JPH05260288A - Image forming device - Google Patents

Abstract

PURPOSE:To improve the resolving power of an image and to record the image with high quality without affecting an adjacent image element. CONSTITUTION:The image element under consideration of read-out image density data is divided into the small image elements, the density of the respective small image elements is executed an RE processing for distributing the density of the image element under consideration in accordance with the distribution of adjacent image element density data adding the image element under consideration, reference wave where a vertex position is displaced in accordance with an obtained density centroid is generated, an image discriminating circuit 231 discriminates the density of the image element under consideration, a character area and a halftone area and the period of reference wave is decided so that modulating circuits 260A, 260B and C modulates the original image density signal of respective kinds of color through the use of the reference wave signal and the image is formed by an obtained recording position modulating signal. At this time, reference wave with the same period as an image period is used when density data of the image element under consideration is between first and second threshold values and is the character area. When it is the halftone area, reference wave with the period twice as large as the image period is used so as to modulate the recording position. When density data is out of the area, the recording position is not modulated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is intended to perform high image quality recording by reflecting the distribution of adjacent pixels on the density distribution of recording pixels of interest. After dividing the image data for one pixel into m × n (horizontal × vertical) small pixels in consideration of the data of adjacent pixels,
The center of gravity is obtained for each row, the phase of the reference wave is displaced according to this center of gravity, and the dot recording consisting of n small scanning lines is performed by the modulation signal obtained by modulating the density data of the pixel by this reference wave signal And a color image forming apparatus for performing halftone reproduction. The recording device is intended for use as a printer device or a display device.

[0002]

2. Description of the Related Art In the field of electrophotographic image forming apparatuses, a document image is read by a scanner as an image signal, and the image signal is subjected to gradation correction, A / D conversion, and shading correction to refer to image density data. A digital image obtained by halftone reproduction by modulating with a wave signal is obtained.

An image signal obtained by reading a document image with a scanner has an edge portion of the image read as a halftone density due to an aperture of a solid-state image pickup device incorporated in the scanner. When a latent image is formed on the photoconductor using the image density data obtained from this image signal, the recording pixels corresponding to the edge portion of the latent image are averagely recorded in the recording pixels when the density is intermediate. Therefore, the sharpness of the image is reduced and the image is recorded. Conventionally, it has been known that the image signal is subjected to MTF correction by sharpening with a differential filter, a Laplacian filter or the like. However, this means that only the edges of the image are emphasized, and the uniformity of the halftone image is relatively reduced.

On the other hand, even if an interpolated character or figure is created from CG or font data, there is a similar problem. That is, when the edge portion is smoothly interpolated by the intermediate density with the interpolation data, the recording pixel corresponding to the edge portion is recorded as the average density in the pixel, so that the resolution of the recorded image is reduced.

[0005]

For the above reasons, it has been necessary to carry out an intermediate density process which effectively works at the image edge portion. Therefore, there is a method of using a triangular wave whose phase is shifted as a reference wave, but this method has a problem that it affects adjacent pixels.

Further, in the color image forming apparatus, when the intermediate density processing is performed for each color, there is a problem that the color tone is changed or characters are unclear.

In view of the above problems, it is an object of the present invention to provide an image forming apparatus which improves the resolution of an image made from a scanner, CG, font data, etc., and performs high quality image recording.

[0008]

In the image forming apparatus that modulates the density of a pixel of interest with a reference wave and converts it into a recording pulse width to perform image recording, the reference wave is used as the reference wave.
This is achieved by an image forming apparatus characterized by using triangular waves having different vertex positions corresponding to the density distribution in the target pixel.

In a preferred embodiment, a specific reference wave is selected from the plurality of reference waves, or a specific pulse width is selected from pulse widths respectively formed from the plurality of reference waves. And an image forming apparatus.

[0010]

EXAMPLE A configuration of an image forming apparatus 400 according to an example of the present invention will be described. FIG. 6 is a perspective view showing a schematic configuration of the image forming apparatus of this embodiment.

The image forming apparatus 400 differentially amplifies an analog image density signal and a reference wave signal obtained by D / A converting digital image density data from a computer or a scanner after uniformly charging the photoconductor. The dot-shaped electrostatic latent image is formed by the spot light whose pulse width is modulated based on the obtained modulation signal, and the dot-shaped toner image is formed by reversal development with this toner, and the charging, exposure and development are performed. Repeat the process to form a color toner image on the photoconductor,
This color toner image is transferred onto recording paper, separated from the photoconductor, and fixed to obtain a color image.

The image forming apparatus 400 includes a drum-shaped photoconductor (hereinafter, simply referred to as a photoconductor) 401 that rotates in the direction of the arrow.
A scorotron charger 402 that applies a uniform charge onto the photoconductor 401, a scanning optical system 430, yellow, magenta,
It includes developing devices 441 to 444 loaded with cyan and black toners, a transfer device 462 composed of a scorotron charger, a separator 463, a fixing roller 464, a cleaning device 470, and a static eliminator 474.

The photosensitive member 401 used in this embodiment is a photosensitive member having a high γ characteristic, and a concrete configuration example thereof is shown in FIG.

As shown in FIG. 17, the photosensitive member 401 comprises a conductive support 401A, an intermediate layer 401B and a photosensitive layer 401C. The thickness of the photosensitive layer 401C is about 5 to 100 μm, preferably 1
It is 0 to 50 μm. As the photoconductor 401, a drum-shaped conductive support 401A made of aluminum having a diameter of 150 mm is used.
0.1μm thick with ethylene-vinyl acetate copolymer on top
Intermediate layer 401B is formed, and a film thickness of 35μ is formed on this intermediate layer 401B.
The photosensitive layer 401C of m is provided.

