JP3066775B2 - Color image forming equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

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

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

[0004] On the other hand, the same problem arises even when an interpolated character or figure is created from CG or font data. That is, when the edge portion is smoothly interpolated with the intermediate density using the interpolation data, the recording pixel corresponding to the edge portion is recorded as an 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 perform an intermediate density process that works effectively at the image edge portion.

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

An object of the present invention is to provide a color image forming apparatus capable of improving the resolution of an image formed from a scanner, CG, font data, and the like and performing high-quality image recording in view of the above problems.

[0008]

An object of the present invention is to divide a target pixel to be recorded based on image data into a plurality of target pixels by assuming that the target pixel has a density distribution.
A color image forming apparatus that performs high-density pixel recording based on density distribution data including divided pixels in the pixel of interest, which is distributed and determined according to density data of a pixel adjacent to the pixel of interest; In the case where the density data of the target pixel is equal to or more than a specific density value as a first threshold value, the dot writing position is displaced and recorded in a high density portion of the density distribution based on the determined density distribution data. Multiple recording position modulation
This is achieved by a color image forming apparatus characterized in that it is performed on a split pixel . A means for image-discriminating the pixel of interest; and, if the image is determined to be a halftone area, only the black component is subjected to recording position modulation based on the density distribution of the adjacent pixels, and if the area is determined to be a character area, the whole is Means for performing the recording position modulation on a color component.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A color image forming apparatus according to one embodiment of the present invention.
The configuration of 400 will be described. FIG. 4 is a perspective view illustrating a schematic configuration of the image forming apparatus of the present embodiment.

A color 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 a photosensitive member. A dot-shaped electrostatic latent image is formed from the spot light pulse-width-modulated based on the modulation signal obtained as described above, and this is reversely developed with toner to form a dot-shaped toner image. The development process is repeated to form a color toner image on the photoreceptor, and the color toner image is transferred onto a recording sheet, separated from the photoreceptor, and fixed to obtain a color image.

The color image forming apparatus 400 is a drum-shaped photosensitive member that rotates in the direction of an arrow (hereinafter simply referred to as a photosensitive member).
401, a scorotron charger 402 for imparting a uniform charge on the photoconductor 401, a scanning optical system 430, developing units 441 to 444 loaded with yellow, magenta, cyan, and black toners, and a transfer including a scorotron charger. It comprises a device 462, a separator 463, a fixing roller 464, a cleaning device 470, and a static eliminator 474.

The photoreceptor 401 used in the present embodiment is a photoreceptor having a high γ characteristic, and a specific configuration example is shown in FIG.

The photosensitive member 401 comprises a conductive support 401A, an intermediate layer 401B and a photosensitive layer 401C as shown in FIG. The thickness of the photosensitive layer 401C is about 5 to 100 μm, preferably 10 to 100 μm.
5050 μm. The photosensitive member 401 uses a drum-shaped conductive support 401A made of aluminum having a diameter of 150 mm.
0.1μm thick ethylene-vinyl acetate copolymer
Of the intermediate layer 401B having a thickness of 35 μm
m photosensitive layer 401C.

As the conductive support 401A, a drum made of aluminum, steel, copper, or the like having a diameter of about 150 mm is used. In addition, a belt-like material obtained by laminating or vapor-depositing a metal layer on paper or plastic film, or an electroconductive support is used. It may be a metal belt such as a nickel belt made by a brazing method. The intermediate layer 401B withstands a high charge of ± 500 to ± 2000 V as a photoreceptor. For example, in the case of a positive charge, it prevents injection of electrons from the conductive support IC, and obtains an excellent light attenuation characteristic due to an avalanche phenomenon. It is desirable that the intermediate layer 401B has a hole mobility, for example, as described in Japanese Patent Application No. 61-1889 previously proposed by the present applicant.
It is preferable to add 10% by weight or less of the positive charge type charge transporting substance described in the specification of Japanese Patent No. 75-75.

As the intermediate layer 401B, for example, the following resins commonly used for photosensitive layers for electrophotography can be used.

