JP3136154B2 - Image forming device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD OF THE INVENTION

The present invention relates to an image forming apparatus such as a digital copying machine, and more specifically, reduces banding and image noise by combining multi-tone writing by one-dot modulation with a minute matrix with a small decrease in resolution. The present invention relates to an image forming apparatus that realizes high-quality image formation.

[0002]

[Prior art]

For example, in a writing process in a digital copying machine, its resolution and gradation are important factors. That is, in the copying process for any document including characters or photographs, fine resolution and gradation that faithfully reproduces halftones are desired. Conventionally, as a method for expressing gradation, an area gradation method using a dither matrix is disclosed in JP-A-54-144126 and JP-A-56-144126.
-17478, JP-A-57-76977 and the like.

However, in the area gradation method, since a pixel is composed of a plurality of dots and density is expressed by the number of dots written to the pixel, the resolution is reduced. In this case, in the binary writing method, assuming that the number of dots forming a pixel is N, the number of gradations can represent N stages of gradations without including the white portion of the background, but generally the resolution is Sex is reduced to 1 / N.

On the other hand, a 1-dot multi-gradation writing method for realizing multi-gradation without lowering resolution has been proposed. this is,
For example, in laser beam writing of an electrophotographic system, the density of one dot to be written is modulated. The light modulation method of the writing laser diode mainly includes a pulse width modulation method for modulating the exposure time and a power modulation method for modulating the exposure intensity. The pulse width modulation method is disclosed in JP-A-62-49776, and the power modulation method is disclosed in JP-A-64-1547. As one condition for improving the image quality of a digital copying machine, high-precision halftone reproduction is required. In order to achieve both resolution and gradation, the one-dot multi-gradation writing method is preferable.

[0005]

[Problems to be solved by the invention]

However, the one-dot multi-tone writing method has a drawback that banding (band-like density unevenness in the main scanning direction) is likely to occur. In a digital copying machine, banding that occurs in a halftone region is caused by various factors such as uneven driving or vibration of a photoconductor and uneven scanning pitch of a writing optical system. The banding mainly appears as a band-like density unevenness continuous in the main scanning direction. In particular, in the one-dot multi-gradation writing method, the scanning beam unevenness in the sub-scanning direction of the laser diode for exposure causes the footprints of the exposure beam in the intermediate exposure area to overlap, thereby causing banding.

[0006] Further, due to the increase in resolution, accuracy for banding is also required. In addition, it is often used now
In the one-dot multi-gradation writing method at about 400 dpi,
In the current electrophotographic process, there is a problem that image noise due to density unevenness occurs in the halftone solid portion regardless of the modulation method, and the halftone is not reproduced smoothly. The present invention has been made in view of the above, and an object of the present invention is to reduce banding and image noise and perform highly accurate image formation with a simple configuration.

[0007]

[Means for Solving the Problems]

In order to achieve the above object, the present invention provides an image forming apparatus which forms and outputs an image by one-dot multi-tone writing for image data input in a main scanning direction and a sub-scanning direction. Extracting means for extracting first and second read dot data adjacent to each other in the main scanning direction from the data; and adding an added value obtained by adding the first and second read dot data extracted by the extracting means to the main scanning direction. Distributing means for distributing the added value to the first and second write dot data while giving priority to the first write dot data when distributing the first write dot data to the first and second write dot data; Outputting means for outputting image data; and outputting the first and second read dot data in accordance with a density difference between the first and second read dot data extracted by the extracting means. Switching means for switching between outputting to a stage and outputting to the output means the first and second writing dot data to which the added value has been distributed by the distribution means. A forming apparatus is provided.

Further, in an image forming apparatus which forms and outputs an image by multi-tone writing of one dot with respect to image data input in the main scanning direction and the sub-scanning direction, in the sub-scanning direction of the input image data Extracting means for extracting the first and second read dot data adjacent to the first and second read dot data, and adding the sum of the first and second read dot data extracted by the extract means to the first and second in the sub-scanning direction. And a distribution unit that distributes the added value to the first and second write dot data while giving priority to the first write dot data when distributing the image data to the first and second write dot data. Means for outputting the first and second read dot data to the output means according to a density difference between the first and second read dot data extracted by the extraction means. Switching means for switching whether to output the first and second write dot data, to which the added value has been distributed by the distribution means, to the output means. Things.