As the conductive support 401A, a drum of aluminum, steel, copper or the like having a diameter of about 150 mm is used. In addition to this, paper, a belt-shaped one in which a metal layer is laminated or vapor-deposited on a plastic film, or an electric electrode is used. It may be a metal belt such as a nickel belt made by the Chu method. Further, the intermediate layer 401B withstands a high charge of ± 500 to ± 2000 V as a photoconductor, for example, in the case of positive charge, it blocks injection of electrons from the conductive support 401C, and has excellent light attenuation characteristics due to avalanche phenomenon. It is desirable to have hole mobility so that the intermediate layer 401B can be obtained, for example, in Japanese Patent Application No. 61-
188975, the positive charge type charge transport material described in
It is preferable to attach 10% by weight or less.

As the intermediate layer 401B, for example, the following resins which are usually used in a photosensitive layer for electrophotography can be used.

(1) Vinyl-based polymers such as polyvinyl alcohol (poval), polyvinyl methyl ether, polyvinyl ethyl ether, etc. (2) Polyvinyl amine, poly-N-vinyl imidazole, polyvinyl pyridine (quaternary salt), polyvinyl pyrrolidone, vinyl Nitrogen-containing vinyl polymers such as pyrrolidone-vinyl acetate copolymer,
(3) Polyether-based polymers such as polyethylene oxide, polyethylene glycol and polypropylene glycol, (4) Polyacrylic acid and its salts, acrylic acid-based polymers such as polyacrylamide and poly-β-hydroxyethyl acrylate, (5) Polymeta Acrylic acid and its salts, methacrylic acid-based polymers such as polymethacrylamide and polyhydroxypropylmethacrylate, (6) Ether fibrin-based polymers such as methylcellulose, ethylcellulose, carboxymethylcellulose, hydroxyethylcellulose and hydroxypropylmethylcellulose, (7) Polyethyleneimine-based polymers such as polyethyleneimine, (8) polyalanine, polyserine, poly-L-glutamic acid, poly- (hydroxyethyl) -L-glutamine, poly-δ-carboxy Cimethyl-L-
Polyamino acids such as cysteine, polyproline, lysine-tyrosine copolymer, glutamic acid-lysine-alanine copolymer, silk fibroin, casein, etc. (9) Starch acetate, hydroxyethyl ethyl starch, starch acetate, hydroxyethyl starch, amine starch, phosphate Starch and other starches and their derivatives, (10) Polyamides such as soluble nylon and methoxymethyl nylon (8 type nylon), which are soluble in a mixed solvent of water and alcohol.

The photosensitive layer 401C basically comprises a phthalocyanine fine particle having a diameter of 0.1 to 1 .mu.m, which is made of a photoconductive pigment, an antioxidant and a binder resin without using a charge transport material together, and a solvent for the binder resin. It is formed by mixing and dispersing to prepare a coating liquid, coating the coating liquid on the intermediate layer, drying and optionally heat treatment.

When the photoconductive material and the charge transport substance are used in combination, the photoconductive pigment and 1 of the photoconductive pigment are used.
/ 5 or less, preferably 1/1000 to 1/10 (weight ratio) of a small amount of a charge transport substance, and dispersed in a photoconductive material, an antioxidant and a binder resin to form a photosensitive layer.
By using such a high-γ photoconductor, a sharp latent image can be formed despite the spread of the beam diameter, and recording with high resolution can be effectively performed.

In this embodiment, the color toner image is transferred to the photosensitive member 401.
Since they are superposed on each other, a photosensitive member having infrared spectral sensitivity and an infrared semiconductor laser are used so that the beam from the scanning optical system is not blocked by the color toner image.

Next, the light attenuation characteristics of the high γ photoconductor used in this embodiment will be described.

FIG. 16 is a graph showing the characteristics of the high γ photoconductor. In the figure, V ₁ is the charging potential (V), V ₀ is the initial potential before exposure (V), and L ₁ is the irradiation amount of the laser beam (μJ / cm) required for the initial potential V ₀ to be attenuated to 4/5. ² ) and L ₂ represent the irradiation light amount (μJ / cm ² ) of the laser beam required for the initial potential V ₀ to be attenuated to 1/5.

The preferred range of L ₂ / L ₁ is _{1.1 <L 2 / L 1 ≦} 1.5.

In this embodiment, V ₁ = 1000 (V), V ₀ = 950
(V), L ₂ / L ₁ = 1.2. The photoconductor potential of the exposed portion is 10V.

The photosensitivity at the position corresponding to the mid-exposure period when the light attenuation curve attenuates the initial potential (V ₀ ) to 1/2 is E.
When the photosensitivity at the position corresponding to the initial exposure stage when the initial potential (V ₀ ) is attenuated to 9/10 is E9 / 10, (E1 / 2) / (E9 / 10) ≧ 2 Preferably, a photoconductive semiconductor that gives a relationship of (E1 / 2) / (E9 / 10) ≧ 5 is selected. Here, the photosensitivity is defined by the absolute value of the potential decrease amount with respect to the minute exposure amount.

In the light attenuation curve of the photoconductor 401, as shown in FIG. 16, the absolute value of the differential coefficient of the potential characteristic, which is the photosensitivity, is small when the amount of light is small, and increases sharply as the amount of light increases. Specifically, as shown in Fig. 16, the light attenuation curve shows a nearly flat light attenuation characteristic in the early stage of exposure due to the poor sensitivity characteristic for a short period of time. The sensitivity becomes an ultra-high γ characteristic that drops almost linearly. Specifically, the photoconductor 401 is considered to obtain a high γ characteristic by utilizing the avalanche phenomenon under the high charge of +500 to + 2000V. That is, the carriers generated on the surface of the photoconductive pigment in the early stage of exposure are effectively trapped in the interface layer between the pigment and the coating resin, and the light attenuation is surely suppressed. It is understood that an avalanche phenomenon occurs.

Next, the color image forming apparatus of the present invention will be described. In this color image forming apparatus, one pixel of the image density data of interest is formed by small pixels of m × n (horizontal × vertical). Replacing the distribution of density data of adjacent pixels including the target pixel with the distribution of m × n small pixels in one pixel, and distributing the data of the target pixel multiplied by a constant P according to the distribution. Based on the image density data of the small pixels obtained by the above, the phase of the reference wave of each row of the small pixels is displaced to displace the writing position of the dots of the n rows to perform image formation. Displacement of the dot writing position is referred to as recording position modulation. The process of converting the pixel of interest into image density data of small pixels obtained by dividing the pixel of interest into m × n is referred to as resolution improving process (RE process). High density recording can be performed by this RE processing. In this case, the high γ photoconductor is particularly effective for forming a latent image in response to the reference wave accurately.

