JP3334010B2 - Image forming device - Google Patents

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,
In particular, the present invention relates to an image forming apparatus that forms an image by reproducing density data of each pixel using a laser beam.

[0002]

2. Description of the Related Art In a recording apparatus of a scanning recording type, various methods are adopted to digitally reproduce the density of an image. Such digital tone reproduction methods include a dither method, an intensity modulation method, and a pulse width modulation method. In the dither method, a plurality of pixels are taken as one unit, and the on / off area ratio of the pixels included therein is changed to express the density of an image. In the intensity modulation method, the light emission intensity of the laser is modulated in multiple stages for one pixel, the laser energy is radiated to the inside of the pixel on average, and the density is changed. The pulse width modulation method modulates the light emission time per unit pixel while keeping the light emission intensity of the laser constant. For example, Japanese Patent Application Laid-Open Nos. Sho 61-225971 and Sho 62-1169.
No. 59 discloses this.

However, in the dither method, since a plurality of pixels are regarded as one unit, moire noise peculiar to the dither method is generated, thereby deteriorating the image quality.

In the intensity modulation method, the density can be basically output one-to-one in accordance with the read image data, and it is possible to obtain a high resolution and a fine smooth gradation characteristic. However, in the highlight portion, fine density control is required, and therefore, the gradation reproducibility is difficult. Further, there is a problem that the graininess of the halftone portion is noticeable due to the influence of the base of the image recording paper.

On the other hand, in the pulse width modulation system, although the problem of granularity in the intensity modulation system has been improved, it is generally employed because the center of gravity of a pixel is moved in units of two pixels, and therefore, for example, a density of 400 DPI is used. When the data is reproduced, the reproducibility is substantially 200 DPI, and the resolution is reduced. On the other hand, if the pulse width modulation method is adopted for each pixel, the emission time of the laser is considerably reduced in a pixel having a low density, and the image quality is not stable in terms of toner adhesion.

[0006] In performing relocation of density data between neighbor pixels in order to solve the above problems, while maintaining the resolution,
Japanese Patent Application No. 4-353263 discloses an image processing apparatus which performs so-called line screen processing for improving the graininess of a halftone portion and further improving the tone reproducibility of a highlight portion.

[0007]

Although the effect is recognized that graininess of a halftone portion of uniform concentration when performing line screen process above Symbol [0006] is improved, in part that changes the concentration continuously There is a problem that a density step is generated and is conspicuous as a pseudo contour. Also has or gradation characteristics change due to variation of the image forming process conditions diameter and photoconductor sensitivity Les Zabimu, such as the saturation of the high-concentration portion becomes faster problem.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and an object of the present invention is to provide an image forming apparatus which can prevent the occurrence of a false contour due to a density step and can cope with a change in gradation characteristics. With the goal.

[0009]

According to a first aspect of the present invention, there is provided an image forming apparatus for forming an image based on density data of each pixel corresponding to an original image, wherein the image forming apparatus reads the original image. Density data receiving means for receiving the density data of each image obtained by the above, and by switching the window of the received density data by two pixels, the density data of one pixel in the adjacent images and the density data of the other pixel
If the difference from the density data of the pixel is less than the predetermined value,
With reducing at least a portion of the density data of the pixel, to apply reduced density data into density data of the other pixels, a data processing unit for processing, the pixel density data after being added by the data processing means An upper limit value setting unit for setting an upper limit value; and a density correction unit for converting the processed density data into light emission amount data based on the tone correction data and outputting the light emission amount data. In the density region, there are at least a continuous data group having a slope of less than 1 as a starting point, and in a high density region including a maximum density, a continuous data group having a slope of 1 or more having at least the maximum density as an end point. , Continuous data having at least one inflection point between the low-density area and the high-density area, and the upper limit of the density data set by the upper limit setting means. Is input density data following at the inflection point.

According to a second aspect of the present invention, there is provided an image forming apparatus for forming an image based on density data of each pixel corresponding to a document image, wherein each image obtained by reading the document image is provided. Density data receiving means for receiving the density data of one pixel and the density data of one pixel and the density data of the other pixel in adjacent images by switching the window by two pixels.
If the difference from the data is less than the specified value, the density data of one pixel
With reducing at least a portion of the data, to apply reduced density data into density data of the other pixels, and data processing means for processing, detecting means for detecting the variation of the electrophotographic process, the data processing means Upper limit value setting means for setting an upper limit value to the density data of the pixel after the addition ;
Based on the detected variation of the electrophotographic process,
A change unit for changing the set upper limit value and a data output unit for outputting processed density data are provided.

[0011]

According to the first aspect of the present invention, the gradation correction data changes at the inflection point so that the inclination of the gradation correction data is lower in the low density area and the high density area than in the low density area.
Then, the upper limit value of the density data of the pixel after being added by the data processing means is equal to or less than the input density data at the inflection point.

In the second aspect of the present invention, the value set as the upper limit value is changed based on the detected fluctuation of the electrophotographic process.

[0013]

FIG. 1 is a sectional structural view of a digital copying machine main body according to an embodiment of the present invention.

Referring to FIG. 1, a printer body 1 includes a desk 4
The photosensitive drum 10 is set at substantially the center so as to be rotatable in the direction of arrow a. Around the photosensitive drum 10, a charging charger 11, magnetic brush type developing devices 12 and 13, a transfer charger 14, a paper separating charger 15, a cleaning device 16 for residual toner, an eraser lamp 17 for residual charge, and the like are provided. . The image is exposed on the photosensitive drum 10 by the laser beam scanning optical system 2 immediately after the charging process. Since the printing process using these elements is well known, the description is omitted.

On the other hand, three automatic paper cassettes 21, 22, and 23 are provided on the left side of the printer main body 1 in the drawing, and an optional automatic paper feeding unit 24 of an elevator type is provided on the desk 40. . The size or weighing of the paper loaded in each cassette and paper feeding unit is detected by sensors SE11 to SE14, respectively. Sheets are selectively fed one by one from each cassette and a sheet feeding unit by respective sheet feeding rollers 25 to 28. The bold line in the drawing indicates a paper passing path, and each sheet is held at one end by a timing roller 30 and sent to a transfer section in synchronization with an image formed on the photosensitive drum 10.
The sheet after the transfer of the toner image is conveyed to a fixing device 32 by a conveying belt 31, where the toner is heated and fixed.
The sheet is discharged from the discharge roller 33 to the outside of the main body, and is introduced into the sheet reversing unit 50.