(1) polyvinyl alcohol (povar),
Vinyl polymers such as polyvinyl methyl ether and polyvinyl ethyl ether; (2) polyvinylamine, poly-
Nitrogen-containing vinyl polymers such as N-vinylimidazole, polyvinylpyridine (quaternary salt), polyvinylpyrrolidone, vinylpyrrolidone-vinyl acetate copolymer, (3) polyether polymers such as polyethylene oxide, polyethylene glycol and polypropylene glycol, (4)
Acrylic polymers such as polyacrylic acid and salts thereof, polyacrylamide and poly-β-hydroxyethyl acrylate; and (5) methacrylic acids such as polymethacrylic acid and its salts, polymethacrylamide and polyhydroxypropyl methacrylate Polymer, (6) methylcellulose, ethylcellulose, carboxymethylcellulose, hydroxyethylcellulose, ether cellulose-based polymer such as hydroxypropylmethylcellulose,
(7) polyethyleneimine polymers such as polyethyleneimine, (8) polyalanine, polyserine, poly-L-glutamic acid, poly- (hydroxyethyl) -L-glutamine,
Poly-δ-carboxymethyl-L-cysteine, polyproline, lysine-tyrosine copolymer, glutamic acid-lysine
-Polyamino acids such as alanine copolymer, silk fibroin, casein, (9) starch and derivatives thereof such as starch acetate, hydroxyethyl ethyl starch, starch acetate, hydroxyethyl starch, amine starch, phosphate starch, and (10) polyamide. A polymer soluble in a mixed solvent of water and alcohol, such as a certain soluble nylon and methoxymethyl nylon (8 type nylon).

The photosensitive layer 401C is basically formed by using a phthalocyanine fine particle of 0.1 to 1 μm in diameter made of a photoconductive pigment, an antioxidant, and a binder resin without using a charge transport material in combination with a binder resin solvent. A coating solution is prepared by mixing and dispersing, the coating solution is applied to an intermediate layer, dried, and if necessary, heat-treated to form a coating solution.

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

In this embodiment, the color toner image is transferred to the photosensitive member 401.
A photoconductor and an infrared semiconductor laser having a spectral sensitivity on the infrared side are used so that the beam from the scanning optical system is not blocked by the color toner image because the beam is superimposed on the top.

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

FIG. 11 is a graph showing characteristics of the high γ photoreceptor. In the figure, V ₁ is a charging potential (V), V ₀ is an initial potential (V) before exposure, and L ₁ is an irradiation light amount (μJ / cm) of a laser beam required for the initial potential V ₀ to attenuate to 4/5. ² ), L ₂
Represents the irradiation light amount (μJ / cm ² ) of the laser beam required for the initial potential V ₀ to attenuate to １ ／.

The preferred range of L ₂ / L ₁ is _{1.0 <L 2 / L 1 ≦} 1.5.

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

The light sensitivity at the position corresponding to the middle stage of exposure where the initial potential (V ₀ ) is attenuated to １ ／ in the light decay curve is expressed by E
When the light sensitivity at the position corresponding to the initial exposure in which the initial potential (V ₀ ) is attenuated to 9/10 is E9 / 10, (E1 / 2) / (E9 / 10) ≧ 2 Preferably, a photoconductive semiconductor which gives a relationship of (E1 / 2) / (E9 / 10) ≧ 5 is selected. Here, the light sensitivity is defined by the absolute value of the amount of potential decrease with respect to the minute exposure amount.

In the light decay curve of the photosensitive member 401, as shown in FIG. 11, the absolute value of the differential coefficient of the potential characteristic, which is the photosensitivity, is small when the light amount is small, and increases sharply as the light amount increases. Specifically, as shown in FIG. 11, the light decay curve shows an almost flat light decay characteristic due to poor sensitivity characteristics for a short period of time in the early stage of exposure as shown in FIG. 11, but it turns out to be extremely high in the middle to late stages of exposure. The sensitivity becomes an ultra-high γ characteristic that decreases almost linearly. It is considered that the photoconductor 401 specifically obtains a high γ characteristic by utilizing an avalanche phenomenon under high charging of +500 to + 2000V. In other words, the carriers generated on the surface of the photoconductive pigment in the early stage of the exposure are effectively trapped in the interface layer between the pigment and the coating resin, and the light attenuation is surely suppressed. As a result, extremely rapid after the middle stage of the exposure. It is understood that an avalanche phenomenon occurs.