In the present invention, this RE processing is performed when the density data of the pixel of interest is equal to or higher than the first threshold value, that is, the recorded optical reflection density is 0.1 or more, that is, the first density
Above the threshold of. That is, in many areas corresponding to the highlight portion, the RE processing is not performed on the background portion of the document, and the m × n small pixels have uniform density. In the case of a CRT, this data can be displayed.

However, in the case of laser recording, which will be described later, it is difficult to perform uniform display. Therefore, a reference wave whose concentration center is in the center is selected. As a result, it is possible to prevent the generation of a noisy image while maintaining the uniformity in the highlight portion.

On the other hand, in the case of the high density portion, if the density gradient is large, dots are formed across adjacent pixels if a reference wave that does not have the density recording position in the center is selected.

In order to prevent density fluctuations due to this and crushing of recorded dots between pixels, the density center is at the center when the density is equal to or higher than a specific second threshold value of 0.5 from the recorded optical reflection density even in the high density portion. Select the reference wave. Even if only one of these threshold conditions for recording position modulation is used, it is effective.

In the case of a CRT, since uniform display is possible, m × n small pixels are processed as uniform density. That is, RE processing is not performed.

That is, in the color image forming apparatus for performing high-density pixel recording, the specific density data of the target pixel is low according to the density distribution data in the target pixel determined corresponding to the density data of the pixel adjacent to the target pixel. A color image forming apparatus is characterized in that recording position modulation is performed from the determined density distribution for a case where the density portion is equal to or higher than a first threshold value, or that the specific density data of the pixel is high density. It is preferable that the image forming apparatus is characterized in that the recording position modulation is performed based on the determined density distribution in the case of the second threshold value or less.

FIG. 7A is a plan view showing the above target pixel as m5 and adjacent pixels including the target pixel m5 as m1 to m9 when the target pixel m5 is divided into 3 × 3.
(B) is an enlarged view showing a case where each small portion when the target pixel m5 is divided into 3 × 3 small pixels is represented by s1 to s9. m1-m9 and s1-s9 shall also represent the density | concentration of the part.

The RE process will be described in detail.
Taking the case of dividing 5 into 3 × 3 small pixels as an example, the density of the small pixel si is determined by the following equation.

Si = (9 × m5 × P × mi / A) + (1−P) × m5 where i = 1, 2, ... 9, and P is a constant that should be called the strength of RE processing. And a numerical value in the range of 0.1 to 0.9 is used. A is the sum of m1 to m9.

In the above equation, (9 × m5 × P × mi /
The term A) is obtained by dividing the product of the density of the target pixel m5 and P according to the ratio of the density of the adjacent pixels.
The term P) × m5 is obtained by evenly distributing the remaining densities of the target pixel m5 to the respective small pixels, which means that a blur factor is incorporated.

In FIG. 8, the pixel of interest m5 is divided into 3 × 3 pixels, and P
8A shows an example of the density distribution of adjacent pixels including the target pixel m5, and FIG. 8B shows the density distribution in the target pixel m5 calculated as P = 0.5. FIG.

Next, an example of dividing the target pixel m5 into 2 × 2 is shown in FIGS.

FIG. 9A is a diagram showing an example in which the target pixel m5 is divided into 2 × 2, and FIG. 9B shows an example of adjacent pixels related to the small pixels s1 to s4 in the target pixel. It is a figure.

The calculation of the concentrations of s1, s2, s3, and s4 is given by
Is done according to.

[0042]

[Equation 1]

In FIG. 10A, the target pixel m5 is 2 × 2.
FIG. 10B is a diagram showing another example in the case of being divided into, and FIG. 10B is a diagram showing another example of the adjacent pixels related to the small pixels s1 to s4 in the target pixel. Concentration calculation of s1, s2, s3, s4 is Equation 2
Is done according to.

[0044]

[Equation 2]

FIG. 1 is a block diagram showing an embodiment of an image processing circuit used in the color image forming apparatus of the present invention (an example in which a target pixel is divided into 3 × 3), and FIG. 2 is shown in FIG.
FIG. 4 is a circuit diagram showing an example of the triangular wave generation circuit of FIG.
FIG. 5 is a block diagram showing the E processing circuit, and FIG. 5 is a block diagram showing the modulation circuit of FIG.

The image processing circuit 1000 of this embodiment is a circuit which constitutes a drive circuit of a scanning optical system, and comprises an image data processing circuit 100, a modulation signal generating circuit 200 and a raster scanning circuit 300.

The image data processing circuit 100 is a circuit for interpolating and outputting the edge portion of the font data, and comprises an input circuit 110 and a font data generating circuit 12 formed of a computer.
0, font data storage circuit 130, interpolation data generation circuit 14
Consists of 0, the character code signal from the input circuit 110,
The size code signal, the position code signal and the color code signal are sent to the font data generating circuit 120. The font data generation circuit 120 selects an address signal from four types of input signals and sends it to the font data storage circuit 130. The font data storage circuit 130 sends outline font data corresponding to one character corresponding to the address signal to the font data generation circuit 120. The font data generation circuit 120 sends the outline font data to the interpolation data generation circuit 140. Interpolation data generation circuit 1
40 is an image density data composed of a frame memory as 8-bit image data by interpolating jaggedness or skipping of the image density data generated at the edge portion of the outline font data, which has been generated by the conventional binary expansion, by using the intermediate density. It is sent to the memory circuit 210. As for the generated color, the corresponding color is converted into density data of yellow (Y), magenta (M), cyan (C), and black (BK) according to the color code. Masking and UCR may be performed before being sent to the image density data storage circuit 210, and after processing, each density data may be sent again. In this way, the fonts are bitmap-developed in each frame memory in the state where each color has the same shape but different density ratios.