The paper reversing unit is a roller 3 for a double-sided printer that prints on the other side of a single-sided printed sheet or a composite print that prints over the same side.
6 and 37, and a face-up discharge (non-reversal mode) for conveying the paper straight to the paper discharge tray 60, and a face-down discharge (non-reversal mode) for reversing the paper. (Reversal mode).

In order to achieve the above functions, the sheet reversing unit includes a receiving roller 51, a sending roller 52, forward / reverse switching rollers 53 and 54, and a switchback path 5.
Eight. The switching claws 56 and 57 can be switched between two rotation angles by a solenoid (not shown).

In the non-reversing mode, the sheet is guided from the receiving roller 51 on the upper surface of the switching claw 56, and sent out from the sending roller 52 to the sheet tray 60 in a face-up state. In the reversing mode, the sheet is guided from the receiving roller 51 on the left side surface of the switching claw 56, and the forward end of the sheet reaches the switchback path by the normal rotation of the roller 54. When the trailing edge of the sheet reaches the reversal point Q, the rollers 53 and 54 are switched to reverse rotation. here,
The sheet is guided by the right side of the switching claw 56 with the rear end up to that point as the leading end, and is sent out from the sending roller 52 to the sheet tray in a face-down state.

On the other hand, in the double-sided printing mode, the sheet is conveyed to the switchback path in the same manner as in the reversing mode, and when the rear end of the sheet reaches the reversing point P, the roller 54 is switched to the reverse rotation. Here, the sheet is guided by the lower left surface of the switching claw 57 with the rear end up to that point as the leading end,
The sheet is fed from the sheet re-feed rollers 36 and 37 to the sheet re-feed path 35. In the composite print mode, the paper is
From 3, the sheet is guided by the upper left surface of the switching claw 57, and is fed from the re-feed rollers 36 and 37 to the re-feed path 35.

An image reader (IR) optical system 110 exposes and scans a document placed on a platen glass 118, and converts reflected light from the document to electric signals by photoelectric conversion elements 116 and 117 using, for example, a CCD array. It is something to convert. The photoelectric conversion elements 116 and 117 convert an image of a specific color such as black, for example, and an image of another color such as red into electric signals, respectively.

An image reader (IR) optical system 110 is attached to a scanner 119 which moves in parallel with a platen glass 118 by a scan motor M2, and an exposure lamp 111 for irradiating a document and a reflection mirror 112 for changing the direction of light reflected from the document. , Two mirrors 113a and 113b for changing the optical path from the reflection mirror 112, a lens 124 for condensing the reflected light, and a color discrimination based on the wavelength of the reflected light, and reflecting or transmitting the color to obtain two photoelectric conversion elements. 11
A half mirror 115 for guiding reflected light to 6 and 117;
Photoelectric conversion element 1 generated by an electric signal according to received light
16 and 117, and performs exposure scanning of the original when the scanner 119 moves leftward as indicated by the arrow.

FIG. 2 is a perspective view showing the configuration of the laser optical system in the optical unit 2 of FIG.

Referring to the figure, a laser beam emitted from semiconductor laser element 161 in response to a drive signal passes through collimator lens 155 and cylindrical lens 153 and enters one surface of polygon mirror 152. The beam reflected by this surface passes through the f-θ lens 156, is reflected by the mirror 159, goes out of the optical unit 2 through the slit 114, enters the photosensitive drum 10, and exposes the photosensitive drum 10. As the polygon mirror 152 rotates, the emission direction of the beam reflected from one surface of the polygon mirror changes as shown in the figure, and the photosensitive drum 10 is scanned in the axial direction. At the start of the scan to synchronize this axial scan, the laser beams are mirrored 158 and 16
0, reflected by photodiode (SOS sensor) 16
3 is incident. When disposing the optical system, mirrors 158 and 160 and photodiode 163 are arranged such that the optical path length from polygon mirror 152 to photodiode 163 is substantially equal to the optical path length from polygon mirror 152 to photoconductor drum 10. You.

FIG. 3 is a block diagram showing the configuration of the entire system of the digital copying machine shown in FIG. Referring to the figure, a control processor 20 for controlling the printer main body is provided on the printer side.
0, a control processor 201 for controlling an image reader (IR) optical system, a control processor 202 for controlling a paper feed option if there is one, and a control processor 203 for controlling the paper discharge option if there is one. The print information is transmitted from the optical system control processor 201 to the printer main body control processor 200 via the line screen processing processor 204 which performs density data processing.

The data ROM 223 stores a gamma correction table. The printer main body control processor 200 controls the semiconductor laser driver 263 with reference to the contents of the data ROM 223, and the semiconductor laser driver 263 drives the semiconductor laser 264 to emit light, thereby printing print information.

A signal such as a print mode is transmitted to the interface control processor 215 via the control line 214. This interface control processor 215
Communicates with the processors 200 to 203 in various modes via the serial interface 216. Further, the interface control processor 215 controls on / off of the operation panel display unit 217 on the printer main body. The display unit displays various information based on an instruction from the processor 215.

FIG. 4 is a system configuration diagram around the line screen processor 204 of FIG.

Referring to the figure, image data (8 bits) read by image reader IR is written into first memory (FIFO) 250 line by line. Along with the image data, a main scanning valid signal A indicating the presence of a document in the main scanning direction shown in FIG. 7 is also sent from the image reader IR, and the signal is sent to the timing controller 252 and the first memory 250.
Is input to A sub-scanning effective area signal indicating the presence of a document in the scanning direction shown in FIG. After a predetermined time, a main scanning effective area signal B is generated by the timing control section 252 and is provided to the processor 204 and the first memory 250. With this signal, the processing shown in the flowchart of FIG. 6 (to be described later) is executed, and the image data for one line is processed for each window corresponding to every two pixels as shown in FIG. An upper limit value (described later), which is a parameter of the line screen processing, is set as upper limit data by the upper limit value setting means 280, and the line screen processing processor 204 executes the above processing while referring to the upper limit value. Go. Then, the image data for every two pixels processed by the line screen processor 204 is written to the second memory 251. The processed image data stored in the second memory 251 is output as 8-bit image data to the printer control processor at the timing of printing, and the laser printer forms an image based on the data.