Next, a color image forming apparatus of the present invention will be described. In this color image forming apparatus, one pixel of interest of image density data is formed by m × n (horizontal × vertical) small pixels. Replacing the distribution of density data of adjacent pixels including the target pixel with the distribution of m × n small pixels in the one pixel, and distributing data of the target pixel multiplied by a constant P in accordance with the distribution. The image formation is performed by displacing the phase of the reference wave of each row of the small pixel based on the image density data of the small pixel obtained by the above, thereby displacing the writing position of the dot of the nth row. Displacing the dot writing position is referred to as recording position modulation. Further, the process of converting the target pixel into image density data of small pixels divided into m × n is referred to as a resolution improvement process (RE process). High density recording can be performed by this RE processing. In this case, a high γ photoreceptor is particularly effective for forming a latent image accurately in response to a reference wave.

In the present invention, this RE processing is performed when the density data of the target pixel is equal to or higher than the first threshold, that is, equal to or higher than a specific density, that is, equal to or higher than the first threshold. In other words, in many areas corresponding to the highlight part, the RE processing is not performed on the background part of the document, and the m × n small pixels have uniform density.
In the case of a CRT, this data display is possible.

However, in the case of laser recording described later, since uniform display is difficult, a reference wave having a central density center is selected. Thus, it is possible to prevent occurrence of noise maintaining the uniformity in the high- write unit.

[0029] hand, when the high density portion, when the concentration gradient is large, thus the dots are formed across adjacent pixels and selects a reference wave recording position of the density is not in the center.

In order to prevent the density fluctuation and the collapse of the recording dots between pixels due to this, a specific second density is used even in the high density area.
If the threshold value is equal to or larger than the threshold value, the reference wave whose density center is at the center is selected.

In the case of a CRT, since uniform display is possible, m × n small pixels are processed as a uniform density. That is, no RE processing is performed.

That is, in a color image forming apparatus for performing high-density pixel recording, specific density data of a pixel of interest is determined by density distribution data within the pixel of interest determined in correspondence with density data of a pixel adjacent to the pixel of interest. A color image forming apparatus which performs printing position modulation based on the determined density distribution for a case where the threshold value is equal to or more than one threshold value.

Further, a color image forming apparatus is preferably characterized in that, when the specific density data of the pixel is equal to or less than a second threshold value, the recording position is modulated based on the determined density distribution.

FIG. 5A is a plan view showing the 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 to m9 and s1 to s9 also represent the concentration of that portion.

The RE processing 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 which can be called the intensity 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 of (A) is obtained by multiplying the density of the target pixel m5 by P and allocating it according to the ratio of the density of the adjacent pixel.
The term of (P) × m5 is obtained by equally distributing the remaining density of the target pixel m5 to each of the small pixels, which means that the blur factor is taken into account.

FIG. 6 shows that the target pixel m5 is divided into 3 × 3,
FIG. 6A shows an example of a density distribution of an adjacent pixel including the target pixel m5, and FIG. 6B shows a density distribution in the target pixel m5 calculated with P = 0.5. FIG.

Next, FIGS. 7 and 8 show an example in which the target pixel m5 is divided into 2 × 2.

FIG. 7A shows an example in which the target pixel m5 is divided into 2 × 2, and FIG. 7B shows an example of adjacent pixels related to the small pixels s1 to s4 in the target pixel. FIG.

The calculation of the concentrations of s1, s2, s3, and s4 is performed according to Equation 1.