The modulation signal generation circuit 200 includes an image density data storage circuit 210, a read circuit 220, a latch circuit 230, an image discrimination circuit 231, an MTF correction circuit 232, a γ correction circuit 233, and an RE.
The processing circuit 240 includes triangular wave generation circuits 250A to 250C that are means for generating a reference wave, modulation circuits 260A to 260C, a reference clock generation circuit 280, a frequency doubled clock generation circuit 281, a selection circuit 282, and the like.

The image density data storage circuit 210 is a normal page memory (hereinafter simply referred to as page memory 210), a RAM (random access memory) for storing in page units, and at least one page (for one screen). It has a capacity for storing corresponding multi-valued image density data. Further, if it is an apparatus adopted for a color printer, it will have a page memory for storing image density signals corresponding to a plurality of color components, for example, yellow, magenta, cyan and black color components.

The reading circuit 220 uses the index signal that determines the start timing of raster scanning as a trigger to generate continuous image density data in units of one scanning line in synchronization with the reference clock DCK ₀ , and an image density data storage circuit (page memory). It is read from 210 and sent to the RE processing circuit 240, the image discrimination circuit 231, and the MTF correction circuit 232.

The latch circuit 230 is an RE processing circuit 2 which will be described later.
40 is a circuit for latching the image density data only while the processing of the 40 and the triangular wave generating circuits 250A to 250C is being executed.

The reference clock generation circuit 280 is a pulse generation circuit, and generates the reference clock DCK ₀ which is a pulse signal having the same repetition period as the pixel clock and reads out the read circuit 2
20, output to the doubled clock generation circuit 281, the selection circuit 282.

The frequency-divided clock generation circuit 281 generates a frequency-divided clock DCK ₁ having a period twice the period of the pixel clock based on the reference clock DCK ₀ and selects the selection circuit 282.
Output to.

The select circuit 282 selects one of the input reference clock DCK ₀ and doubled clock DCK ₁ by a selection signal from an image discrimination circuit 231 which will be described later, and the triangular wave generation circuits 250A to 250C and the modulation circuit. Circuit 260A-260
Output to C etc. The triangular wave generation circuits 250A to 250C have the same circuit configuration as shown in FIG. 2, and the center of gravity of the sawtooth wave from the sawtooth wave generation circuit of the circuit 251 and the center of gravity position data of each small scanning line from the RE processing circuit 240 are used. A reference wave composed of triangular waves having different vertex positions corresponding to the position data is generated and sent to the modulation circuits 260A to 260C.

FIG. 3 shows the triangular wave generating circuit 250 shown in FIG.
It is a time chart which shows the signal waveform in each part of A-250C. Triangular wave generation circuit according to this time chart
The operation of 250 will be described.

In FIG. 3, (a) shows the clock signal waveform at the point a, and (b) shows the detection signal obtained by detecting the rising portion of the clock signal by the rising detection circuit at the point b of the circuit 251. Signal by Tr2
Is turned on for a short time, C1 is once discharged, and then charging is started via Tr1, and a sawtooth wave shown in (c) is generated at point c. In the figure, VR1 is a variable resistor for adjusting the charging current, and VB1 is a DC variable power source for adjusting the DC level of the generated sawtooth wave. (D) is the RE processing circuit 240
Shows the waveform obtained by converting the 8-bit center-of-gravity position data from (4) to an analog signal by the D / A conversion circuit of the circuit 251.
(E) shows the analog signal and the sawtooth wave input from the sawtooth wave generation circuit of the circuit 251 to the comparator 252.
Is an output signal that has been compared with. This signal is input to the bases of Tr4 and Tr5 via a capacitor and becomes a signal for turning on Tr4 and Tr5. The analog signal of the center-of-gravity position data is input to the center point of Tr5, R5, R6, Tr6 connected in series between the power source Vcc and -Vcc with the direct current level adjusted by VB3. As a result, the signal waveforms at points f and g are as shown in (f) and (g). The level of a ₁ and a _{2 in} (f) is the capacitor C
The magnitude of the charging current of ₂ is determined, and the levels of b ₁ and b ₂ in (g) determine the magnitude of the discharging current of the capacitor C ₂ .
At the point h in FIG. 2, a triangular wave whose vertex position as shown in (h) is determined corresponding to the gravity center position data is obtained.

The triangular wave displacing the apex position corresponding to the above-described density centroid position can also be generated by the circuit shown in FIG. 15 (a). That is, the CPU 255 selects the rising clock for generating the reference wave from the table of the ROM 258 based on the 8-bit center-of-gravity position data input from the RE processing circuit 240. This allows the oscillator 25
The pulse from 6 is counted, and when it reaches a predetermined number (when the reference wave voltage reaches the predetermined voltage Vp), the counter 257 is counted down. Next, the reference wave falling clock determined by the center-of-gravity position data is selected from the table of the ROM 258 and counted down by this clock. As a result, the waveform shown in FIG.
The / A conversion circuit 259 converts the analog signal to obtain a triangular wave having the same repetition frequency as the pixel clock and displacing the vertex position corresponding to the center-of-gravity position data.

The modulation circuits 260A to 260C have the same circuit configuration as shown in FIG. 5, and have a D / A conversion circuit 261, a comparator 262, and a triangular wave input section T corresponding to the center-of-gravity position data. The image density data input through the latch circuit 230 is D / A converted by the D / A conversion circuit 261 in synchronization with the clock DCK ₀ or DCK ₁ , and the triangular wave generation circuit 2
It is a circuit for obtaining a pulse width modulation signal by comparing the triangular wave input from 50A to 250C as a reference wave.

The image discrimination circuit 231 discriminates whether the inputted image density data is a character area or a halftone area, and the select circuit 282 supplies the reference clock DCK ₀ for the character area and the halftone area for the character area. A selection signal for selecting the doubled clock DCK ₁ is output. In addition, a signal for activating and deactivating the above circuits is sent to the MTF correction circuit 232 and the γ correction circuit 233 as described later.