FIGS. 5 and 6 are flowcharts showing the control contents of the print processing performed by the line screen processing processor 204 of FIG.

Before describing this flowchart in detail, an outline of the control will be described with reference to FIGS.

FIG. 7 is a diagram showing the ON / OFF relationship of the main scanning direction effective area signal and the sub-scanning effective area signal for the original to be read. As shown in the figure, the main scanning effective area signal is a signal that changes when a document is present in the line direction of the document, that is, in the main scanning direction, and the sub-scanning effective area signal is in the scanning direction or the paper passing direction. On the other hand, it is a signal that changes when a document exists.

FIG. 8 is a diagram showing the relationship between the main scanning effective area signal, each image data, and the window number. In the figure, as an example, one line of image data is described for N + 1 pixels from 0 to N. One window number is assigned to each two adjacent pixels. That is, the window number W0 corresponds to the image data 0 and 1, and the window number W (N-1) / 2 corresponds to the image data N-1 and N. For convenience of the following description, image data 0 is a window W
0 corresponds to left density data L, and pixel data 1 corresponds to W
This corresponds to right density data R of 0. Similarly, the pixel data N-1 corresponds to the left density data L of W (N-1) / 2 and the image data N corresponds to the right density data R of W (N-1) / 2.

FIG. 9 is a schematic diagram showing the procedure of processing the density data for each pixel. In (1) of FIG. 9,
The basic concept of processing density data according to the present invention is shown. In the figure, the vertical axis indicates 256 gradations (0 to 25).
The density level of 5) is shown, and the upper limit value of the density data after transfer (220 in this case) is set. The horizontal axis shows the left density data L and the right density data R of a certain window. In this example, the density value of the left density data L is large, and there is a difference from the density value of the right density data R by the value _DLR . Is smaller than the threshold D _TH there is this difference D _LR, the value of the right density data R is, on the left density data L, are moved as indicated by the broken line. In this example, as a result, the value of the left density data L increases to the position indicated by the broken line, and the value of the right density data R becomes 0.

In the case of FIG. 9 (2), a process is shown when the difference _DLR between the left density data and the right density data is larger than the threshold value _DTH . In this case, the transfer of the density data as shown in FIG. 9A is not performed. The reason why the relocation is not performed in this case is, for example, in the case of an image in which an edge portion such as character data is clearly shown,
This is because if the transfer is performed as shown in (1) of FIG. 9, the position of the edge part moves, which is not preferable in terms of image reproducibility.

FIG. 9C shows another example of the transfer of image data. In this example, since the difference _DLR between the left density data L and the right density data R is smaller than the threshold value _DTH , the density data is basically transferred as shown in (1). However, in this example, the left density data L
Therefore, if all the values of the right density data R are relocated, the value exceeds the upper limit value 220 of the set density level. Therefore, in this case, the transfer amount is determined from the right density data R as indicated by a broken line so that the upper limit of the left density data L is the upper limit value 220. Therefore, as a result of the relocation, the value of the left density data L becomes the upper limit value 220, the value of the right density data R does not become 0, and remains as the density data at the position indicated by the broken line. By doing so, the amount of toner adhered to the pixel corresponding to the left density data L becomes the upper limit value, and the density level of the whole pixel constituting this window is maintained.

FIG. 9 (4) shows still another example. The difference D _LR of the left density data L and the right density data R in this example is smaller than the threshold D _TH, since already exceeds the density level limit 220 of the left density data L is set No relocation is performed.

Even if both the left density data L and the right density data R exceed the upper limit of the density level as shown in FIG. 9 (5), no relocation is performed.

If the difference between the left density data and the right density data is small as shown in (4) and (5) of FIG. 9, and at least one of the density data exceeds the set upper limit, a high density line is displayed. It is considered an image or a solid image. In the case of such an image, since the graininess does not matter, it is not necessary to transfer the density data.

FIG. 10 is a view showing the result of the data transfer shown in FIG. 9 (1) showing the state of the pixels in the portions corresponding to the windows W0 to W4 and the lines 1 to 3. As shown in the figure, in this example, all the right density data R are moved to the left density data L, so that the densities of the pixels constituting each of the windows W0 to W4 are all moved to the left pixel L. As a result, line screen processing in which every other pixel is emphasized is performed. In this example, the value of the right pixel R after the transfer is set to 0. However, even when the transfer as shown in (3) of FIG. 9 is performed, the density of the left pixel L is emphasized. As a result, the same effect as when the line screen processing is performed is produced.

Next, the flowcharts of FIGS. 5 and 6 will be described. When the line screen processor starts processing, first, in step S501, it is determined whether the sub-scanning effective area signal has changed. When the sub-scanning effective area signal has not changed, that is, when the optical system is not reading a document, the process returns as it is. On the other hand, if it is within the sub-scanning effective area, it is determined in step S502 whether the main-scanning effective area signal has changed. When this signal changes, it means that the optical system has started scanning the original in the main scanning direction. Therefore, the window number is reset to 0 in step S503. on the other hand,
If the main scanning effective area signal has already been changed, the flow jumps from step S503 and proceeds to step S504. In step S504, it is determined whether the document to be read is within the main scanning effective area. If it is out of the area, the program returns without performing line screen processing.

If the object to be scanned by the optical system is within the main scanning effective area, an even-numbered pixel among the pixels stored in the first memory 250 is read into the line screen processor 204 in step S505. . That is, the data of the (window number × 2) -th pixel is stored in the left image data L. Next, in step S506, the odd-numbered pixels in the image data stored in the first memory 250 are read. That is, in the right image data R,
The data of the (window number × 2 + 1) -th pixel is stored.

Next, in step S507, step S50
In steps 5 and 506, the absolute value of the difference between the stored image data L and image data R is calculated. That is, step S
In step 507, a process for determining the variation in density data between adjacent pixels is performed.

Next, in step S508, it is determined whether the obtained difference is less than a predetermined value. If the difference is not less than the predetermined value, it corresponds to the state of (2) in FIG. 9 and the flow returns to step S516. in this case,
Since the image data is not relocated, the first memory 25
The values of the image data L and R read from 0 are output to the second memory 251 as they are and stored there (S51).
6). Then, in step S517, after the window number is incremented in order to process the image data for the next window, the flow returns to step S504.