[0042]

(Equation 1)

FIG. 8A also shows a pixel of interest m5 of 2 × 2.
FIG. 8B is a diagram illustrating another example of adjacent pixels related to the small pixels s1 to s4 in the target pixel. The density calculation of s1, s2, s3, and s4 is performed according to Equation 2.

[0044]

(Equation 2)

FIG. 1 is a block diagram showing an embodiment of an image processing circuit used in a color image forming apparatus of the present invention (an example in which a pixel of interest is divided into 3 × 3), and FIG. FIG. 3 is a block diagram showing a modulation circuit of the present embodiment.

The image processing circuit 1000 of this embodiment is a circuit constituting a driving circuit of a scanning optical system, and includes an image data processing circuit 100, a modulation signal generation 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 includes an input circuit 110 composed of a computer and a font data generation circuit 12.
0, font data storage circuit 130, interpolation data generation circuit 14
0, a 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 generation circuit 120. The font data generation circuit 120 selects an address signal from the four types of input signals and sends it to the font data storage circuit 130. The font data storage circuit 130 sends 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 font data to the interpolation data generation circuit 140.

The interpolation data generation circuit 140 interpolates the jaggedness or jump of the image density data generated at the edge portion of the font data using the intermediate density and sends it to the image density data storage circuit 210 composed of a frame memory. 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. In this way, the font is bitmap-developed in each frame memory in a state where each color has the same shape and the density ratio is different.

The modulation signal generation circuit 200 includes an image density data storage circuit 210, a read circuit 220, a latch circuit 230, an image determination circuit 231, an MTF correction circuit 232, a γ correction circuit 233, a reference wave phase determination circuit 240, and a selection circuit 250A. ~ 250C, modulation circuit 2
60A to 260C, a reference clock generation circuit 280, a triangular wave generation circuit 290, a delay circuit group 291 and the like.

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

The reading circuit 220 reads the image density data of consecutive one scan line units continuously in synchronization with the reference clock DCK ₀ with an index signal as a trigger from the image density data storage circuit (page memory) 210, reference wave phase The information is sent to the decision circuit 240, the image determination circuit 231 and the MTF correction circuit 232.

The latch circuit 230 is a circuit that latches the image density data for the time during which the process of the reference wave phase determination circuit 240 described later is executed.

The reference clock generation circuit 280 is a pulse generation circuit that generates a pulse signal having the same repetition cycle as the pixel clock and outputs the pulse signal to the readout circuit 220, the triangular wave generation circuit 290, the delay circuit group 291 and the modulation circuits 260A to 260C. I do. For convenience the clock referred to as a reference clock DCK _0.

Reference numeral 290 denotes a triangular wave generation circuit which is a reference clock DCK _0.
Performing waveform shaping of the triangular wave phi ₀ reference is the reference wave with the same period and pixel clock based on. Further, in the delay circuit group 291, the reference clock DCK _{0 is} fixed at regular intervals (in this example, 1/6
A plurality of clocks DCK ₁ to DCK having a phase difference are generated at intervals of one cycle, and based on this, triangular waves φ _{1 to} φ ₄ which are reference waves having different phases (here, triangular waves φ ₁ ,
A triangular wave φ ₂ delayed by / 6 cycle, a triangular wave φ ₃ advanced by 1/6 cycle, and a triangular wave φ ₄ advanced by 2/6 cycle are output.

The select circuits 250A to 250C have input portions for the triangular waves φ _{1 to} φ ₄ out of phase with the reference triangular wave φ ₀ ,
One of the triangular waves is selected by a selection signal from a reference wave phase determination circuit 240, which will be described later, and the modulation circuits 260A to 260C
To the input terminal T.

[0056] The modulation circuit 260A~260C has the same circuit configuration as shown in FIG. 3, the triangular wave shifted D / A conversion circuit 261, a comparator 262, the reference triangular wave phi ₀ or 1/6 cycle out of phase Of the image density data input via the latch circuit 230,
In synchronism with CK ₀ and D / A converted by D / A conversion circuit 261 is a circuit for obtaining a pulse width modulated signal to the comparator as the reference wave to said triangular wave inputted from the select circuit 250A through 250C.

