JPH05167811A - Image forming device - Google Patents

Abstract

PURPOSE:To improve the resolution of images generated by a color scanner, a CG and font data, etc., and to provide an image forming device by which high definition image recording can be performed. CONSTITUTION:Image density data from an image density data memory circuit 210 is read out at a circuit 220, and the density of each small picture element in which a remarked picture element is divided is allocated corresponding to the distribution of neighboring picture element density data including the remarked picture element, and the reference wave of a phase in accordance with found density centroid is selected by a reference wave phase decision circuit 241 and select circuits 250A-C. The density of the remarked picture element, and a character area and a halftone area are discriminated by an image discrimination circuit 231, and the cycle of the reference wave is decided, and the original image density signal of each color is modulated by a selected reference wave signal at a modulation circuit 260, then, the image can be formed by an obtained recording position modulation signal. At this time a reference wave with the same cycle as that of the image is used when density data of the remarked picture element is within first and second threshold values and it is in the character area, and recording position modulation is performed by using the reference wave with double cycles when it is in the halftone area.

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.

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]

An object of the present invention is to provide an image forming apparatus for performing high-density pixel recording by using density data of small pixels in a pixel of interest determined corresponding to density data of a pixel adjacent to the pixel of interest. According to the density of the target pixel and the density distribution of the adjacent pixels, when the recording position of each color is modulated in the main scanning direction and the sub-scanning direction, the target pixel determines the recording position of each color from the density distribution of the adjacent pixels. This is achieved by an image forming apparatus. In this image formation, by making the cycle of the reference wave variable depending on the image, it is possible to realize preferable resolution and gradation depending on the image. Further, means for discriminating the image of the pixel of interest, and when it is discriminated as a halftone region by this image discrimination, recording position modulation is performed from the density distribution of the adjacent pixel using a reference wave having a long cycle, and discriminated as a character region. And a means for performing the recording position modulation by using a reference wave having a short cycle.

[0009]

EXAMPLE A configuration of an image forming apparatus 400 according to an example of the present invention will be described. FIG. 4 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 a 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. 14, the photoconductor 401 comprises a conductive support 401A, an intermediate layer 401B and a photoconductive 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 material obtained by laminating or vapor-depositing a metal layer on a plastic film, or an electric electrode. 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 from the conductive support 401C of the electron and obtains excellent light attenuation characteristics due to avalanche phenomenon. As described above, it is desirable that the intermediate layer 401B has hole mobility.
No. 8975 describes positively chargeable charge transport materials.
It is preferable to attach less than or equal to wt%. 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 solution, coating the coating solution 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 one of the photoconductive pigments 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. 13 is a graph showing the characteristics of the high γ photoconductor. In the figure, V ₁ is a charging potential (V), V ₀ is an initial potential before exposure (V), and L ₁ is a laser beam irradiation light amount (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 ⅕.

A 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), 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 A photoconductive semiconductor that gives a relationship of (E1 / 2) / (E9 / 10) ≧ 5 is preferably 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. 13, 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. 13, the light attenuation curve shows a nearly flat light attenuation characteristic in the early stage of exposure due to poor sensitivity characteristics 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 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 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, equal to or higher than a specific density, that is, equal to or higher than the first threshold value. 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, if a reference wave in which the density recording position is not in the center is selected, dots are formed across adjacent pixels.

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

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 determined by 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 characterized in that recording position modulation is performed based on the determined density distribution when the threshold value is 1 or more, and when the specific density data of the pixel is the second threshold or less. Preferably, the image forming apparatus is characterized in that recording position modulation is performed 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-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 formula.

Si = (9 × m5 × P × mi / A) + (1−P) × m5 Here, 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. 6, the target pixel m5 is divided into 3 × 3 pixels, and P
6A shows an example of the density distribution of adjacent pixels including the target pixel m5, and FIG. 6B shows the density distribution in the target pixel m5 calculated as 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 of dividing the target pixel m5 into 2 × 2, and FIG. 7B 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 performed according to equation 1.

[0040]

[Equation 1]

In FIG. 8A, the target pixel m5 is 2 × 2.
FIG. 8B is a diagram showing another example in the case of being divided into, and FIG. 8B is a diagram showing another example of the adjacent pixels related to the small pixels s1 to s4 in the target pixel. The concentrations of s1, s2, s3, and s4 are calculated according to Equation 2.