On the other hand, if the difference is smaller than the predetermined value in step S508, the flow advances to the next step S509. In step S509, it is determined whether or not the image data R exceeds a preset upper limit. If the upper limit is exceeded, the flow returns to step S516. If the image data R is less than the upper limit in step S509, the process proceeds to the next step S510. In step S510, it is determined whether or not the image data L exceeds the upper limit.
If it exceeds the upper limit, the flow proceeds to step S516
Return to

If it is determined in step S510 that the image data L is smaller than the upper limit, the flow proceeds to step S51 in order to process the image data (1) or (3) in FIG.
Go to 1 or less. In step S511, the image data L
, A value obtained by adding the image data L and the image data R is stored. Then, in step S512, the image data L
Is higher than the upper limit value, that is, in FIG. If the image data L does not exceed the upper limit, a value of 0 is stored in the image data R in step S514 in order to process the image data as (1) in FIG. Then, the image data L and R determined in step S511 and step S514 are
In step S516, the data is output to and stored in the second memory 251. After the window number is incremented in step S517, the flow returns to step S504.

On the other hand, in step S512, step S5
If the image data L obtained in step 11 exceeds the maximum value, processing corresponding to (3) in FIG. 9 is performed. That is, in step S513, the image data R is added to the image data L.
Store the value obtained by subtracting the maximum value from. That is, as shown in (3) of FIG. 9, this is a process of obtaining the value of the position of the broken line of the right image data R. Then, in step S515, the upper limit value 220 is stored in the image data L. Thereafter, similarly, the processing of step S516 and subsequent steps is performed, the window number is incremented so as to process the image for the next window number, and the flow returns to step S504.

FIG. 11 shows the printer main unit control processor 20.
It is a system configuration diagram around 0. Referring to the figure, the processed image data stored in the second memory is output as 8-bit image data to the printer main processor at the timing of printing, and the laser printer converts the image based on the data. Form. The printer main body control processor 200 includes a line screen processor 20.
4 is connected by an image data bus, and based on an image density signal received via the image data bus, controls the semiconductor laser driver 263 with reference to the contents of the data ROM 223 storing the γ correction table. ing. The light emission of the semiconductor laser 264 is driven by the semiconductor laser driver 263. Tone expression is semiconductor laser 2
This is performed by modulating the emission intensity of 64.

The 8-bit image data from the line screen processor 204 is transmitted to the interface unit 241.
To a first-in / first-out memory (hereinafter, referred to as a “FIFO memory”) 242 via a. The FIFO memory 242 is a line buffer memory capable of storing gradation data of a predetermined number of lines of images in the main scanning direction, and absorbs a difference in operation clock frequency between the image reader unit and the copying unit. Is provided. F
The data in the IFO memory 242 is input to the gamma correction unit 253. As described later in detail, the data ROM 223
The gamma correction data of the gamma correction table of
It is sent from the laser exposure control unit 220 within 0 to the γ correction unit 253, and the γ correction unit 253 corrects the input data RD and sends the output level to the D / A conversion unit 254.

The D / A converter 254 converts the input digital data into an analog voltage, and converts the converted analog voltage via the amplifier 255, the variable attenuator 266, the drive I / O 261 and the semiconductor laser driver 263. Is output to a semiconductor laser 264 having a semiconductor laser diode LD, thereby causing the semiconductor laser 264 to emit light at an intensity corresponding to the digital data. Here, the amount of attenuation of the variable attenuator 266 is changed in eight steps in accordance with the gain switching signal input from the laser exposure control unit 220, whereby the power of the laser light emitted by the semiconductor laser 264 is changed in eight steps. Be changed.

Further, clock generators 270a and 270
b generates clock signals having different clock frequencies from each other, and outputs the clock signals to the semiconductor laser driver 2 via the a side and b side of the switch SW and the parallel I / O 262, respectively.
63. The switch SW is switched by a clock switching signal output from the laser exposure control section 220, whereby each of the clock signals is selectively input to the semiconductor laser driver 263.

Here, the γ correction method in the conventional intensity modulation method and the problem that occurs when the γ correction method is applied to the line screen processing will be described in detail below.

According to this type of gradation method, an image density having a gradation corresponding to the gradation of the image data to be reproduced on a one-to-one basis can be reproduced in principle. The density of the original to be reproduced and the density of the reproduced image (hereinafter referred to as "image reproduction density") are not exactly proportional due to the entanglement of the photosensitive characteristics of the photoconductor and the characteristics of the toner. It shows shifted characteristics. The characteristic deviating from the proportional characteristic is generally called a γ characteristic, which is a major factor that particularly lowers the fidelity of a reproduced image for a halftone original.

Therefore, in order to improve the fidelity of the reproduced image, conventionally, the read document density is converted using a predetermined gamma correction conversion table, and a digital image is formed based on the converted document density. Thus, so-called gamma correction is performed so that the relationship between the document density and the image density satisfies the proportional characteristic. As described above, normally, by performing the γ correction, the image can be faithfully reproduced according to the density of the document density.

FIG. 12 is a graph showing sensitometry including a light quantity-density characteristic, an image reproduction characteristic, a γ correction characteristic and an image reading characteristic in a conventional digital copying machine of the intensity modulation system. Note that, in FIG. 12 and FIGS. 19 and 22 referred to below, the image reproduction density I
D is the density ID of the background of the paper even if the original density OD is 0
u has been measured.

Here, the .gamma. Correction characteristic can be prepared in advance in a known manner based on the light quantity-density characteristic DC so that the target image reproduction characteristic shown in the first quadrant of FIG. 12 can be obtained. .

In the conventional intensity modulation method, the above γ
By performing the correction, the image can be faithfully reproduced. However, the inventor of the present application has found that if similar gamma correction is performed on image data that has been subjected to line screen processing, which is a premise of the present invention, there is a disadvantage that a gradation step occurs.

Hereinafter, the cause of the occurrence of the gradation step will be described with reference to FIGS.