[0009] The density difference between the first and second read dot data extracted by the extraction means is calculated by a subtraction circuit. Further, the subtraction circuit is configured to output the first signal extracted by the extraction unit.
And the first and second extracted based on the spatial frequency of the second read dot data or extracted by the extracting means.
The density difference is calculated based on the density difference between pixels of the read dot data.

The image forming apparatus according to the present invention operates as follows. First and second read dot data adjacent in the main scanning direction or the sub-scanning direction of the input image data are extracted, and the added value obtained by adding the extracted first and second read dot data to the main scan is used. When allocating the first and second write dot data to the first and second write dot data in the scanning direction or the sub-scanning direction,
Are distributed to the write dot data. Then, according to the density difference between the extracted first and second read dot data, the first and second read dot data are output to the output means or the first and second read dot data to which the added value is distributed are output. It switches whether to output the writing dot data to the output means.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings in the order of the configuration of a digital copying machine, the modulation method of a writing laser diode, image reading signal processing, image processing, and a two-dot multi-level circuit.

FIG. 1 shows a digital copying machine composed of a laser printer to which general laser writing means is applied and a document reading device. In FIG. 1, a contact glass 111 for placing a read original is illuminated by a light source 112, and reflected light from an image surface of the read original is transmitted through mirrors 113, 114, 115 and a lens 116 to a CCD image sensor 117. Is formed on the light receiving surface of The light source 112 and the mirror 113 are mounted on a traveling body 118 that moves on the lower surface of the contact glass 111 in parallel with the contact glass 111.

The main scanning is executed by the solid-state scanning of the CCD image sensor 117. Original image is 1 by CCD image sensor 117
The original is scanned dimensionally, and the entire surface of the document is scanned by moving the optical system (sub-scanning). In this example, the density of the reading process is set to 400 dpi in both the main scanning and the sub-scanning, so that a document of A3 size (297 mm × 420 mm) can be read.

Next, a laser printer constituting the digital copier will be described. The document reading device and the laser printer may be integrally configured (this embodiment) or may be separately configured and only electrically connected. The laser printer is integrally configured with a laser writing system, an image reproducing system, a paper feeding system, and other systems.

As shown in FIGS. 1, 2, and 3, the laser writing system includes a laser output unit 219, an imaging lens group 120, and a mirror 121. The laser output unit 219 is provided with a laser diode LD as a laser light source, and the writing unit is provided with a polygon mirror (polygon mirror) 219a that rotates at a high speed and a constant speed by a motor. The laser light output from the laser writing system is applied to the photosensitive drum 122 provided in the image reproducing system.

As shown in FIG. 1, around the photosensitive drum 122, a charger 1 for uniformly charging the photosensitive drum 122 is provided.
23, a developing unit that visualizes the formed electrostatic latent image
125, a transfer charger 126 for transferring the image of the photosensitive drum 122 to the transported recording paper, a separation charger 127 and a separation claw 128 for separating the recording paper from the photosensitive drum 122,
A cleaning unit 129 for cleaning the surface of the photosensitive drum 122 after the transfer process is provided.

A beam sensor 330 that generates a main scanning synchronization signal (PMSYNC) is arranged at a position near one end of the photosensitive drum 122 where the laser beam is irradiated (see FIG. 3). Reference numeral 131 denotes a transport belt, 132 denotes a fixing unit, 133 and 134 denote paper feed cassettes, 135 and 136 denote paper feed rollers, and 137 denotes a registration roller.

The operation of the above configuration will be described. The surface of the photosensitive drum 122 is uniformly charged to a high potential by the charger 123. When the surface of the photosensitive drum 122 is irradiated with the laser beam, the irradiated portion has a lower potential. Since the laser light is ON / OFF controlled in accordance with the black / white of the recording pixels, a potential distribution corresponding to the recorded image, that is, an electrostatic latent image is formed on the photosensitive drum 122 by the irradiation of the laser light.