As shown in FIG. 4, the RE processing circuit 240 comprises a 1-line delay circuit 242, a 1-clock delay circuit 243, and an arithmetic processing circuit 241.
For the image density data for the first one scanning line of the image density data sent for each scanning line, a delay of two line scanning time is set for the image density data for the first scanning line, and for the image density data for one scanning line in the middle, one is set. The line scanning time is delayed, and the image density data for the last one scanning line is not delayed. Further, each image density data is delayed by two reference clocks or one reference clock by the one-clock delay circuit 243, and all the image density data of the pixels including the target pixel and adjacent to the target pixel are calculated at the same time. It is sent to the processing circuit 241. Note that the reference clock DCK ₀ input is omitted in FIG.

In the arithmetic processing circuit 241, the RE processing is performed to obtain the density data of the small pixels. First, the case where the density distribution within one pixel is obtained will be described.

The density data of the small pixels obtained is s in FIG.
Small scan lines including 1, s2, s3 ...
5, s6, small scan lines and s7, s8, s
9 is divided into small scan lines, and 3 small scan lines of these small pixels correspond to 1 scan line of the original pixel.

The arithmetic processing circuit 241 further performs an arithmetic operation for obtaining the average density of each small scanning line and the barycentric position of the density data in each small scanning line, and outputs an analog signal of the average density data from the output terminals O4, O5, O6. Laser driver 301A-301
The center of gravity position data is output from the output terminals OA to OC to the triangular wave generating circuits 250A to 250C. That is, when the density centroid of s1, s2, s3 (first small scanning line) of the pixel m5 is at the left end of s1, the maximum value digital signal, when the density centroid is at the center of s2, the intermediate value digital signal, From the output terminal OA, the triangular wave generation circuit 250 outputs centroid position data corresponding to the above-described concentration centroid position, which produces a minimum digital signal when the concentration centroid is at the right end of s3.
Output to A. Similarly, from the output terminal OB, s of the pixel m5
The gravity center position data of the second small scanning line (in this case, the small scanning line at the center) determined by the density gravity center positions of 4, s5, and s6 is supplied to the triangular wave generation circuit 250B, and the pixel m is output from the output terminal OC.
The center-of-gravity position data of the third small scanning line determined from the density center-of-gravity positions of s7, s8, and s9 of 5 are triangular wave generation circuit 250C.
Output to. FIG. 11 is a diagram showing an example of the relationship between the triangular wave having different vertex positions and the pixel of interest.

The arithmetic processing circuit 241 also determines the laser driver 301 according to the average density in the pixel m5 of each small scanning line.
The light emission output of A to 301C is controlled. For example, S1, S2
The semiconductor laser 301A is controlled to emit laser light in proportion to the average density of S3. FIG. 20 is a graph showing an example of the relationship between the drive current of the semiconductor laser and the laser emission output.

Next, a case will be described in which the density distribution within two pixels, which is necessary when the image is a halftone region, is obtained.
The obtained density data of the small pixels are s4 and s4, which are small scanning lines including s1, s2, s3 ... Of m5 and m6 in FIG.
small scan lines including s5, s6 ... and s7, s8,
It is divided into small scanning lines including s9 ..., and the three small scanning lines of the small pixels correspond to one scanning line in units of two pixels of the original pixel.

When the arithmetic processing circuit 241 receives a signal indicating that it is in the halftone area from the image discrimination circuit 231, the density centroid position including two pixels of each small scanning line and the original one pixel of each small scanning line. Then, an analog signal of the average density data is output from the output terminals O4, O5, O6 to the laser drivers 301A to 301C, and the center-of-gravity position data is output from the output terminals OA to OC. Output to 250C. That is, s1 of m6 adjacent to the pixel m5
When the density center of gravity of s2, s3 (first small scan line) is at the left end of s1 of m5, the maximum value digital signal, and when the center of gravity is at the boundary point between s3 of m5 and s1 of m6, intermediate value digital When the signal and the center of gravity are at the right end of s3 of m6, the center of gravity position data that provides the minimum digital signal is output from the output terminal OA to the triangular wave generation circuit 250A. Similarly, from the output terminal OB, s4, s5 of pixels m5 and m6
The center-of-gravity position data of the second small scanning line determined by the density center of gravity of s6 is input to the triangular wave generation circuit 250B, and from the output terminal OC of the third small scanning line determined by the density center of gravity of s7, s8, and s9 of the pixels m5 and m6. The center-of-gravity position data is output to the triangular wave generation circuit 250C. FIG. 13 is a diagram showing an example of the relationship between the triangular wave having different vertex positions and the pixel of interest.

As described above, the arithmetic processing circuit 241 outputs an analog signal corresponding to the average density in the pixels m5 and m6 of each small scanning line to each of the laser drivers 301A to 301C and controls the light emission output thereof. For example, the semiconductor laser 301A is controlled to emit laser light in proportion to the average density of one pixel of S1, S2, and S3. FIG. 20 is a graph showing an example of the relationship between the drive current of the semiconductor laser and the laser emission output.

On the other hand, the image discrimination circuit 231 compares the image data of the pixel of interest with a predetermined first threshold value having a low value and a predetermined second threshold value having a high value, and is in an area outside the first and second threshold values. If it is determined that the center-of-gravity position data for all color components is the center of the vertex position, the RE processing circuit 240 determines the triangular wave generation circuit 25.
The signal to be sent to 0A to 250C is sent to the MTF correction circuit 23.
2 does not work. As a result, the image density data read by the read circuit 220 is not corrected by the MTF correction circuit 232, is corrected by the γ correction circuit 233, and is then sent to the modulation circuits 260A to 260C via the latch circuit 230.

As a result, since MTF correction and position modulation are not performed in the highlight and high density areas, a highly uniform image without noise can be formed.