The reference wave phase determination circuit 240 comprises a one-line delay circuit 242, a one-clock delay circuit 243, and an arithmetic processing circuit 241 as shown in FIG.
A delay of two scanning times is applied to the first one scanning line of image density data of three scanning lines of the image density data sent one scanning line at a time, and to an intermediate one scanning line of image data. A delay of one line scanning time is applied (no delay is applied to the image data of the last one scanning line). Further, each image data has a one-clock delay circuit 24.
A delay of 2 reference clocks or 1 reference clock is applied by 3 and all image density data of pixels including the target pixel and adjacent to the target pixel are simultaneously sent to the arithmetic processing circuit 241.

The arithmetic processing circuit 241 performs the above-mentioned RE processing to obtain density data of small pixels. The obtained small pixel density data includes a small scanning line including s1, s2, s3,..., A small scanning line including s4, s5, s6, and s7, s8, s9,. The three small scanning lines of the small pixel correspond to one scanning line of the original pixel.

The arithmetic processing circuit 241 further calculates the barycenter of the density data in the original one pixel of each small scan line, and outputs different selection signals as follows from the output terminal OA depending on the barycenter position. Output to 250A to 250C. That is, when the center of gravity of s1, s2, and s3 (first small scanning line) of the pixel m5 is near the center of s2, a signal for selecting the reference triangular wave φ ₀ having no phase shift is given by s.
When near the boundary between 2 and s1, select a signal that selects a triangular wave φ ₁ whose phase is delayed by 1/6 cycle. When the center of gravity is near the center of s1, select a triangular wave φ ₂ whose phase is delayed by 2/6 cycle. When the center of gravity is near the boundary between s2 and s3, a signal for selecting a triangular wave φ ₃ having a phase advanced by ６ cycle is
When the center of gravity is near the center of s3, a signal for selecting the triangular wave φ ₄ advanced by 2/6 cycle is output from the output terminal OA to the select circuit 250.
Output to A. Similarly, s of the pixel m5 is output from the output terminal OB.
The triangular wave selection signal of the second small scanning line determined from the density centroids of 4, 4, s5, and s6 is supplied to the select circuit 250B by the output terminal O.
C outputs a triangular wave selection signal for the third small scanning line determined from the density centroids of s7, s8, and s9 of the pixel m5 to the selection circuit 250C. FIG. 9 is a diagram showing an example of the relationship between the triangular waves having different phases and the pixel of interest.

On the other hand, the image discriminating circuit 231 discriminates whether the image data of the pixel of interest is the first or second threshold, and determines that the image data is outside the first and second thresholds. In this case, the reference wave phase determination circuit 240
There triangular wave selected is not output, and sends a selection signal to output only the reference triangular wave phi ₀ to the select circuit 250A through 250C. The MTF correction circuit 232 does not operate the circuit. As a result, the image density data other than those read out by
After being corrected by the TF correction circuit 232 and corrected by the γ correction circuit 233, the modulation circuit 260
A to 260C.

As a result, a highly uniform and noise-free image can be formed in the highlight and high density area.

On the other hand, the image determination circuit 231 further determines whether the image is a character area or a halftone area under the above conditions. This discrimination is performed by 5 × including the pixel of interest.
This is performed by changing the density in five pixels. When the density conversion is large, the pixel of interest is determined to be a character area, and when the density conversion is small, it is determined to be a halftone area. If it is determined that it is a character area of a character or a line drawing, a selection signal for outputting the triangular wave selected by the reference wave phase determination circuit 240 to the modulation circuits 260A to 260C for all color components is output to the selection circuits 250A to 250C. , The MTF correction circuit 232 and the γ correction circuit 233 are disabled and the image density data is sent to the modulation circuits 260A to 260C via the latch circuit 230 without processing. As a result, clear characters and edge portions having no change in color tone are reproduced. When it is determined that the halftone area, and outputs the same selection signal and character area only for the black color data, the triangular reference wave phase determination circuit 240 has selected for the other color components is not output, the reference a selection signal to output only triangular wave phi ₀ is sent to select circuit 250A through 250C, MTF correction circuit 232, gamma
The correction circuit 233 is operated. As a result, the image density data other than black read by the reading circuit 220 is transferred to the MTF correction circuit 232.
And the latch circuit 2 after being corrected by the gamma correction circuit 233.
The signal is sent to the modulation circuits 260A to 260C via 30.