When the portion on which the electrostatic latent image is formed passes through the developing unit 125, toner adheres according to the level of the potential, and a toner image is formed by visualizing the electrostatic latent image. The recording paper is conveyed at a predetermined timing to the portion where the toner image is formed, and overlaps the toner image.

After the toner image is transferred to the recording paper by the transfer charger 126, the recording paper is separated from the photosensitive drum 122 by the separation charger 127 and the separation claw 128. The separated recording paper is conveyed by a conveyance belt 131, is thermally fixed by a fixing unit 132 having a built-in heater, and is then discharged to a paper discharge tray (not shown).

In the digital copying machine shown in FIG. 1, the paper feed system is composed of two systems. One sheet feeding system is provided with a sheet feeding cassette 133, and the other sheet feeding system is provided with a sheet feeding cassette 134. The recording paper in the paper feed cassette 133 is fed by the paper feed roller 135. The recording paper in the paper cassette 134 is
Fed by 36.

The fed recording paper is temporarily stopped in a state where it is in contact with the registration roller 137, and is conveyed to the photosensitive drum 122 at a timing synchronized with the progress of the recording process. Although not shown, each paper feed system has a size detection sensor for detecting the size of the recording paper in the cassette.

FIG. 4 is a block diagram of a power modulation system of a laser diode (LD) according to an embodiment of the present invention. And input to the second current conversion means 441.

The command signal of the light emission level in the first current converting means 440 is converted into light emission level instruction signal current (output current) I _S in response to the intensity. Output current I _S of the first current converting means 440, the input current (I _S of the difference between a photoelectric current I _L which is proportional to the optical output P ₀ generated in the light receiving element 442 of the laser diode LD1 -
I _L ) and input to the current amplifier 443.

The current amplifier 443 outputs an output current A (I _S −I _L ) obtained by multiplying the input current (I _S −I _L ) by A. On the other hand, light emission level instruction signal by the second current converter 441 is converted into an output current I ₁ to emit the set amount of light P _S. I ₁ + A (I _S −I _L ), which is the sum of the output current I ₁ and the output current A (I _S −I _L ) of the current amplifier 443, is the forward current of the laser diode LD1.

As described above, the laser diode LD 1 has the forward current I
Obtaining a light output P ₀ determined by _{1 + A (I S -I L} ). That is, the following relational expression holds. P ₀ = P ｛I ₁ + A (I _S −I _L )｝ P: light output of laser diode LD1-function representing forward current characteristic

Here, since I ₁ is set so that I _S ≒ I _L , it can be approximated as follows. _{P 0 = P (I 1)} + [δp / δI] I = I1 · A (I S -I L) = P S + η · A · (I S -I L)

Assuming that the radiation sensitivity S of the light receiving element and the coupling efficiency with the laser diode LD1 are δ, P ₀ = P _S + η · A · (I _S −P ₀ · S · δ), and P ₀ = P _S / (1 + ηδSA) + ηA · I _S / (1 + ηδSA).

Assuming that the crossover frequency of the photoelectric negative feedback loop is f ₀ , the step response of the optical output P ₀ can be expressed recently as follows. P ₀ = I _S / δS + {P _S −I _S / δS} · exp (−2πf ₀ t)

The P _S set by the second conversion means 441 is set to be equal to I _S / δS. For example, when P _S fluctuates by 5% due to the droop characteristic, f ₀ = 40 MHz. even, time is about 10ns required for the error of P ₀ is 0.4% or less.

The error of the total light quantity (integrated value of light output ∫P _OUT ) from immediately after changing the light output P ₀ to the set time τ ₀ is 0.4.
% The crossover frequency f ₀ to the following case of the tau _{0 =} 50 ns, may be a f ₀ ≧ 40 MHz, can be easily realized if the degree of cross-frequency. As described above, this method enables high-speed, high-precision,
A high-resolution laser diode control method can be realized.

By power modulating the laser diode LD 1 using this method, 256 kinds of analog signals are input to the light emission level command signal, and one dot is output by the laser printer.
Image output of 256 gradations is realized.

