description, according to the present invention, (1) as a method of changing the area gradation at the time of printing, the laser gradation setting value of the image recording apparatus is fixed,
On the halftone processing side, there are provided a mode for changing the quantization threshold value and the quantization value from the high-value image density gradation to the low-value image gradation according to the image, and a mode having a fixed threshold value and a quantization value. Since it can be switched, simple operation is possible, image quality is improved, and fixed thresholds and quantization values are fixed as in the past, and it is complicated to perform halftone by setting the laser gradation. However, it also eliminates the drawback of poor image quality. Furthermore, even if the reference data changes due to changes in the environment, the threshold value and the quantized value are corrected based on the comparison with the first stored reference data. A constant image without is obtained. In addition, a reference toner image is formed on the photoconductor, this is optically detected, and the potential difference is determined by controlling the charging output voltage while the developing bias voltage is fixed so that fog does not occur at low density. However, the absolute voltage is controlled while maintaining the potential difference so that the predetermined concentration is obtained at high concentration, and at least two or more threshold values in the halftone concentration are controlled within the controlled low concentration and high concentration ranges. Since the quantized value is determined, stable copy image quality is obtained even when the environment changes or the thin film thickness on the charged body changes over a long period of time, and multiple points at the halftone level are independent. Then fine adjustment is possible.

[Brief description of drawings]

FIG. 1 is a sectional view showing an overall configuration of an embodiment of a digital copying machine according to the present invention.

FIG. 2 is a plan view showing an example of an operation panel of the digital copying machine 10 shown in FIG.

FIG. 3 is an explanatory diagram showing a copy mode etc. setting screen of a display unit 50a of the operation panel of FIG.

4 is a block configuration diagram of an image processing unit and each control system included in the digital copying machine 10 shown in FIG.

FIG. 5 is a diagram showing an example of a quantized value of image density and a threshold value.

FIG. 6 is a diagram showing a mode gradation configuration.

FIG. 7 is a diagram showing a gradation change and a density curve of FIG.

FIG. 8 is a diagram for explaining an example of halftone processing when the input density values in A mode and B mode according to the present invention are changed.

FIG. 9 is a diagram showing a distribution method of step 1 in which a quantization error ε of a pixel of interest to be processed is distributed to surrounding pixels.

FIG. 10 is a diagram showing a distribution method of step 2 in which the processing target pixel quantization error ε is distributed to surrounding pixels.

FIG. 11 is a block diagram of a halftone processing circuit of the image recording apparatus of the present invention.

FIG. 12 is a diagram for explaining error distribution in a step 1 processing unit.

FIG. 13 is a diagram for explaining error distribution in a step 2 processing unit.

FIG. 14 is a diagram for explaining halftone processing.

FIG. 15 is a diagram showing an exposure operation of the copying machine.

FIG. 16 shows an example of a change over time of reference data (white image).

FIG. 17 is a diagram for explaining resetting of a quantized value and a threshold value.

FIG. 18 is a flowchart for changing a quantized value and a threshold value when a reference value changes due to the environment according to the present invention.

FIG. 19 is a flow chart for explaining another embodiment of the image processing apparatus according to the present invention (flow chart: No. 1).

FIG. 20 is a flow chart for explaining another embodiment of the image processing apparatus according to the present invention (flow chart: No. 2).

FIG. 21 is a diagram showing an example of the relationship between quantization and threshold values.

FIG. 22 is a diagram showing an example of laser gradation.

FIG. 23 is a diagram showing an example of a four-valued laser gradation structure.

FIG. 24 shows a density line for the gradation of FIG. 22.

[Explanation of symbols]

1 ... Intermediate processing circuit, 2 ... step1 processing unit, 3 ... step2 processing unit, 10 ... Digital copying machine, 11 ... Scanner, 12
... Laser printer section, 13 ... Multi-stage paper feeding unit, 14 ...
Sorter, 15 ... Original document table, 16 ... Automatic document feeder (RDF) for double-sided, 20 ... Unit, 22 ... Photoelectric conversion element (CCD), 23 ... Reflective mirror, 50 ... Operation panel,
50a ... Display unit, 51 ... Magnification, 52 ... Paper size area, 53 ... Copy density, 54 ... Number of copies, 55 ... Sorter, 56 ... Plus function, 60 ... Image data input unit, 61
Image processing unit 62 Image data output unit 63 Memory 64 Image processing central processing unit (CPU) 10
0 ... Optical reading means.

Claims (3)

[Claims] 1. A fixed method for setting a low gradation fixed threshold value to high gradation digital data output from an image data output means, and setting a low gradation quantization value based on the fixed threshold value. The mode setting means, a variable mode setting means for setting a variable threshold value based on the image quality of the image, and a quantization value based on the variable threshold value, and the fixed mode or the variable mode setting means. Halftone processing means for distributing an error between the variable threshold value and the quantized value of the low gradation and the digital data of the high gradation to the pixels around the processing target pixel for each processing target pixel; and the fixed mode. An image processing apparatus comprising: a mode switching unit that switches between a setting unit and a variable mode setting unit. 2. A first reference data storage means for reading and storing the first image reference data, a second reference data storage means for reading and storing the reference data read thereafter, and the first reference data storage means. Comparing means for comparing the first reference data with the second reference data stored in the second storage means and outputting the comparison value as an error; the error output by the comparing means; and the error and the reference. Compare with the value,
The image processing apparatus according to claim 1, further comprising a discrimination correction unit that corrects a quantization value and a threshold value based on the comparison value. 3. A density detecting means for optically detecting the density of a reference toner image formed on a photoconductor, and a developing bias voltage fixed based on the density of the reference toner image detected by the density detecting means. First, the charging output is controlled so that the density of the reference toner image formed as described above becomes a predetermined value, and the potential difference between the developing bias voltage and the charging output voltage is determined.
Electrophotographic process control means, second electrophotographic process control means for controlling the absolute value of the voltage when the potential difference between the developing bias voltage and the charging output voltage is constant, and a plurality of halftone densities of the reference toner image. An image processing apparatus comprising: a third electrophotographic process control means for determining a threshold value and a quantization value.