Further, the image discrimination circuit 231 further discriminates whether the image is a character region or a halftone region under the above conditions. This determination is made by the density change in the 16 × 16 pixels including the target pixel. When the density change is large, the pixel of interest is determined to be a character area, and when the density change is small, it is determined to be a halftone area. Further, when the determination area result is different only in the minute area, for example, in the character area, when the halftone area exists independently, it is determined as a character. The same determination is made in the case of the halftone area. If it is determined that the area is a character area of a character or a line drawing, the reference circuit D is set to the select circuit 282 so that the reference wave is equal to the image clock cycle.
The selection signal for outputting CK ₀ is output, and the MTF correction circuit 23
2. The γ correction circuit 233 is inoperative, and the image density data is not processed and the modulation circuits 260A to 260C are processed through the latch circuit 230.
To send. As a result, clear characters and edge portions with no change in color tone are reproduced. When it is determined that the reference tone is in the halftone region, the selection signal for outputting the frequency-divided clock DCK ₁ is output to the selection circuit 282 so that the reference wave has twice the image clock cycle.

By the above processing, an image having a high gradation can be formed in the halftone region, and an effect of giving sharpness and tightness to the character image is produced.

The image density data used for determining the vertex position of the reference wave is the density of a specific color, for example, R + 2G + B (where R is red density data, G is green density data, and B is blue density data). It is converted to data.
For convenience, the density data of (R + 2G + B) is represented by N.

By using the vertex position of the reference wave in common for each recording color, it is possible to guarantee the gradation of the image and prevent the change of the tint. It should be noted that it is preferable to use G components that are visually coincident or achromatic data having G components to determine the vertex positions of the reference waves. Further, the data used for the image discrimination circuit 231 is also common to each color for the same reason.

In the modulation circuits 260A to 260C, a signal of the image density data input through the latch circuit 230 is modulated by the triangular wave whose vertex position is changed corresponding to the density center of gravity position to generate a pulse width modulated signal. , The raster scanning circuit 300 with three small scanning lines (one line of the original image density data) of these modulation signals continuous in parallel as one unit.
To send to.

Next, the operation of the modulation signal generation circuit 200 will be described.

FIGS. 12 (a) to 12 (d) are time charts showing respective signals of the modulation signal generating circuit when the recording position is modulated in the area determined as the character area. The period of the reference wave is the same as that of the original pixel.

In FIG. 12, (a) shows the page memory 210.
Reference clock DC from the index signal as a trigger
The image density data read out based on K ₀ is partially converted into an analog value by the D / A conversion circuit 261. The higher level shows a lighter density, and the lower level shows a darker density.

(B) shows a triangular wave which is a reference wave sequentially output from the triangular wave generating times 250A to 250C.

(C) shows the triangular wave (solid line) and the image density signal (dotted line) converted into the analog value, and shows the modulation operation in the modulation circuits 260A to 260C.

(D) shows a pulse width modulation signal generated by being compared by the comparator 262, and shows that the recording position modulation for displacing the recording position according to the density center of gravity position is performed.

From the modulation signal generation result, the low density portion,
In the case of the pixel in the high density portion, the recording position modulation is not performed, and in the area where the density change is large, the position of the small dot of the nth row in the pixel of interest is larger than that of the original character or line drawing from the density data of the original adjacent pixel. As a result of the recording position modulation that moves to a position along the line direction, characters and line drawings are clearly reproduced.

Further, FIGS. 14A to 14D are time charts showing signals of respective parts of the modulation signal generating circuit when the recording position is modulated in the area judged as the halftone area. The cycle of the reference wave is twice the cycle of the original pixel as shown in FIG.

In FIG. 14, (a) shows the page memory 210.
Reference clock DC from the index signal as a trigger
The image density data read out based on K ₀ is partially converted into an analog value by the D / A conversion circuit 261. The higher level shows a lighter density, and the lower level shows a darker density. The image data is image density obtained by averaging two pixels in the main scanning direction.

(B) shows a triangular wave which is a reference wave sequentially output from the triangular wave generating circuits 250A to 250C.

(C) shows the triangular wave (solid line) and the image density signal (dashed line) converted into the analog value, and shows the modulation operation in the modulation circuits 260A to 260C.

(D) shows a pulse width modulation signal generated by being compared by the comparator 262.

From the modulation signal generation result, the low density portion,
In the case of the pixels in the high density portion, the recording position modulation is not performed, while in the area where the density change is large, the position of the n-th small dot in the pixel of interest is in the direction where the density change is larger than the density data of the original adjacent pixel. As a result of the recording position modulation that moves to a position along the line, even halftones such as photographs can be reproduced without loss of sharpness.

Further, by sequentially shifting the phase of the reference wave in the sub-scanning direction, dots corresponding to halftone dots with a screen angle can be formed. For example, the screen angle is 45 ° for the yellow component and 2 for the magenta component.
By setting 6.6 °, -26,6 ° for the cyan component and 0 ° for the black component, it is possible to improve the uniformity of color reproduction and prevent the occurrence of moire fringes.

Particularly, by setting the black component to 0 °, there is an advantage that the recording position modulating means can be used as it is without being changed.

The raster scanning circuit 300 includes a delta delay circuit 311, 2
A delta delay circuit 312, laser drivers 301A to 301C, an index detection circuit (not shown), a polygon driver and the like are provided.

The laser drivers 301A to 301C are modulation circuits 26.
A plurality of modulation signals from 0A to 260C (3 in this embodiment)
The semiconductor laser array 431 having (a) individual laser emission units 431A to 431C is oscillated, and a signal corresponding to the light amount of the beam from the semiconductor laser array 431 is fed back and driven so that the light amount becomes constant.

The index detection circuit detects the surface position of the rotary polygon mirror 434 which rotates at a predetermined speed by the index signal from the index sensor 439 of FIG. 6, and is modulated by the raster scanning method according to the cycle in the main scanning direction. Optical scanning is performed by the image density signal. Scanning frequency 220
4.72Hz, effective print width 297mm or more, effective exposure width 306mm or more.

The polygon mirror driver uniformly rotates the DC motor at a predetermined speed to rotate the rotary polygon mirror 434 to 16535.4.
It is rotated at rpm.