FIG. 13 is an enlarged graph of the low density portion of the γ correction characteristic T of FIG. As is clear from FIG.
The γ correction characteristic T created in the conventional example has a characteristic in which the laser exposure amount rises rapidly with a large gradient from 0 to the image reproduction start light amount a, and gradually changes with a relatively small inclination after the image reproduction start light amount a. . Here, the image reproduction start light amount is a laser exposure amount when an image is reproduced for the first time when the laser exposure amount is changed from 0 to a larger direction.

FIGS. 14 and 15 show two different density data. (A) Exposure amount for each pixel given for each pixel. (B) Integrated exposure amount on photoreceptor including leaked light from adjacent pixels. (C) The distribution of the surface potential on the photoreceptor after exposure at the above exposure amount, and (d) the distribution of the amount of toner adhered on the paper after development, transfer and fixing.

FIG. 14 shows the density data after performing the above-mentioned line screen processing on the density data of a uniform density level (110 in the figure). The right pixel density data R indicated by the broken line is all transferred to the left pixel density data L,
The left pixel L has an upper limit of 220 and the right pixel R has a value of 0.

(A) shows the exposure amount for each pixel. The left pixel L is exposed to an exposure amount corresponding to the density data 220 via the conversion tables for gamma correction in FIGS. The right pixel R is not exposed because the density data is 0. However, the light amount distribution of the laser beam usually has a Gaussian distribution, and further, since scanning exposure is performed, an ideal rectangular exposure amount distribution cannot be given. For this reason, light leaking from the adjacent left pixel L also occurs in the right pixel R that has not been exposed.

(B) shows the distribution of the integrated exposure amount on the photoreceptor (the sum of the exposure amounts received at each point). The right pixel R is exposed with the light amount obtained by adding the leakage light from the left pixel L adjacent to the left and right.

(C) shows the distribution of the surface potential of the photosensitive member in the exposed portion. Before exposure, the surface potential is charged to V ₀ .
V _B indicates a potential as a development start threshold. Although the density data is 0 in the right pixel R, the surface potential is reduced by V ₀ ′ at the maximum due to the leakage light described above.

FIG. 9D shows the distribution of the amount of toner adhered to the paper with respect to the electrostatic latent image. Although the right pixel R has caused a reduction potential, the toner is not developed since it does not exceed the development start threshold V _B.

Next, FIG. 15 shows that the density data is の of the upper limit value.
It shows the state when exceeding. FIG. 15 shows that the left pixel L is 11
0 shows a case where density data of 111 is alternately given to the right pixel R. By performing the above line screen processing, the upper limit value 2 of the right pixel density data R indicated by the broken line
The margin 110 up to 20 is moved to the left pixel density L.
Since the right pixel density data R is 111, the density data after the transfer has the upper limit value 220 for the left pixel L and 1 for the right pixel R.

In FIG. 9A, an exposure amount corresponding to the density data 1 is given to the right pixel R, and the density data 220 is given to the left pixel L.
Is provided. Leakage light occurs in the same manner as in FIG. The exposure amount is determined for both the right pixel R and the left pixel L using the γ correction conversion tables shown in FIGS. 12 and 13. Therefore, as described above, in the low-density portion, the gamma correction characteristic T sharply rises from the exposure amount of 0, so that the density data 1 of the right pixel R
Is exposed with a certain amount of exposure.

(B) shows the integrated exposure dose distribution. Concentration day
The exposure amount corresponding to data 1 sharply changes from 0 as shown in FIG.
Has been determined, the right pixel R
Exposure amount also increased sharply compared to (b) of FIG.
U. The exposure amount corresponding to the density data 1 is set large
Is the surface potential V₀At a stretch the development start threshold V _B
To be able to reproduce the image from density data 1.
In order to

(C) shows the surface potential distribution. Right pixel R
The surface potential is already V in the state of FIG. ₀To V₀′
Has been dropped. In FIG. 15, the density is further increased in this state.
Since the exposure amount of data 1 is added, the surface potential is
V₀''. The surface potential of the right pixel R is dark
Since the degree data is 1, the development start threshold V_BSlightly
Although it is desirable to lower the potential to
Causes the potential to drop too much below the desired potential.

(D) shows the distribution of the amount of toner adhered. FIG.
In FIG. 4, no toner adheres to the right pixel R, but in FIG. 15, an amount of toner corresponding to the potential difference of V _B −V ₀ ″ attaches at once.

A comparison between FIG. 14 and FIG. 15 shows that although the density data before the line screen processing has increased only by one, the image density sharply increases, and as an originally continuous tone portion, A gradation step occurs in an area to be reproduced. This is recognized as a false contour and the image becomes very unsightly.

In FIG. 15, the density data of the left pixel L is 11
1, the density data of the right pixel R has been described as 110,
If the density data of both the left and right pixels is 111, the only difference is that the density data 2 is left unloaded on the right pixel R, and a gradation step occurs similarly.

FIG. 16 is a diagram showing the overall image reproduction characteristics. The horizontal axis shows the density data before the line screen processing,
The vertical axis represents a macro average image density of an image when the density data is printed by performing line screen processing.
Here, a case where the density data of the right pixel R and the density data of the left pixel L are the same is illustrated, and an image having a uniform density is assumed.

If the density data before the line screen processing is from 0 to 110 (１ ／ of the upper limit), the density data of the right pixel R becomes 0 by the line screen processing.
The density data of the left pixel L increases by two. Since the image density increases only by the continuous increase in the density of the left pixel L, a smooth gradation characteristic is obtained in this region.

At the point (point A) where density data first occurs in the right pixel R, a gradation step occurs due to the cause described with reference to FIGS. The density data of the point A before the line screen processing is 111, the density data of the right pixel R is 2 and the density data of the left pixel L is 220 (upper limit) by the line screen processing. When the substantial density of the right pixel R sharply increases, the average image density also sharply increases, and the gradation characteristic becomes discontinuous at the boundary between the density data 110 and 111.

When the density data before the line screen processing is from 111 to 220, the density of the left pixel L is kept at 220 (upper limit) and the density data of the right pixel R is from 2 to 2 by the line screen processing. Increase by one. Since the image density increases only by the continuous increase in the density of the right pixel R, a smooth gradation characteristic can be obtained even in this area.