[0042]

[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 target pixel is divided into 3 × 3), and FIG. 2 is shown in this embodiment. 3 is a block diagram showing the reference wave phase determination circuit of FIG. 3, and FIG. 3 is a block diagram showing the modulation circuit of the present embodiment.

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 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 comprises an input circuit 110 and a font data generating circuit 12 which are formed by 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 the 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 jaggedness or jumps of the image density data generated at the edge portion of the font data using the intermediate density and sends the interpolated data to the image density data storage circuit 210 composed of a frame memory. Also, regarding the generated color, the corresponding color is set to each yellow (Y), according to the color code.
Convert to density data of magenta (M), cyan (C), and black (BK). 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 readout circuit 220, a latch circuit 230, an image discrimination 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 the 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 read circuit 220 uses the index signal as a trigger and synchronizes with the reference clock DCK ₀ in succession 1
Continuous image density data for each scanning line is read from the image density data storage circuit (page memory) 210 and sent to the reference wave phase determination circuit 240, the image determination circuit 231, and the MTF correction circuit 232.

The latch circuit 230 is a circuit for latching the image density data only while the processing of the reference wave phase determination circuit 240 described later is being executed.

The reference clock generation circuit 280 is a pulse generation circuit, which generates a pulse signal having the same repetition period as the pixel clock and outputs it to the read circuit 220, the triangular wave generation circuit 290, the delay circuit group 291, and the modulation circuits 260A to 260C. To do. For convenience, this clock is referred to as a reference clock DCK ₀ .

Reference numeral DC is a reference clock DC
Based on K ₀ , a reference triangular wave φ ₀ ′ which is a reference wave having the same period as the pixel clock and a triangular wave φ ₀ ″ which is a reference wave having a double period.
Waveform shaping. Further, in the delay circuit group 291, a plurality of clocks DCK ₁ ′ to DCK ₄ ′ and DCK ₁ ″ having a phase difference with respect to the reference clock DCK ₀ and a clock having a double cycle by a constant period (1/6 period in this example). ~ DCK ₄ ″
Based on this, triangular waves φ ₁ ′ to φ ₄ ′ and φ ₁ ″ to φ ₄ ″ (here 1/6
Periodically advanced triangular wave φ ₁ ′, φ ₁ ″, 2/6 period advanced triangular wave φ ₂ ′, φ ₂ ″, 1/6 period delayed triangular wave φ ₃ ′,
φ ₃ ″, triangular wave φ ₄ ′, φ ₄ ″) that is delayed by 2/6 period is output.

[0052] select circuit 250A~250C has an input portion of the reference triangular wave φ ₀ '"~φ_4' and φ _1, φ ₀ a triangular wave phi ₁ out of phase '" ~φ ₄ ", reference will be described later One of the triangular waves is selected by the selection signal from the wave phase determination circuit 240 and sent to the input terminals T of the modulation circuits 260A to 260C.

The modulation circuits 260A to 260C have the same circuit configuration as shown in FIG. 3, and the D / A conversion circuit 261, the comparator 262, the reference triangular wave φ ₀ ′ and the phase thereof are shifted by 1/6 cycle. Triangular wave and triangular wave with double period φ ₀ ″
And a triangular wave input section T whose phase is shifted by 1/6 cycle
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 reference clock DCK ₀ , and the image density data input from the selection circuits 250A to 250C is input. This is a circuit that obtains a pulse width modulation signal by comparing a triangular wave as a reference wave.

The reference wave is selected by the image discrimination circuit 231 from among φ ₀ ′ to φ ₄ ′ for the character area and from φ ₀ ″ to φ ₄ ″ for the halftone area.

The reference wave phase determination circuit 240 comprises a 1-line delay circuit 242, a 1-clock delay circuit 243, and an arithmetic processing circuit 241, as shown in FIG.
In the image density data for the first one scanning line for the three scanning lines of the image density data sent for each one scanning line, a delay of two line scanning time is added to the image density data for one intermediate scanning line. Does not delay the image density data for the last one scanning line. Further, each image density data is delayed by 2 reference clocks or 1 reference clock by the 1-clock delay circuit 243, and all image density data of pixels including the target pixel and adjacent to the target pixel are calculated at the same time. Processing circuit
Send to 241.