Next, a laser diode (LD) power modulation system according to another embodiment using a plurality of constant current power supplies will be described. The drive control method of the laser diode in the present embodiment is based on the forward current (I) of the laser diode shown in FIG.
And the light emission intensity (L) (IL characteristic). The IL characteristic of this laser diode is based on the threshold current (It
h) The above-described forward current is almost linear, and the differential quantum efficiency (n) at that time is treated as constant.

In the control method, as shown in FIG. 6, the forward current is driven by the total current of a plurality of constant current sources 641, 642, 643, and 644, and is driven by switches 645, 646, and 647 according to write data. To switch. A bias current larger than the threshold current is supplied by the constant current source 641, and the constant current sources 642, 643, and 644 weighted so as to have a current value of 1: 2: 4. Control to a value. The current values at that time are I ₁ , I ₂ , I _{3 respectively} , and switches 645, 646,
Minimum value of the bias current is not driven 647 is I _0. Therefore, the light emission intensity (light amount) by each of the currents I _{0 to} I ₃ is the fifth.
Amount by all combinations of current I ₀ ~I ₃ were as shown in the figure eight to L ₀ ~L ₇ is obtained equal light amount difference.

The setting procedure at that time is executed as follows. (A) The laser diode emission intensity range is set to P _{0 to} P _max (where P ₀ ≒ 0). (B) Laser diode minimum emission intensity P ₀ ← Determine laser diode forward current I ₀ . (C) The laser diode maximum emission intensity P _max ← I _max is determined by the laser diode forward current I ₀ + I _max . (D) I ₁ = (1/7) · I _max , I ₂ = (2/7) · I _max , I ₃ =
(4/7) ・ I _max .

As described above, assuming that the number of constant current sources is n, a light emission intensity of 2 ⁿ can be obtained. For example, if eight constant current sources are used and switching is performed using 8-bit light emission data, 256 laser diodes Is obtained.

Image Read Signal Processing FIG. 7 shows a detailed block diagram of the image read signal processing. The CCD (Charge Coupled Device) 117 can read about 5000 pixels and 400 dpi, and simultaneously reads reflected light of the document in the main scanning direction. The optical data stored in the CCD 117 is converted into an electric signal (photoelectric conversion), waveform correction such as clamping, amplification, and A / D conversion are performed, and the digital signal is output to a IPU (image processing device) as a 6-bit digital signal.

More specifically, the analog data output of the CCD 117 is subjected to photoelectric conversion (photoelectric conversion), and then divided and output into two systems of EVEN and ODD for high-speed transfer.
Each is amplified (signal amplified) in 703 and input to a switching IC 703 composed of an analog switch. here,
The signal is synthesized into a serial analog signal (signal synthesis). The analog signal synthesized by the switching IC 703 is amplified (variable amplification) by the amplifier 704 and is A / D converter
Enter 705. The image transfer speed of one pixel after composition is about 10M
In Hz, the A / D converter 705 converts the signal into a 6-bit 64 gradation digital signal in synchronization with the signal (signal digitization). In addition, the (variable) amplifier 704 reads a reference white plate before scanning the original and controls the amplification degree to an appropriate value in order to correct the light amount fluctuation of the exposure fluorescent lamp.

Image Processing The digital signal for each pixel indicating the density of the original is obtained by IPU
(Image processing device) The image is input to 800 and subjected to image processing. IPU
FIG. 8 shows the flow of image processing by 800. The IPU 800 is composed of a plurality of LSIs and executes the following control based on the image processing in addition to the image processing.

I. Shading Correction Since a linear light source such as a fluorescent lamp is used and light is condensed by a lens, the light amount becomes maximum at the center of the CCD 117 and decreases at the end. Further, the CCD 117 has a variation in sensitivity of each element. In both cases, the original read data is corrected based on the reference white plate read data for each pixel.

Ii. MTF Correction In an optical system using a lens or the like, the read output by the CCD 117 is read as if the peripheral pixel information affected the performance of the lens or the like. Therefore, when one piece of pixel data is obtained, an image having high reproducibility is obtained by performing correction based on the peripheral pixel level.