When the density data before the line screen processing is from 221 to 255, since the density data of both the right and left pixels exceed the upper limit, the transfer of the density data by the line screen processing does not occur. Right pixel R and left pixel L
Increases by one while taking the same value, so that a smooth gradation characteristic can be obtained even in this region.

At the boundary between the density data 220 and 221,
The density data increases by one for each of the right and left pixels, but the right pixel R
Since the density of the left pixel L is continuously increased, the gradation characteristic becomes smooth.

As described above, with reference to FIGS.
It has been described that when the γ correction method in the conventional intensity modulation method is applied to the line screen processing, a problem that a gradation step occurs in an intermediate density portion occurs. In the present invention, this problem is solved by improving the γ correction characteristic.

FIG. 17 and FIG. 18 show the improved γ correction characteristics. In this embodiment, the density data 1
Is not increased rapidly to the image reproduction start light amount, but is continuously increased with a gentle inclination. FIG. 18 shows the γ correction characteristic T of FIG.
5 is an enlarged graph of a low concentration portion of FIG.

As shown in the figure, it can be seen that the laser exposure amount gradually rises from 0 with a small slope and continuously increases. Further, an inflection point is provided in the middle of the γ correction characteristic, and the density data beyond that is increased in inclination to continuously change the maximum exposure amount. In FIG. 17, the inflection point is set to the density data 220 and the LD exposure level 120, but this is an example.

In FIG. 19, a desirable setting range of the inflection point is shown by oblique lines. The value of the density data is 80 to 95 of the maximum density data.
% Is suitable. LD exposure level is the maximum image density of 7
An exposure amount that can provide an image density of 5 to 90% is suitable.
The set value of the inflection point and the upper limit of the line screen processing are closely related, and will be described later in detail.

Referring to FIG. 20, it will be described that the gradation step is eliminated by using the γ correction characteristic of FIG. The state in which right pixel R is 0 and left pixel L is at the upper limit is the same as in FIG. 14, and thus description thereof will not be repeated here. FIG. 20 shows a state where density data is first generated in the right pixel R.

The density data of the right pixel R in (a) is 1 as in FIG. 15, but the exposure amount is smaller than that in (a) in FIG. This is because the exposure amount corresponding to the density data 1 is small because the γ correction characteristic is set so as to gradually rise as shown in FIG.

The integrated exposure amount distribution shown in FIG.
14B, the exposure amount only slightly increases from the state shown in FIG. 14B.

(C) shows the surface potential distribution. The potential of the right pixel R is a state exceeding the development start threshold V _B little, it will not excessively reduced as in (c) of FIG. 15.

(D) shows the distribution of the amount of toner adhered. Since the surface potential of the right pixel R is almost equal to the threshold value for starting development, the toner slightly adheres.

A comparison between FIG. 14 and FIG. 20 shows that the right pixel R
Since the amount of toner adhering only slightly increases, the image density becomes almost continuous, and the gradation step does not occur.

FIG. 21 is a diagram showing improved image reproduction characteristics. There is no step in the gradation characteristics at the boundary between the density data 110 and 111, and the image reproduction characteristics are smoother than in FIG. Since the exposure amount corresponding to the density data is set to be small in the low density portion, the rise is slightly delayed, but the influence on the image is small. This delay in the rise of the low-density portion can be easily corrected by adjusting the IR image reading characteristics as described below.

FIG. 22 is a graph showing sensitometry showing the correction method and the correction effect in the IR image reading characteristic.

The image reading characteristics are set so that the value of the image reading data RD is output to be larger than the proportional relationship with respect to the low density portion of the document density OD. As a result, the density data of the low density portion can be converted into a large value, so that the rise can be improved even if the γ correction characteristic is gentle. As a result, the image reproduction characteristics can maintain linearity over the entire image density area.

Next, the relationship between the upper limit value set in the line screen processing and the inflection point of the γ correction characteristic will be described. In this embodiment, both the upper limit value and the density data of the inflection point are described as 220, but the upper limit value may be any value as long as it is equal to or less than the inflection point. Several cases will be described below with reference to FIGS.

FIG. 23 shows a case where the upper limit value is not set for the line screen processing and the γ correction characteristic of the conventional example is used.

In the figure, (a) shows the density data before and after the line screen processing. Since the upper limit is not set, the left pixels L are stacked up to the maximum value 255.

(B) is the gamma correction characteristic. As described above, the rise of the low density is set to be steep in order to reproduce the image from the density data 1.

(C) is an image reproduction characteristic. A gradation step occurs due to the cause already described in the halftone section.

FIG. 24 shows a case where only the gamma correction characteristic in FIG. 23 is changed to that of the embodiment of the present invention. The upper limit of the line screen processing is not set as in FIG.

Referring to the figure, the density data of FIG.
3, but the low-density part of the gamma correction characteristic in (b) is set gently, so that the gradation step in the halftone part of the image reproduction characteristic in (c) is eliminated. However, in a high-density portion, the density saturation is fast, and the image reproduction characteristics are impaired.

The left pixels L stacked up to the density data 255 are exposed at the maximum exposure amount indicated by the broken line in FIG. 9B, so that the toner adhesion amount is the saturated development amount.

An image when the left pixel L is the maximum value (255) and the right pixel R is 0 should ideally be a vertical line every other pixel, but in fact it tends to be crushed. Such collapse tends to be caused by electrophotography processes such as a large amount of development, an increase in toner scattering at the transfer section, and an increase in fatness at the fixing section. This is due to well-known characteristics.

When the density data of the right pixel R rises from that state, the image density quickly saturates, so that the gradation characteristic of the high density portion is impaired.

FIG. 25 shows a case in which an upper limit value is further set for the line screen processing in FIG. 24, and corresponds to one embodiment of the present invention. The gradation step has already been eliminated by using the γ correction characteristic of FIG.

Referring to the figure, the exposure amount corresponding to the upper limit density data (220) set in the line screen processing in (a) is suppressed low as shown by the broken line in (b). By setting the exposure amount at this time to an exposure amount at which an image density of 75 to 90% of the maximum image density can be obtained, the left pixel L
Can be prevented and the concentration can be ensured (FIG. 1).
9).