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 calculates the average density of each small scanning line and the barycentric position of the original density data in one pixel of each small scanning line to obtain the average density data of the laser drivers 301A to 301C. Based on the gravity center position data, different selection signals are output from the output terminals OA to OC to the select circuits 250A to 250C. That is, when the center of gravity of s1, s2, s3 (first small scan line) of the pixel m5 is near the center of s2, a signal for selecting the reference triangular wave φ ₀ ′ having no phase displacement is generated. When it is near the boundary, a triangular wave φ ₁ ′ whose phase is advanced by ⅙ cycle is selected. When the center of gravity is near the center of s1, the triangular wave φ ₂ ′ is advanced by 2/6 cycle.
A signal for selecting a signal for selecting a triangular wave phi ₃ 'whose phase is delayed 1/6 period when the center of gravity is in the vicinity of the boundary s2 and s3, when the center of gravity is in s3 center near 2/6 cycle delay A signal for selecting the triangular wave φ ₄ ′ is output from the output terminal OA to the select circuit 250A. Similarly, from the output terminal OB, the selection circuit 250 outputs the triangular wave selection signal of the second small scanning line determined by the density centroids of s4, s5, and s6 of the pixel m5.
A triangular wave selection signal of the third small scanning line determined from the density centroid of s7, s8, and s9 of the pixel m5 is output from the output terminal OC 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.

Further, the arithmetic processing circuit 241 causes each 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. 17 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 of obtaining the density distribution within two pixels will be described. The density data of the small pixels obtained are m5 and m6 in FIG.
Small scan lines including s1, s2, s3 ...
Small scan lines including s4, s5, s6 ... and s7, s
.., s9 ..., and is divided into 3 small scanning lines of the small pixels, and 1 in 2 pixel units of the original pixel.
This corresponds to scanning lines.

The arithmetic processing circuit 241 further calculates the average density of each small scanning line and the barycentric position of the original density data in one pixel of each small scanning line to obtain the average density data of the laser drivers 301A to 301C. Based on the gravity center position data, different selection signals are output from the output terminals OA to OC to the select circuits 250A to 250C. That is, s1, s2 of m6 adjacent to the pixel m5
The center of gravity of s3 (first small scan line) is m5, s3 and m6
Of the reference triangular wave φ ₀ ″ with no phase displacement when it is near the center of s1 and when the center of gravity is near the boundary between s2 and s1 of m6, the triangular wave φ whose phase advances by 1/6 cycle
_When the center of gravity is near the boundary between s2 and s3 of m6, the signal for selecting ₁ ″ is a triangular wave φ ₂ ″ whose phase is advanced by 2/6 period.
When the center of gravity is in the vicinity of the boundary between s2 and s3 of m5, the signal for selecting the triangular wave φ ₃ ″ whose phase is delayed by 1/6 period is in the vicinity of the boundary between s1 and s2 of m5. At this time, a signal for selecting the triangular wave φ ₄ ″ delayed by 2/6 period is output from the output terminal OA to the select circuit 250A. Similarly, s4, s5, s of pixels m5 and m6 are output from the output terminal OB.
The triangular wave selection signal of the second small scanning line determined by the density gravity center of 6 is sent to the select circuit 250B, and the triangular wave of the third small scanning line determined by the density gravity center of s7, s8, s9 of the pixels m5 and m6 from the output terminal OC. Select signal to select circuit 250
Output to C. FIG. 11 is a diagram showing an example of the relationship between the triangular waves having different phases and the pixel of interest.

Further, the arithmetic processing circuit 241 controls the light emission output of each of the laser drivers 301A to 301C according to the average density in the pixels m5 and m6 of each small scanning line. For example, S1, S
The semiconductor laser 301A is controlled to emit laser light in proportion to the average density between the two pixels S2 and S3. FIG. 17 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 discriminating circuit 231 discriminates whether the image data of the pixel of interest is the first threshold value or the second threshold value, and it is discriminated that it is the area outside the first and second threshold values. If the reference wave phase determination circuit 240
Does not output the triangular wave selected by, and the reference triangular waves φ ₀ ′, φ ₀ ″
Only the selection signal for outputting only the selection signal is sent to the selection circuits 250A to 250C, and the MTF correction circuit 232 is not operated. As a result, the image density data other than that read by the read circuit 220 is not corrected by the MTF correction circuit 232.
After being corrected by 33, it is sent to the modulation circuits 260A to 260C via the latch circuit 230.