Iii. Magnification in the main scanning direction In this embodiment, the image reading and writing resolutions are the same 400 dpi, but the reading pixel frequency is about 10 MHz.
Since z and the writing pixel frequency are different at about 12 MHz, frequency conversion is performed. Clock conversion is realized by reading / writing of a two-line memory, and main scanning magnification is calculated by calculation using peripheral pixel data in the main scanning direction.

Iv. Γ Correction Both the density data conversion characteristic of the optical system using the CCD 117 (γ characteristic of the scanner) and the density reproduction characteristic of the laser printer using the electrophotographic method (γ characteristic of the printer) are not linear, and are not linear. Does not faithfully reproduce the original density.
Although the above may be individually corrected, the present image forming apparatus executes a conversion process in consideration of both.
Also, at the time of manual density adjustment, density adjustment is realized by changing this value.

In addition to the above, the IPU (image processing apparatus) 800 also performs control of AGC and the like, image conversion such as masking, trimming, mirroring, black-and-white inversion, detection of document size and density, detection of images such as markers, and the like. .

In the present embodiment, 1 is obtained by power modulation of a laser diode.
This is a combination of a 256-dot dot output and a matrix of two dots in the main scanning and backward scanning directions. FIG. 9 (a) shows a 1 × 2 matrix and FIG. 9 (b) shows a 2 × 1 matrix optical writing system. In the low density portion, the exposure power is increased from one of the dots, and when the increase value is reached, the exposure power of the next dot is increased.

The density reproduction is executed using two dots in the main scanning or sub-scanning direction as target pixels. The reading density of the CCD 117 is proportional to the amount of received light. Therefore, the amount of light received by the CCD 117 is linear with respect to the original reflection density, the density data of two dots is added to the digital value, the added value is subjected to γ conversion, and converted to write density data by the above method.

As a result, two dots in the main scanning and sub-scanning directions are equivalent to 51 dots.
Two gradations are realized. The chart of the formed halftone density region is generated as shown in FIG. In the figure, the density is filled from the EVEN dots. 1 × 2 of FIGS. 10A and 10C for performing area gradation in the sub-scanning direction
The matrix is a horizontal line tone in a continuous intermediate density area, and the 2 × 1 matrix in FIGS. 10B and 10D in which area gradation is performed in the main scanning direction is a vertical line tone in a continuous intermediate density area.

FIGS. 10 (c) and (d) show FIGS. 10 (a) and (b), respectively.
Are alternately changed in writing phase, two dot lines are formed in the main scanning and the sub-scanning, and an image of 100 lines is formed. As a result, the number of gradations does not change, but the lines are concentrated,
The apparent resolution is reduced by half.

FIG. 11A is a block diagram of a one- and two-dot multi-valued circuit, and is a line memory connected in series for inputting a 6-bit signal input from a scanner. 1101, 1102, rats 1103, 1104, adders 1105 connected to the line memories 1101, 1102 and latches 1103, 1104 via switches SW1 to SW5, respectively, and connected to the adder 1105
It comprises a ROM 1106, a delay circuit 1110, and a subtraction circuit 1111. Reference numerals 1120 to 1125 denote data lines. The output of the ROM is output to a printer as an 8-bit data signal.

Hereinafter, i.1 × 2 matrix, ii.2 × 1 matrix, ii
i. It will be described in detail separately for dot concentration. i.1 × 2 matrix When area gradation is executed with two dots in the sub-scanning direction (1
(× 2 matrix) uses two line memories 1101 and 1102 to delay read data for two main scanning lines.
Thereafter, the two 6-bit data are added by the adder 1105, and the 7-bit data is input to the ROM 1106 for γ conversion. In ROM1106, one table consists of 256 bytes, the first 128 bytes of which are EVEN and the latter 128 bytes are ODD.
Data.

The first addition data is input to the address bus of the ROM 1106, and EVEN data indicated by the address is output as write data. The same data is added in the next line, and the ODD data is output from the data bus as write data. EVEN,
The ODD is switched in synchronization with the line cycle (PMSYNC). Thereafter, the process proceeds to the next two dots and the processing is sequentially repeated.