This makes it possible to form an image free of moiré and color skipping in the halftone area, while producing an effect of giving the image sharpness and tightness by the black image.

The density data is converted into density data 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) for use in determining the phase of the reference wave. Use something. For convenience (R +
The density data of (2G + B) is represented by N.

By using the phase of the reference wave in common for each recording color, it is possible to guarantee the gradation of an image and prevent a change in tint .

The data used in the image discriminating circuit 231 uses data common to each color for the same reason.

The modulation circuits 260A to 260C modulate the image density data signal input through the latch circuit 230 with the triangular wave, which is the selected reference wave, to generate a pulse width modulated modulation signal. Are sent to the raster scanning circuit 300 as three units of three consecutive small scanning lines (one line of the original image density data).

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

FIGS. 10A to 10D are time charts showing signals of each part of the modulation signal generating circuit when recording position modulation is performed.

FIG. 10A shows a page memory 210.
From the reference clock DCK _{0 using the} index signal as a trigger
3 shows a part of the image density data read based on the D / A conversion circuit 261 converted into an analog value. A higher level indicates a lighter density, and a lower level indicates a higher density.

FIG. 7B shows a triangular wave which is a reference wave sequentially output from the select circuit 250 and includes a delayed signal.

(C) shows the triangular wave (solid line) and the image density signal (dot-dash line) converted to the analog value.
The modulation operation in the modulation circuits 260A to 260C is shown.

FIG. 9D shows a pulse width modulation signal generated by comparison by the comparator 262.

According to the modulation signal generation result, the low density portion,
In the case of a pixel of a high density part, the recording position modulation is not performed. As a result of performing the recording position modulation for moving to the position along the line, characters and images are clearly reproduced. Further, in the above-described recording position modulation, only the black component is performed in the halftone region to prevent a change in color tone, and the other color components are modulated by a triangular wave having no phase shift.

Further, by sequentially shifting the reference wave phase in the sub-scanning direction, a dot corresponding to a halftone dot having a screen angle can be formed. For example, the screen angle is 45 ° for the yellow component and 26.6 for the magenta component.
°, -26.6 ° for the cyan component and 0 ° for the black component to improve the uniformity of color reproduction and prevent the occurrence of moiré fringes.

In particular, setting the black component to 0 ° has the advantage that the recording phase modulation means can be used without any change.

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

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

The index detecting circuit detects the surface position of the rotating polygon mirror 434 which rotates at a predetermined speed based on the index signal from the index sensor 439, and modulates the modulated image density signal by the raster scanning method according to the period in the main scanning direction. Optical scanning is performed. Scan frequency 2204.72Hz
The effective printing width is 297 mm or more, and the effective exposure width is 306 mm.
That is all.

The polygon mirror driver rotates the DC motor uniformly at a predetermined speed and rotates the rotary polygon mirror 434 at 16535.4 rpm.

As shown in FIG. 13, a semiconductor laser array 431 in which three light-emitting portions 431A to 431C are arranged in an array at equal intervals is used. Since it is usually difficult to make the distance d between the light emitting units equal to or less than 20 μm, the axis passing through the center of each of the light emitting units 431A to 431C is parallel to the rotation axis of the rotary polygon mirror 434 and the main scanning direction as shown in FIG. Install at an angle to the direction. Thus, the semiconductor laser array 431
Spot s on the photoconductor 401 of the laser beam by the laser beam
As shown in FIG. 14, a, sb, and sc can be scanned vertically and closely. However, for this reason, the positions of the respective laser spots sa, sb, sc in the scanning direction are shifted with respect to the scanning direction. To correct this shift, a 2δ delay circuit 311 is provided between the modulation circuit 260A and the laser driver 301A, and the modulation circuit 260B and the laser driver
A deviation is corrected by inserting a δ delay circuit 312 between the semiconductor laser array 301B and the delta delay circuit 312 and delaying them by an appropriate amount to take a timing, and the laser spots sa, sb, and sc emitted from the semiconductor laser array 431 with respect to the scanning direction. Vertically aligned s
a ', sb', and sc 'can be recorded.
When the RE process is performed by dividing the pixel of interest into 2 × 2 small pixels, a semiconductor laser array having two light emitting units is used.