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

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. When it is determined to be a character area of a character or a line drawing, the reference wave is selected from φ ₀ ′ to φ ₄ ″, which is the same as the image period, and the triangular wave selected by the reference wave phase determination circuit 240 for all color components is modulated. Circuit 260A-26
Select circuit 250A to 250C for selecting signal output to 0C
The MTF correction circuit 232 and the γ correction circuit 233 are inoperative, 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 with no change in color tone are reproduced. When it is determined that the region is the halftone region, the reference wave is selected from φ ₀ ″ to φ ₄ ″, which is twice the image period.

As a result, 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.

Converted to 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. I am using one. For convenience (R +
The density data of 2G + B) will be 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 the image and prevent the tint from changing. In addition, to determine the phase, G
It is preferable to use achromatic data having a component or a G component.

The data used in the image discrimination circuit 231 is also common to each color for the same reason.

In the modulation circuits 260A to 260C, the signal of the image density data input through the latch circuit 230 is modulated by the triangular wave which is the selected reference wave to generate a pulse width modulated signal, and these modulated signals are generated. Of three small continuous scanning lines in parallel (one line of the original image density data) is sent to the raster scanning circuit 300 as one unit.

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

FIGS. 10A to 10D are time charts showing the signals of the respective parts of the modulation signal generating circuit when the recording position is modulated in the area determined as the character area. The cycle of the reference wave is the same as that of the recording pixel.

In FIG. 10, (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 including a delayed wave which is sequentially output from the select circuit 250.

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

(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 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. 12A to 12D are time charts showing the signals of the respective parts of the modulation signal generating circuit when the recording position is modulated in the area determined as the halftone area. The cycle of the reference wave is twice the cycle of the recording pixel as shown in FIG.

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. 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 including delayed ones which are sequentially output from the select circuit 250.

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

(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 reference wave phase 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 that rotates at a predetermined speed by the index signal from the index sensor 439, and the image density signal modulated by the raster scanning method according to the cycle in the main scanning direction. Optical scanning is performed. Scan frequency 2204.72Hz
The effective print 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 the semiconductor laser array 431, as shown in FIG. 15, 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. 15, 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. 16, 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 process 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. The average density of each small scanning line obtained by the reference wave phase determination circuit 240 is used as density information as shown in FIGS. 20 and 21, and is input to the modulation circuits 260A to 260C by each reference wave and each laser driver 301A.
A configuration for modulating ~ 301C can also be 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. 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 developed by the first developing device 441, and the 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 cleaning device 470 which is retracted without being transferred to the recording paper, and is charged again on the photoconductor 401 by the scorotron charger 402.

Next, 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 and
Toner image (magenta toner image) is formed. In the same manner as described above, the third developing device 443 sequentially develops to form a third toner image (cyan toner image).
A three-color toner image is sequentially formed on the first layer. Finally, the fourth toner image (black toner image) is formed, and the photoconductor 1
A four-color toner image that is sequentially stacked on top is formed.

According to the image forming apparatus 400 of this embodiment, the photoconductor 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 onto the recording paper supplied from the paper feeding device by the action of the transfer device 462.

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 example, as a result of various experiments in which the value of the coefficient P of the RE process was 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 edge portion of the character or the line drawing is overemphasized. Therefore, the preferable value range of P is 0.3 to 0.7. Was found to be in the range. As a result, when the manuscript is a character or a line drawing, 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 this method, P can be used with a constant 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, preferably 0.9 to 0.4, and when the image is a halftone area, the value of P is small.
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. 18 is a graph showing an example of converting the relationship between the print position and the center of gravity in the main scanning direction, and FIG. 19 is a graph showing an example of converting the average density in the sub-scanning direction.

In the arithmetic processing circuit 241, the result obtained by the arithmetic processing from the image density data is converted into a recording position by using a ROM 245 which is built-in or externally attached according to a preset conversion formula as shown in FIG. Can be changed. Similarly, the average density in the sub-scanning direction can be converted as shown in FIG.