In the block diagram of the one- and two-dot multi-level circuit shown in FIG. 11A, the switch SW1 and EVEN / ODD are set to the main scan 1
Switch for each line. FIG. 11B is an explanatory diagram showing a combination (1 × 2 matrix) with the area gradation in the sub-scanning direction. The two sub-scanning dots for reading correspond to the two sub-scanning dots for writing.

Ii. 2 × 1 Matrix When area gradation is performed with two dots in the main scanning direction (2
X1 matrix) uses two latches 1103 and 1104 to delay read data for two dots in the main scanning direction.
Hereinafter, as in the case of the 1 × 2 matrix, the addition processing, γ
Execute the conversion process and output the write data. EVEN, ODD
Is executed in synchronization with the write dot cycle (WRITECLK). Thereafter, the process proceeds to the next two dots and the processing is sequentially repeated.

In the block diagram of the one- and two-dot multi-valued circuit shown in FIG. 11A, the switch SW2 and EVEN / ODD are switched every writing clock. FIG. 11 (c) is an explanatory diagram showing a combination (2 × 1 matrix) with the area gradation in the main scanning direction. The two main scanning dots for reading correspond to the two main scanning dots for writing.

Iii. Concentration of dots The phase in writing is converted to concentrate the dots.
In order to form an image of 100 lines, the switching cycle of EVEN and ODD is performed by dividing each by two. As described above, loss of gradation information does not occur in all modes. The 1 × 2 and 2 × 1 matrices can be separately selected or designated.

Switching between the one-dot multi-tone writing method and the method combining the one-dot multi-tone writing and the minute matrix is performed by inputting the document selection key 1 of the operation unit shown in FIG. When the document selection key 1 is pressed, the mode is switched between the automatic document mode, the character mode, and the character / photo mode, and the mode indicators 4, 3, and 2 are turned on. In the automatic document mode, one-dot multi-tone writing and two-dot multi-tone writing are switched for each pixel according to document information. The character mode is one-dot multi-gradation writing with little resolution degradation, and the character / photo mode is one in which halftones are reproduced smoothly and resolution degradation is small.
Processing is performed by a combination of dot multi-gradation writing and a fine matrix, and as a result, high-quality image formation is realized for a document in which characters, photographs, or both are mixed.

Next, the automatic document mode will be described. Since the method of the present embodiment is a method that includes the area gradation only in the main scanning or sub-scanning direction, the spatial frequency (for example, the area gradation Density distribution between dots to be executed)
Judging the original easily, the part with high spatial frequency (mainly text) is multi-tone 1-dot writing processing, and the part with low spatial frequency (mainly photograph)
Executes a combined process of one-dot multi-tone writing and a minute matrix.

In the automatic original mode, the determination of the switching between the one-dot multi-tone writing process based on the spatial frequency of the image information and the process combining the one-dot multi-tone writing and the minute matrix is described in FIG. 11A. Subtraction circuit 11 in 2 multi-level circuit
Run in 11. The steepness of the density change is determined from the difference between the density data of the dots that execute the area gradation. The subtraction circuit 1111 is composed of logic such as an adder, the comparator determines the density difference as a reference value, selects data input to the ROM 1106 from the detected value, selects write data, and checks the density conversion table. Switch according to the upper address.

In FIG. 11A, density data of a target pixel of two dots used for area gradation is input to the subtraction circuit 1111 from the data lines 1124 and 1125. The subtraction circuit 1111 obtains the density difference. For example, if the difference is 16 or more in 64 steps, the difference is determined to be the edge portion of the image, and the one-dot multi-gradation writing process is executed. The threshold for performing the determination is variable depending on the system, and the value can be changed depending on the density even in the same original.

In the subtraction circuit 1111, the switching line 11
SW5 is operated by 20 to switch the data lines 1122 and 1123. The data line 1122 is 6-bit data for one-dot processing, and the data line 1123 is 7-bit data for processing for performing one-dot multi-tone writing for two dots.

At the same time, the conversion table corresponding to the processing is addressed by the γ table selection line 1121. Delay circuit 1110
Are line memories 1101, 1102 or latches 1103, 1104
Is corrected, and the dots at the time of process switching are synchronized.