As a result, the left pixel L is set to the upper limit (220).
The image when the right pixel R is 0 is reproduced as a vertical line every other pixel without being collapsed. Even if the density data of the right pixel R rises from this state, density saturation hardly occurs, and the gradation characteristics can be maintained even in a high density portion.

FIG. 26 shows a case where the upper limit of the line screen processing is set higher than that of FIG. This upper limit is 230. Referring to the figure, if the upper limit value of the left pixel L is set to a value larger than the density data at the inflection point of the γ correction characteristic, the exposure amount is compared with that in FIG. Increase rapidly. As a result, although not as much as in FIG. 24, the image still tends to be crushed, and the gradation characteristics of the high-density portion are impaired.

As described above with reference to FIGS. 23 to 26, there is a close relationship between the upper limit value of the line screen processing and the value of the density data at the inflection point of the γ correction characteristic. In order to maintain the gradation characteristics, the former value is desirably set to be equal to or less than the latter value. The density data at the inflection point also has an optimal value, and 80 to 95% of the maximum density data is suitable. The upper limit is desirably 95% or less of the maximum density data in order to avoid variations in IR image reading characteristics, and the lower limit is desirably 80% or more of the maximum density data in order to secure gradation characteristics in a high density portion.

The upper limit of the line screen processing may be changed below the density data at the inflection point of the gamma correction characteristic.

In the electrophotographic process, the LD beam diameter,
There are many factors that vary, such as the photosensitivity characteristics of the photoreceptor, the charge amount of the toner, and the toner concentration. When the LD beam diameter is larger than the set value, both the exposure amount of the left pixel L and the amount of leaked light to the right pixel R increase, and the toner adhesion amount increases in both the left and right pixels. As for the photosensitivity characteristics of the photoreceptor, when the sensitivity is high, the drop of the potential becomes large with respect to the same exposure amount, so that the amount of toner attached increases.

When the charge amount of the toner decreases, the developing efficiency increases, and the toner adhesion amount increases for the same electrostatic latent image.

When the toner density becomes high, the developing efficiency becomes high, and the toner adhesion amount also increases.

When the above-mentioned electrophotographic process fluctuates particularly in the direction in which the amount of adhered toner increases, the density saturation of the high-density portion becomes faster as a gradation characteristic.

As described above, this is the same as the phenomenon that occurs when the upper limit is set high in the line screen processing. This can be solved by lowering the setting of the upper limit.

FIG. 27 shows the difference in the state of toner adhesion depending on the state of the electrophotographic process.

With reference to the figures, (a) shows the toner adhesion state when the electrophotographic process is in the standard state, and (b) shows the toner adhesion state when the electrophotographic process is crushed due to variations in the electrophotographic process. Is shown. In (b), the amount of toner attached to the left pixel L is large, and the vertical line is difficult to separate.

FIG. 28 shows a part of the sensitometry.
It is a graph. Referring to the figure, the upper part is the light intensity-density characteristic,
The lower part is a gamma correction characteristic. Upper limit of line screen processing
To L₁To L_Two, The exposure amount of the left pixel L becomes E₁
To E _TwoAnd the toner adhesion amount is M₁To M_TwoReduced to
Will do.

FIG. 29 shows the density data and the toner adhesion state before and after the upper limit value is changed.

Referring to the figure, when the upper limit value of the line screen processing is changed from L ₁ to L ₂ , the density data of the left pixel L decreases, but the density data of the right pixel R becomes (L ₁ −L _2). )
Only increase. Although the sum of the density data of the right pixel R and the left pixel L are the same, the toner adhesion amount of the left pixel L as shown in FIG. 28 vertical lines separated as a whole to reduce the M ₁ to M ₂ It will be easier. This reduces crushing,
The gradation characteristics of the high density portion are improved.

That is, by making the upper limit value changeable, the above-described variation in the electrophotographic process can be absorbed.

The upper limit is changed by changing upper limit data set by upper limit setting means 280 shown in FIG. Referring to FIG. 3, the upper limit value is input from an operation panel or the like, transmitted to interface control processor 215 via control line 214, and further transmitted to line screen processor 204 via serial interface 216. The line screen processor 204 in FIG. 4 transmits the set value of the upper limit to the upper limit setting unit 280. The upper limit value setting means 280 changes the upper limit data to the transmitted set value, and thereafter the line screen processor 204 performs line screen processing with reference to the changed upper limit data. More simply, the upper limit value setting means may be provided with a dip switch or the like to perform direct switching.

A means for detecting the fluctuation of the electrophotographic process may be provided, and the upper limit value data may be automatically changed based on the detection result. The LD beam diameter can be detected by a CCD, and the photosensitivity characteristics of the photoreceptor can be detected by measuring the surface potential before and after exposure with a potential sensor at a known exposure amount.
Further, fluctuations in the toner charge amount and the toner concentration can be detected by measuring the development efficiency using a potential sensor and a toner adhesion amount sensor. The toner concentration can be directly detected using a magnetic sensor. By automatically changing the upper limit value based on the detection results of these known detection means, it is possible to prevent a change in image quality due to the use environment and an image deterioration due to long-term use.

Although the line screen processing has been described as a method of performing processing in units of two pixels, an upper limit value is set for the amount of toner adhered to a pixel on which density data is accumulated, and the upper limit value is set below the inflection point of the γ correction characteristic. By setting, the same effect can be obtained for a line screen processing method in which processing is performed on three or more adjacent pixels which are well known. That is, for example, in the line screen processing in which the total value of the density data of each of the three adjacent pixels is held and the density data of one of the pixels is increased, the upper limit value is set to the toner adhesion amount of the increased pixel. It will be good if it is provided.

[0121]

According to the first aspect of the present invention, as described above,
In the slope of the gradation correction data low density region and the high concentration region, as well as changes in boundary inflection points so than lower for low concentration region, in addition the data processing means
Since the upper limit value of the density data of the pixel after the setting is set to be equal to or less than the input density data at the inflection point, the occurrence of a false contour due to the density step is prevented, and the image quality is improved.

[0122] As described second aspect of the invention above, the value set as the upper limit, since the change based on the variation of the detected electrophotographic process, the gradation characteristics by a change in the use environment It becomes easy to flexibly cope with the change and maintain a constant image quality.

[Brief description of the drawings]

FIG. 1 is a sectional structural view of a digital copying machine main body according to a first embodiment of the present invention.