Next, the image forming apparatus 400 shown in FIG.
The image forming process will be described.

First, the photoconductor 401 is uniformly charged by the scorotron charger 402. An electrostatic latent image corresponding to yellow is stored on a drum-shaped photoconductor 401 in an image density data storage circuit 210.
The laser beam modulated by yellow data (8-bit digital density data) from the inside is formed by irradiation through a cylindrical lens 433, a rotating polygon mirror 434, an fθ lens 435, a cylindrical lens 436, and a reflection mirror 437. The electrostatic latent image corresponding to the yellow is developed by a first developing device 441, and a dot-shaped first toner image (yellow toner image) having extremely high sharpness is formed on the photoconductor 401.
Is formed. The first toner image passes under the retracted cleaning device 470 without being transferred to the recording paper, and is again charged on the photoconductor 401 by the scorotron charger 402.

Next, the laser beam modulated by the magenta data (8-bit digital density data) is irradiated onto the photosensitive member 401 to form an electrostatic latent image. This electrostatic latent image is developed by the second developing device 442,
(Magenta toner image) is formed. In the same manner as described above, the toner is sequentially developed by the third developing device 443 to form a third toner image (cyan toner image).
A three-color toner image sequentially stacked on the first color image is formed. Finally, a fourth toner image (black toner image) is formed.
A four-color toner image sequentially stacked on the first color image is formed.

According to the image forming apparatus 400 of the present embodiment, the photosensitive member 401 has excellent high γ characteristics, and this excellent high γ characteristic allows the charging, exposure and development steps to be performed many times from above the toner image. The latent image is formed stably even when the toner image is repeatedly formed over a long period of time. That is, even if a laser beam is irradiated from above the toner image based on the digital signal, a high-sharpness, high-dot electrostatic latent image without fringes is formed, and as a result, a high-sharpness toner image is obtained. Can be.

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

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

In this embodiment, as a result of experiments in which the value of the coefficient P of the RE processing was variously changed, the value of P was 0.1 to 0.9.
A good image was obtained within the range. 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 character or the line drawing is excessively emphasized is obtained. Therefore, the preferable range of the value of P is 0.3 to 0.7. Turned out to be in the range. As a result, when the original is a character or a line drawing, the edge portion clearly appears, and even small characters can be reproduced in detail. In addition, there was no adverse effect on the low density portion or the high density portion. This is because the method stops recording position modulation for these pixels, and effectively sets P = 0.

In the present invention, it is possible to use P constant, but it is preferable to use P by changing it according to the image (character area or halftone area). The value in the case of the character region and P _1, when the case of the halftone area and P _2, it is preferable that the P _1> P _2. That is, when the image is a character or the like, the value of P is large and preferably 0.9 to 0.4, and when the image is halftone, the value of P is small and 0.6 to 0.

Note that P = 0 corresponds to not performing recording position modulation.

The flow of the image data has been described as a laser printer which outputs data once stored in the page memory 210. However, the present invention is not limited to this. If a circuit for inputting image density data from a scanner and performing image processing is used instead of the image data processing circuit 150 including the D conversion circuit 152, the density conversion circuit 153, the masking UCR circuit 154, etc. The present invention can be applied to an image forming apparatus.