As described above, determination is made in real time based on image data, and write data is generated. FIGS. 13 (a), (b) and (c) show examples of the γ conversion table used in the present apparatus. FIG. 13 (a) shows a γ conversion table for one-dot multi-tone writing, FIG. 13 (b) shows a γ conversion table used for a 1 × 2 matrix, and FIG. 13 (c) shows a γ conversion table used for a 2 × 1 matrix. γ
It is a conversion table.

All are set so that the copy density is substantially equal to the document density. The two-dot multi-level writing unit exposes only one EVEN dot up to the intermediate density, increases the input data, and
When the dot reaches the maximum value, the exposure intensity of the ODD dot is increased. As a result, the dots are concentrated while maintaining the density information of the two dots. Further, γ can be freely controlled by this table, and the way of increasing 2 dots can be changed. Further, since the density output characteristic changes depending on the combination method with the area gradation, the γ conversion data is selected or the γ conversion data is rewritten by using the RAM for the conversion table.

Generally, the inverse conversion of the printer's γ characteristic represented by the print density with respect to the writing exposure light amount is converted into a table value, so that the γ characteristic of the printer alone can be made linear. The one- and two-dot multi-valued circuit shown in FIG.
It converts the image data for each dot from the scanner and sends it to the writing system.

In this embodiment, since output density characteristics for input data are different from each other, independent density conversion is executed, and both processes are set so as to faithfully reproduce the original density. Therefore, the switching density point is determined from the read data.

In addition, as in the present embodiment, independent density conversion is performed for each method, and the converted contents are configured so that the writing density is the same for the same input data. There is no density jump or reversal of the switched part and smooth reproduction is achieved.

Using the method of performing one-dot multi-tone writing for one dot constituting one pixel according to the present embodiment, an image at each density is output, and the effect of the occurrence of banding can be confirmed.

For image output, banding is generated by adding 1% rotation speed unevenness to the photosensitive drum at a pitch of 2 mm. In the sensory evaluation, as shown in FIG. 14, banding was greatly reduced in an image in which one dot multi-gradation was applied to two dots as compared with a one-dot multi-gradation image by power modulation of a laser diode. . As can be seen from FIG. 14, the occurrence of banding differs depending on the density of the halftone even in the area gradation of the 1 × 2 and 2 × 1 matrices.

As described above, in this embodiment, a portion having a high spatial frequency is written by one dot multi-gradation without degradation in resolution, and a low portion is reproduced by one dot multi-gradation writing in which halftone is reproduced smoothly and resolution degradation is small. And a micro matrix.

In the present embodiment, even if the switching detection error occurs, since the processing is performed for at most a few dots, there is little difference in images due to the processing, no change in density occurs, and image deterioration rare. Also, in comparison with the conventionally performed pictographic character separation processing, the present discrimination method has the same fineness as the character part in the pictorial part.
Dot processing is performed, resulting in excellent resolution.

[0071]

【The invention's effect】

As described above, according to the image forming apparatus of the present invention, of the input image data, the first and second read dot data adjacent in the main scanning direction or the sub scanning direction are extracted and the extracted first and second read dot data are extracted. When distributing an addition value obtained by adding the first and second read dot data to the first and second write dot data in the main scanning direction or the sub-scanning direction, the first write dot data is added with priority. The values are distributed to the first and second write dot data, and the first and second read dot data are output to the output means according to the density difference between the extracted first and second read dot data. Or outputting the first and second write dot data to which the added value is distributed to the output means, so that when the read dot data is output to the output means, the resolution is high, and , Add Gradation is high in the case of outputting the write dot data subjected to distribution of values in the output unit, resulting in possible reduced binding and image noise,
In particular, since the added value is distributed to the same number of write dots as the number of extracted read dots, it is possible to achieve highly accurate image formation with a simple configuration.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a configuration of a digital copying machine to which an image forming apparatus according to the present invention is applied.

FIG. 2 is an explanatory diagram showing a configuration of a laser writing system in the digital copying machine shown in FIG.