FIG. 2 is a perspective view showing a configuration of an optical system in the optical unit 2 shown in FIG.

FIG. 3 is a block diagram showing a configuration of an entire system of the digital copying machine shown in FIG. 1;

FIG. 4 is a system block diagram showing a configuration around a line screen processor of FIG. 3;

FIG. 5 is a part of a flowchart showing control contents of the line screen processor of FIG. 4;

FIG. 6 is another part of the flowchart showing the control contents of the line screen processing processor of FIG. 4;

FIG. 7 is a diagram showing a change in a main scanning effective area signal and a sub-scanning effective area signal with respect to a document according to an embodiment of the present invention.

FIG. 8 is a diagram showing a relationship among a main scanning effective area signal, pixel data, and a window number according to one embodiment of the present invention.

FIG. 9 is a schematic diagram showing a data processing procedure by transferring density data in adjacent pixels according to an embodiment of the present invention.

FIG. 10 is a diagram showing a state in which density data transfer processing has been performed on each pixel according to an embodiment of the present invention, in relation to a window number and a line number.

FIG. 11 is a system configuration diagram around a printer main body control processor 200 of FIG. 3;

FIG. 12 is a graph showing sensitometry including a light quantity-density characteristic, an image reproduction characteristic, a γ correction characteristic, and an image reading characteristic of a conventional digital copying machine.

FIG. 13 is a graph showing, in an enlarged manner, a low density portion of the γ correction characteristic in FIG.

FIG. 14 is an explanatory diagram of a toner adhering amount when uniform density data is 上限 of an upper limit value, for explaining a problem of a conventional digital copying machine.

FIG. 15 is an explanatory diagram of a toner adhesion amount when uniform density data exceeds 1/2 of an upper limit value, for explaining a problem of a conventional digital copying machine.

FIG. 16 is a diagram showing a relationship between image reproduction characteristics and density data when a gradation step occurs, with reference to FIGS. 14 and 15;

FIG. 17 is a diagram showing a γ correction characteristic in one embodiment of the present invention.

18 is a graph showing, in an enlarged manner, a low density region of the γ correction characteristic in FIG.

FIG. 19 is an explanatory diagram of setting an inflection point of the γ correction characteristic in one embodiment of the present invention.

FIG. 20 is an explanatory diagram showing that a gradation step does not occur in one embodiment of the present invention.

FIG. 21 is a diagram showing a relationship between image reproduction characteristics and density data in one embodiment of the present invention.

FIG. 22 is a graph showing sensitometry including a light quantity-density characteristic, an image reproduction characteristic, a γ correction characteristic, and an image reading characteristic of the digital copying machine in one embodiment of the present invention.

FIG. 23 is a diagram showing a relationship between an upper limit value and an inflection point when a conventional digital copier does not set an upper limit value in line screen processing and also uses a conventional gamma correction characteristic. It is.

FIG. 24 is an explanatory diagram when the γ correction characteristic is changed to that of the embodiment of the present invention in the example of FIG. 23;

FIG. 25 is an explanatory diagram in the case where an upper limit value of line screen processing is set in the example of FIG. 24;

FIG. 26 is an explanatory diagram in the case where the upper limit value of the line screen processing is set high in the example of FIG. 25;

FIG. 27 is an explanatory diagram showing the influence of variations in the toner adhesion state in a general electrophotographic process.

FIG. 28 is a graph showing sensitometry for explaining the effect of changing the upper limit in conventional line screen processing.

FIG. 29 is an explanatory diagram showing an improved toner adhesion state by changing the upper limit in conventional line screen processing.

[Explanation of symbols]

 204 line screen processing processor 223 Data ROM 250 First memory 251 Second memory 252 Timing control unit 253 γ correction unit 264 Semiconductor laser Note that the same reference numerals in the drawings denote the same or corresponding parts.

──────────────────────────────────────────────────の Continuing from the front page (72) Inventor Koichi Eto 2-3-13 Azuchicho, Chuo-ku, Osaka-shi Osaka International Building Minolta Camera Co., Ltd. (72) Inventor Toshikazu Kawaguchi 2-chome Azuchicho, Chuo-ku, Osaka-shi No. 13 Osaka International Building Minolta Camera Co., Ltd. Examiner Minoru Matsunaga (56) References JP-A-5-336360 (JP, A) JP-A-63-124674 (JP, A) (58) Fields investigated (Int .Cl. ⁷ , DB name) H04N 1/40-1/409

Claims (2)

(57) [Claims] An image forming apparatus for forming an image based on density data of each pixel corresponding to a document image, comprising: density data receiving means for receiving density data of each image obtained by reading a document image; By switching the window of the received density data by two pixels at a time, the difference between the density data of one pixel and the density data of the other pixel in the adjacent images is found.
If below value, it said with reducing at least a portion of the density data of one pixel, to apply reduced density data into density data of the <br/> other pixel, and a data processing means for processing the data Upper limit value setting means for setting an upper limit value for the density data of the pixel after being added by the processing means; and processing the processed density data based on the gradation correction data.
A density correction means for converting the data into light emission amount data and outputting the data; wherein the gradation correction data includes, in a low density region including the density 0, a continuous data group starting from the density 0 and having a slope of less than 1; The high-density region including the density includes at least one continuous data group having a slope of 1 or more ending at the maximum density, and at least one data group between the low-density region and the high-density region.
Continuous data having two inflection points, wherein the upper limit value of the density data set by the upper limit value setting means is equal to or less than the input density data at the inflection point,
Image forming device. 2. An image forming apparatus for forming an image based on density data of each pixel corresponding to a document image, comprising: density data receiving means for receiving density data of each image obtained by reading a document image; By switching the window of the received density data by two pixels at a time, the difference between the density data of one pixel and the density data of the other pixel in the adjacent images is found.
If below value, it said with reducing at least a portion of the density data of one pixel, to apply reduced density data into density data of the <br/> other pixel, and a data processing means for processing an electrophotographic a detecting means for detecting a variation of the process, the upper limit setting means for setting an upper limit to the density data of the pixel after being added by prior Symbol data processing means, based on the variation of the detected electrophotographic process, Previous
An image forming apparatus comprising: changing means for changing the set upper limit; and data output means for outputting the processed density data.