As the semiconductor laser array 431, as shown in FIG. 18, one in which three light emitting portions 431A to 431C are arranged in an array at equal intervals is used. Since it is difficult to set the distance d between the light emitting units to 20 μm or less, as shown in FIG. 18, the axis passing through the centers of the light emitting units 431A to 431C is parallel to the rotation axis of the rotary polygon mirror 434, and the main scanning is performed. Install at an angle to the direction. In this way, the semiconductor laser array 431
Laser beam s on the photoconductor 401
As shown in FIG. 19, a, sb, and sc can be intimately scanned vertically. However, for this reason, the positions of the laser spots sa, sb, and sc in the scanning direction deviate from the main scanning direction. In order to correct this shift, a delta delay circuit 311, a modulation circuit 260C and a laser driver are provided between the modulation circuit 260B and the laser driver 301B.
The 2δ delay circuit 312 is inserted between the 301C and the 301C, and each of them is delayed by an appropriate amount to correct the shift, and the laser spots sa, sb, and sc emitted from the semiconductor laser array 431 are adjusted in the main scanning direction. And can be recorded vertically as sa, sb ', and sc'.

When the RE processing is performed by dividing the target pixel into 2 × 2 small pixels, a semiconductor laser array having two light emitting portions is used.

In the previous embodiment of the present invention, the average density in the main scanning direction is the laser emission output as the density information in each sub-scanning direction, and the image data from the reading circuit 220 is used. As shown in FIGS. 23 and 24, the average density of each small scanning line obtained by the RE processing circuit 240 is used as density information and is input to the modulation circuits 260A to 260C by each reference wave and each laser driver 301A to 301C.
Can be used to modulate Next, the image forming process of the image forming apparatus 400 shown in FIG. 6 will be described.

First, the photoconductor 401 is uniformly charged by the scorotron charger 402. The electrostatic latent image corresponding to yellow is formed on the drum-shaped photoconductor 401 by the image density data storage circuit 210.
The laser beam modulated by the yellow data (8-bit digital density data) from the inside is formed by irradiation through the cylindrical lens 433, the rotary polygon mirror 434, the fθ lens 435, the cylindrical lens 436, and the reflection mirror 437. The electrostatic latent image corresponding to the yellow is the first
And a dot-shaped first toner image (yellow toner image) with extremely high sharpness is formed on the photoconductor 401. The first toner image is not transferred to the recording paper, passes under the retracted cleaning device 470, and is again on the photoconductor 401 on the scorotron charger 402.
Is charged by.

Then, the laser beam modulated by the magenta data (8-bit digital density data) is applied to the photoconductor 401 to form an electrostatic latent image. This electrostatic latent image is developed by the second developing device 442 to form a second toner image (magenta toner image). In the same manner as described above, the third developing device 443 sequentially develops,
A third toner image (cyan toner image) is formed, and
A three-color toner image is sequentially formed on 401. Finally, a fourth toner image (black toner image) is formed, and a four-color toner image sequentially formed on the photoconductor 1 is formed.

According to the image forming apparatus 400 of this embodiment, the photosensitive member 401 has excellent high γ characteristics, and this excellent high γ characteristic is used for charging, exposing and developing a toner image many times. The latent image is stably formed even when the toner images are repeatedly formed over each other. That is, even if a laser beam is irradiated from above the toner image based on a digital signal, a dot-shaped electrostatic latent image with high fringes and high sharpness is formed, and as a result, a toner image with high sharpness can be obtained. You can

These four color toner images are transferred by the transfer device 462 onto the recording paper supplied from the paper feeding device.

The recording paper carrying the transferred toner image is separated by a separator.
The sheet is separated from the photoconductor 401 by 463, is conveyed by a guide and a conveyance belt, is carried into a fixing roller 464, is heated and fixed, and is discharged to a sheet discharge tray.

In this embodiment, as a result of an experiment in which the value of the coefficient P for RE processing was variously changed, the value of P was 0.1 to 0.9.
Good images were obtained in the range of. However, when P is small, the sharpness of the character is insufficient, and when P is large, the result that the edge portion of the line drawing or the halftone dot is overemphasized is obtained. Therefore, the preferable value range of P is 0.3 to It was found to be in the range of 0.7. As a result, when the original is a line drawing or halftone dots, the edge part appears clearly, and even small characters can be reproduced in detail. Moreover, the low-density portion and the high-density portion were not adversely affected. This is because the method stops recording position modulation for these pixels and effectively sets P = 0.

In the present method, P can be used with a fixed value, but it is preferable to use P in accordance with the recording cycle, that is, by changing P according to the image (character area or halftone area). Recording period is short, i.e. the value in the case of the character region and P _1, long recording period, that is, in the case of halftone area and P _2, it is preferable that the P _1> P _2. That is, when the image is a character area, the value of P is large and preferably 0.9 to 0.4, and when the image is a halftone area, the value of P is small and is 0.6 to 0.1.

P = 0 corresponds to no recording position modulation.

Further, in the present invention, the rate of RE processing can be changed.

FIG. 21 is a graph showing an example in the case of converting the relationship between the vertex position of the triangular wave corresponding to the recording position in the main scanning direction and the center of gravity, and FIG. 22 is an example in the case of converting the average density in the sub scanning direction. It is a graph.

In the arithmetic processing circuit 241, the result obtained by arithmetic processing from the image density data is used by using the ROM 245 which is built-in or externally attached, and the coefficient according to the preset conversion formula as shown in FIG. The recording position can be changed by multiplying by the data and converting. Similarly, the average density in the sub-scanning direction can be converted as shown in FIG. Note that “0” in FIG. 21 indicates the center position of the dye.

In particular, it is preferable to change the conversion ratios shown in FIGS. 21 and 22 according to the case where the reference wave period is changed. When the reference wave period is large, gradation is emphasized and recording position modulation is performed. It is preferable to reduce the ratio of ∘ and the gradation γ.