FIG. 3 is an explanatory diagram showing a configuration of a laser writing system in the digital copying machine shown in FIG.

FIG. 4 is a block diagram showing a power modulation method of a laser diode (LD) used in the digital copying machine shown in FIG.

FIG. 5: Forward current (I) and emission intensity (L) of a laser diode
6 is a graph showing the relationship (IL characteristic) with the graph.

FIG. 6 is a circuit diagram showing a control method of a laser diode.

FIG. 7 is a block diagram illustrating each unit that executes image reading signal processing.

FIG. 8 is a block diagram illustrating a flow of image processing performed by the image processing apparatus.

FIGS. 9A and 9B are explanatory diagrams showing a 1 × 2 matrix and 2 × 1 matrix optical writing system, respectively.

FIGS. 10 (a), (b), (c), and (d) are chart diagrams showing a halftone density region to be formed.

FIG. 11 (a) is a view showing the components 1 and 2 used in the image forming apparatus according to the present invention.
FIG. 4B is a block diagram of a dot multi-valued circuit, FIG. 4B is an explanatory diagram showing a combination with an area gradation in the sub-scanning direction, and FIG. 4C is an explanatory diagram showing a combination with an area gradation in the main scanning direction.

FIG. 12 is an explanatory diagram showing a part of the operation unit.

FIGS. 13A, 13B, and 13C are tables each showing γ conversion.

FIG. 14 is a graph showing a ranking of banding.

[Explanation of symbols]

 117 CCD image sensor 122 Photosensitive drum 219 Laser output unit 330 Beam sensor 440 First current converting means 441 Second current converting means 442 Light receiving elements 641, 642, 643, 644: Constant current source 645, 646, 647: Switch 702, 704: Amplifier 703: Switching IC 705: A / D converter 800: IPU (image processing device) 1101, 1102: Line memory 1103, 1104 Latch 1105 Adder 1106 ROM 1110 Delay circuit 1111 Subtraction circuit 1120 Switching line 1121 γ table selection line 1122 to 1125 Data line SW1 to SW5 Switch

Continuation of the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) H04N 1/40-1/409 H04N 1/46 H04N 1/60

Claims (5)

(57) [Claims] 1. An image forming apparatus for forming and outputting an image by multi-tone writing of one dot with respect to image data input in a main scanning direction and a sub-scanning direction, wherein the main scanning direction of the input image data is Extracting means for extracting first and second read dot data adjacent to the first and second read dot data; and adding the first and second read dot data extracted by the extract means to the first and second read dot data in the main scanning direction. When allocating the first and second write dot data to the first and second write dot data,
A distribution unit for distributing the first and second read dot data extracted by the extraction unit; a distribution unit for distributing the first and second read dot data extracted by the extraction unit; Switching means for switching whether to output the read dot data to the output means or to output the first and second write dot data to which the added value has been distributed by the distribution means to the output means. An image forming apparatus comprising: 2. An image forming apparatus for forming and outputting an image by multi-tone writing of one dot with respect to image data input in a main scanning direction and a sub-scanning direction, wherein: Extracting means for extracting first and second read dot data adjacent to the first and second read dot data; and adding the sum of the first and second read dot data extracted by the extract means to the first and second in the sub-scanning direction. When allocating the first and second write dot data to the first and second write dot data,
A distribution unit for distributing the first and second read dot data extracted by the extraction unit; a distribution unit for distributing the first and second read dot data extracted by the extraction unit; Switching means for switching whether to output the read dot data to the output means or to output the first and second write dot data to which the added value has been distributed by the distribution means to the output means. An image forming apparatus comprising: 3. The image forming apparatus according to claim 1, wherein a difference in density between the first and second read dot data extracted by the extracting unit is calculated by a subtraction circuit. 4. The image according to claim 3, wherein said subtraction circuit calculates said density difference based on a spatial frequency of said first and second read dot data extracted by said extraction means. Forming equipment. 5. The apparatus according to claim 3, wherein the subtraction circuit calculates the density difference based on a density difference between pixels of the first and second read dot data extracted by the extraction means. The image forming apparatus as described in the above.