In particular, it is preferable to change the conversion ratio shown in FIGS. 18 and 19 depending on the case where the reference wave period is changed. When the period of the reference wave is large, it is preferable to attach importance to the gradation and reduce the ratio of recording position modulation or decrease the gradation γ.

Although the above-described image data flow has been described as a laser printer which outputs data once stored in the page memory 210, 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 reference wave cycle is switched for each pixel according to the image discrimination result, it is also possible to switch the entire screen uniformly by an external command such as a character or photo mode.

[0108]

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 The phase of the reference wave signal is selected from the image data subjected to the RE processing that distributes the density of the target pixel according to the distribution of the density data of adjacent pixels including the pixel, and the density signal of the target pixel is modulated by this reference wave. An excellent recorded image was obtained by generating the recording position modulation signal and not performing the recording position modulation on the low density portion and the high density portion. Further, image discrimination is performed by the image discrimination circuit, and in the case of the character area, the recording position modulation is performed by the reference wave having a short cycle, and in the halftone area, the recording position modulation is performed by the reference wave having a long cycle so that the color image is recorded. Therefore, it is possible to provide an excellent color image forming apparatus capable of improving the sharpness without changing the color tone of a color image formed from a scanner, CG, font data, or the like.

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. 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 block diagram showing an example of a reference wave phase determination circuit of the circuit of FIG.

FIG. 3 is a block diagram showing an example of a modulation circuit of the circuit of FIG.

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

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

FIG. 6 illustrates a pixel of interest for RE processing divided into 3 × 3 pixels, and P = 0.
FIG. 7 is a diagram showing an example of a case of setting 5.

FIG. 7 is a diagram showing an example of dividing a target pixel of RE processing into 2 × 2.

FIG. 8 is a diagram showing another example of a case where a pixel of interest in RE processing is divided into 2 × 2.

FIG. 9 is a diagram for explaining a phase displacement of a reference wave in the case of a character area.

FIG. 10 is a time chart showing each signal in the case of the character area of the modulation signal generation circuit of the embodiment of FIG.

FIG. 11 is a diagram for explaining a phase displacement of a reference wave in the case of an intermediate length region.

FIG. 12 is a time chart showing signals at respective parts in the halftone region of the modulation signal generation circuit of the embodiment of FIG.

FIG. 13 is a graph showing characteristics of the high γ photoconductor used in this example.

FIG. 14 is a cross-sectional view showing a specific configuration example of a high-γ photoconductor used in this example.

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

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

FIG. 17 is a graph showing an example of the relationship between the drive current of the semiconductor laser and the laser emission output.

FIG. 18 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. 19 is a graph showing an example of converting the average density of a small scan line in the sub-scanning direction.

FIG. 20 is a block diagram showing an image processing circuit according to another embodiment of the present invention.

21 is a block diagram showing the reference wave phase determination circuit of FIG. 20. FIG.

[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 230 Latch circuit 231 Image discrimination circuit 232 MTF correction circuit 233 γ correction circuit 240 Reference wave phase determination circuit 241 Operation processing circuit 250A to 250C Select circuit 260A to 260C Modulation circuit 280 Reference clock generation circuit 290 Triangular wave generation circuit 291 Delay circuit group 300 Raster scanning circuit 400 Image forming device

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tetsuya Nizuma 2970 Ishikawa-cho, Hachioji-shi, Tokyo Konica Stock Company

Claims (4)

[Claims] 1. High density pixel recording is performed based on density data of small pixels in the target pixel determined corresponding to density data of a pixel adjacent to the target pixel, and the density data of the target pixel is determined. An image forming apparatus that performs recording position modulation based on density distribution, wherein modulation in the main scanning direction is performed by selection of a reference wave signal and selection of a reference wave cycle according to an image discrimination result. 2. A means for determining the image, and a means for performing the recording position modulation using a reference wave having a period of 2 pixels when the image is determined to be a halftone area. The image forming apparatus according to claim 1. 3. The image discriminating device, and when the image region is discriminated as a character region, the recording position modulation is performed by using a reference wave having a period of a pixel unit. The image forming apparatus according to claim 1. 4. When the density data of the pixel is equal to or more than a first threshold value or less than a second threshold value, recording position modulation is performed from the determined density distribution. Image forming apparatus.