In the present embodiment, the pulse width is formed by comparing the selected reference wave and the density signal. However, a plurality of pulse widths are formed in advance by comparing the plurality of reference waves with the density signal. Alternatively, the pulse width corresponding to a specific reference wave may be selected from the recording position information.

In the present invention, the period and the amplitude are the same,
A plurality of reference waves having different vertex positions are used. The advantage is that the pulse width formed in comparison with the density information is the same, and the pulse width formed is not different from other pulse widths. This has an advantage that an image can be formed corresponding to the density information.

The flow of the image data described above is explained as a laser printer which outputs the data once stored in the page memory 210, but the present invention is not limited to this, and the color scanner 151, A / D conversion circuit 152, density conversion circuit 153, masking UCR circuit 15
Instead of the image data processing circuit 150 composed of 4 or the like, a circuit for inputting image density data from a scanner and performing image processing can be applied to another image forming apparatus such as a copying machine.

Further, although the switching of the reference wave period is performed for each pixel according to the image discrimination result, the entire screen can be uniformly switched by an external command such as a character or photo mode.

The present invention is applied to an edge emitting EL head capable of pulse width modulation in the sub-scanning direction.

[0114]

As described above, for a pixel of interest included in a specific density, the pixel of interest is divided into small pixels corresponding to the density data of the pixel of interest, and the density of each small pixel is A means for generating a reference wave by displacing the apex position of the triangular wave in accordance with the density centroid position of the image data subjected to RE processing that distributes the density of the pixel of interest in accordance with the distribution of the density data of adjacent pixels including pixels The recording position modulation signal is generated by modulating the density signal of the pixel of interest with the reference wave generated by this means. As a result, the reference wave does not extend to other pixels, so that it does not affect other pixels and a recorded image with excellent image quality is obtained. Further, the area of the reference wave does not change, and the pulse width corresponding to the image density data is formed regardless of the reference wave. In addition, the recording position modulation is not performed for the low-density portion and the high-density portion, or the image is discriminated by the image discriminating circuit. In the area, the recording position is modulated by the reference wave having a long cycle, so that the color image is recorded, so that the sharpness is improved without causing the change in the color tone of the color image made from the scanner, CG or font data. It was possible to provide an excellent color image forming apparatus capable of doing so.

In the above method, the recording beam of pixels is 3
Although the case of a book is shown, it is also possible to scan one pixel with one or two recording beams, or to perform recording position modulation only in the main scanning direction only in the sub-scanning direction or by the EL head. The effect can be further improved by using a high γ photoconductor.

[Brief description of drawings]

FIG. 1 is a block diagram of an image processing circuit of an embodiment of an image forming apparatus of the present invention.

FIG. 2 is a circuit diagram showing an example of the triangular wave generation circuit of FIG.

FIG. 3 is a time chart showing signal waveforms of respective parts of the triangular wave generation circuit of FIG.

FIG. 4 is a block diagram showing an example of the RE processing circuit of FIG. 1.

5 is a block diagram showing an example of the modulation circuit of FIG. 1. FIG.

FIG. 6 is a perspective view showing a schematic configuration of an image forming apparatus of the present invention.

FIG. 7 is a diagram for explaining RE processing used for determining a reference wave phase.

FIG. 8: A pixel of interest in RE processing is divided into 3 × 3, and P = 0.
FIG. 7 is a diagram showing an example of a case of setting 5.

FIG. 9 is a diagram showing an example of dividing a target pixel of RE processing into 2 × 2.

FIG. 10 is a diagram showing another example of a case where a pixel of interest in RE processing is divided into 2 × 2.

FIG. 11 is a diagram showing a shape of a reference wave in the case of a character area.

FIG. 12 is a time chart showing signals of respective parts in the case of a character area of the modulation signal generation circuit of the embodiment of FIG.

FIG. 13 is a diagram showing the shape of a reference wave in the case of a halftone region.

FIG. 14 is a time chart showing signals of respective parts in the halftone region of the modulation signal generation circuit of the embodiment of FIG.

FIG. 15 is a block diagram and an output diagram showing another example of the triangular wave generation circuit.

FIG. 16 is a graph showing the characteristics of the high γ photoconductor used in this example.

FIG. 17 is a cross-sectional view showing a specific configuration example of a high γ photoconductor used in this example.

FIG. 18 is a diagram showing the semiconductor laser array of the embodiment of FIG.

FIG. 19 is a diagram showing a scanning locus of a laser spot by the semiconductor laser array of FIG.

FIG. 20 is a graph showing the relationship between the drive current of the semiconductor laser and the laser emission output.

FIG. 21 is a graph showing an example of changing the relationship between the center of gravity of the small scanning line in the main scanning direction and the recording position.

FIG. 22 is a graph showing a case where the average density of a small scan line in the sub-scanning direction is converted.

FIG. 23 is a block diagram showing an image processing circuit according to another embodiment of the present invention.

FIG. 24 is a block diagram showing the RE processing circuit of FIG. 23.

[Explanation of symbols]

 100 Image data processing circuit 200, 201 Modulation signal generation circuit 210 Image density data storage circuit (page memory) 220 Read circuit 231 Image discrimination circuit 232 MTF correction circuit 233 γ correction circuit 240 RE processing circuit 250A to 250C Triangular wave generation circuit 260A to 260C Modulation circuit 280 Reference clock generation circuit 281 Double frequency clock generation circuit 282 Select circuit 300 Raster scanning circuit 400 Image forming device

Claims (3)

[Claims] 1. An image forming apparatus that modulates the density of a pixel of interest with a reference wave and converts the pixel density into a recording pulse width to perform image recording, wherein as the reference wave, a peak position corresponding to a density distribution in the pixel of interest is used. An image forming apparatus using different triangular waves. 2. The image forming apparatus according to claim 1, wherein a specific reference wave corresponding to a density distribution is selected from the plurality of reference waves. 3. The image forming apparatus according to claim 1, wherein a specific recording pulse width corresponding to the density distribution is selected from the recording pulse widths modulated by the plurality of reference waves.