[0092]

As described above, for the target pixel included in the specific density, the target pixel is divided into small pixels corresponding to the density data of the target pixel. The phase of the reference wave signal is selected from the image data subjected to the RE processing for distributing the density of the target pixel according to the distribution of the density data of the adjacent pixel including the pixel, and the density signal of the target pixel is modulated by the reference wave. An excellent recorded image was obtained by generating a recording position modulation signal and not performing recording position modulation on a low density portion or a high density portion. Further, image discrimination is performed by an image discrimination circuit, and in the case of a character area, recording position modulation is performed for all color components, and in a halftone area, color image recording is performed using a modulation signal obtained by performing recording position modulation of black only. An excellent color image forming apparatus capable of improving sharpness without causing a change in color tone of a color image formed from a scanner, CG, font data, or the like was provided.

In the above method, the recording beam of the pixel is 3
Although the case of the number 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. The effect can be further improved by using a high γ photoreceptor.

[Brief description of the drawings]

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

FIG. 2 is a block diagram illustrating an example of a reference wave phase determination circuit of the circuit of FIG. 1;

FIG. 3 is a block diagram illustrating an example of a modulation circuit of the circuit of FIG. 1;

FIG. 4 is a perspective view illustrating a schematic configuration of an example of the image forming apparatus of the present invention.

FIG. 5 is a diagram for explaining RE processing used for determining a reference wave phase.

FIG. 6 divides a pixel of interest in RE processing into 3 × 3 pixels, and sets P = 0.
FIG. 9 is a diagram showing an example in the case of 5.

FIG. 7 is a diagram illustrating an example of a case where a pixel of interest in the RE processing is divided into 2 × 2.

FIG. 8 is a diagram illustrating another example in which a target pixel indicated by an RE is divided into 2 × 2.

FIG. 9 is a diagram for explaining a phase displacement of a reference wave.

FIG. 10 is a time chart showing signals of respective parts of the modulation signal generation circuit of the embodiment of FIG. 1;

FIG. 11 is a graph showing characteristics of the high γ photoconductor used in this example.

FIG. 12 is a cross-sectional view illustrating a specific configuration example of a high γ photoconductor used in the present embodiment.

FIG. 13 is a diagram showing a semiconductor laser array of the embodiment of FIG.

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

[Explanation of symbols]

 100 image data processing circuit 200 modulation signal generation circuit 210 image density data storage circuit (page memory) 220 readout circuit 230 latch circuit 231 image discrimination circuit 232 MTF correction circuit 233 gamma correction circuit 240 reference wave phase determination circuit 241 arithmetic processing circuit 250A- 250C Select circuit 260A-260C Modulation circuit 280 Reference clock generation circuit 290 Triangular wave generation circuit 291 Delay circuit group 300 Raster scanning circuit 400 Image forming apparatus

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-63-93269 (JP, A) JP-A-64-22175 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04N 1/40-1/409 H04N 1/46 H04N 1/60

Claims (4)

(57) [Claims] 1. A density of a target pixel to be recorded based on image data is divided into a plurality of target pixels assuming that the target pixel has a density distribution, and corresponds to density data of a pixel adjacent to the target pixel. minute of the noted within the pixel determined allocated Te
In a color image forming apparatus that performs high-density pixel recording based on density distribution data composed of divided pixels, the above-described determination is performed when the density data of the target pixel is equal to or more than a specific density value as a first threshold value. A color image forming apparatus , comprising: performing a recording position modulation for displacing a dot writing position in a high density portion of a density distribution based on density distribution data for recording on a plurality of divided pixels . 2. The image processing apparatus according to claim 1, further comprising: means for determining an image of the target pixel; and means for performing recording position modulation of only a black component based on the density distribution of the adjacent pixels when the image is determined to be a halftone area. 2. The color image forming apparatus according to claim 1, wherein: 3. The color image processing apparatus according to claim 1, wherein said target pixel is image-discriminated, and said recording position modulation is performed on all color components when said image is determined to be a character area. Image forming device. 4. The method according to claim 1, wherein, when the density data of the pixel is equal to or lower than a second threshold value higher than the first threshold value, the recording position is modulated based on the determined density distribution. 1 is a color image forming apparatus.