JP3078313B2 - Image forming device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus applied to a digital copying machine or the like, and more particularly, to a method of reducing resolution in multi-tone writing by one-dot modulation. The present invention relates to an image forming apparatus that realizes high-quality image formation by reducing banding and image noise by combining a small number of small matrices.

[Conventional technology]

For example, in a writing process in a digital copying machine, its resolution and gradation are important factors. That is, fine resolution and gradation that faithfully reproduces halftones are desired in the copying process for any document including characters or photographs.

Conventionally, there is an area gradation method using a dither matrix as a method of expressing gradation (see Japanese Patent Application Laid-Open Nos. 54-144126, 56-17478, and 57-76977). Disclosed).

However, in the area gradation method, since a pixel is constituted by 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 constituting a pixel is N, the number of tones is expressed in N levels without including the background white portion, but in general, the resolution is It drops to 1 / N.

On the other hand, 1 that realizes multiple gradations without lowering resolution
A dot multi-tone writing method has been proposed.

This modulates the density of one dot to be written, for example, in an electrophotographic laser beam writing. The light modulation method of the writing laser diode mainly includes a pulse width modulation method for modulating the exposure time,
There is 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.

[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 occurring in a halftone area is caused by drive unevenness or vibration of a photoconductor, scan pitch unevenness of a writing optical system, and the like. The banding mainly appears as a band-like density unevenness continuous in the main scanning direction. In particular, in the one-dot multi-tone writing method, the scanning beam unevenness in the sub-scanning direction of the exposure laser diode causes the footprint of the exposure beam in the intermediate exposure area to overlap, thereby causing banding.

Further, with the increase in resolution, accuracy for banding is also required.

In addition, at around 400 dpi, which is currently widely used,
In the multi-tone dot writing method, in the current electrophotographic process, there is a problem that image noise due to density unevenness occurs in a halftone solid portion irrespective of the modulation method, and the halftone is not reproduced smoothly.

Further, in a method in which a multi-tone writing by one-dot modulation for reducing banding and image noise is combined with a small matrix with a small decrease in resolution, the resolution in the direction in which the area gradation is executed is reduced. However, there is a problem that the possibility of visualizing fine lines and cracks in characters is high.

The present invention has been made in view of the above, and in a method in which multi-tone writing by one-dot modulation is combined with a small matrix with a small decrease in resolution, based on the type, direction, and size of the original, Switching the direction in which the area gradation is performed, suppressing a decrease in resolution in the direction in which the area gradation is performed, and preventing the visualization of fine lines and cracks in characters to realize high-quality image formation. As a first object.

A second object is to reduce banding in all halftone densities and realize high-quality image formation.

[Means for solving the problem]

In order to achieve the above object, the present invention provides a first means for executing a one-dot multi-tone writing process on a plurality of dots constituting only one pixel at the time of printing and arranged only in the main scanning direction; A second means for executing a one-dot multi-tone writing process for a plurality of dots arranged only in the sub-scanning direction which constitute one pixel during printing, and switching between the first means and the second means An image forming apparatus comprising a switching input unit is provided.

Further, a first means for executing a one-dot multi-tone writing process for a plurality of dots arranged only in the main scanning direction which constitute one pixel at the time of printing, and a sub-scanning which constitutes one pixel at the time of printing Second means for executing a one-dot multi-tone writing process for a plurality of dots arranged only in the direction, switching input means for switching between the first means and the second means, direction and size of the document Wherein the switching input means switches between the first means and the second means based on the direction and size of the document detected by the detecting means. Is provided.

Further, a first means for executing a one-dot multi-tone writing process for a plurality of dots arranged only in the main scanning direction which constitute one pixel at the time of printing, and a sub-scanning which constitutes one pixel at the time of printing A second means for executing a one-dot multi-tone writing process for a plurality of dots arranged only in the direction; and a switching input means for switching between the first means and the second means. The means switches to the second means when the density is lower than the specific reference density, and switches to the first means when the density is equal to or higher than the specific reference density.

Further, it is preferable that the specific reference density is an intermediate density (ID = 0.3) having a reflectance of 50% or a vicinity thereof.

Further, density conversion data storage means for storing density conversion data for equalizing the image density of the original and the image density printed on the recording paper, and are arranged only in the main scanning direction constituting one pixel during printing. Means for executing a one-dot multi-tone writing process on the plurality of dots based on the density conversion data stored in the density conversion data storage means, and a sub-scanning direction forming one pixel during printing. A second means for executing a one-dot multi-gradation writing process based on density conversion data stored in the density conversion data storage means for a plurality of dots arranged only in the first means; An image forming apparatus comprising: a switching input unit that switches between the two units.

(Operation)

The image forming apparatus according to the present invention switches the direction in which the area gradation is executed based on the type or direction, size, or reference density of the document.

In addition, it is set so that the density change at the time of switching is continuous.

Further, banding is reduced at all halftone densities by combining a multi-tone writing by one-dot modulation with a minute matrix having low resolution.

〔Example〕

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 multilevel circuit.

FIG. 1 shows a digital copying machine comprising 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 performed 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 for both main scanning and sub-scanning, so that a document of A3 size (297 mm × 420 mm) can be read.

Next, a laser printer constituting the digital copying machine 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 the like.

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 charging 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 disposed 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 charging charger 123. When the laser beam is irradiated on the surface of the photosensitive drum 122, the irradiated partial potential decreases. 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 surface of the photosensitive drum 122 by the irradiation of the laser light.

When the portion where 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 feeding 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 while being 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 one embodiment of the present invention. Is input to the current conversion means 441.

Command signal of the first current converting means 440 in the light emitting level is converted into light emission level instruction signal current (output current) I _S in response to the intensity. The output current I _S of the first current conversion means 440 is an input current (I _S −) that is different from the photovoltaic current I _L that is proportional to the light output P _O generated in the light receiving element 442 of the laser diode LD1.
I _L ) and input to the current amplifier 443.

It said current amplifier 443 outputs an input current (I _S -I _L) of A multiplied by the output current A (I _S -I _L).

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.

Thus, the laser diode LD1 has the forward current I
Obtain an optical output P _O as determined by _{1 + A (I S -I L} ).

 That is, the following relational expression holds.

P _O = P ｛I ₁ + A (I _S −I _L )｝ P: A function representing the optical output-forward current characteristic of the laser diode LD1. Here, I ₁ is set so that I _S ≒ I _L Therefore, it can be approximated as follows.

_{P O = P (I 1)} + [δp / δI] I = I1 · A (I S -I L) = P S + η · A · (I S -I L) radiation sensitivity of the light receiving element S, a laser diode LD1 placing the coupling efficiency [delta] _{and, P O = P S + η} · a · is expressed as _{(I S -P O · S ·} δ), P O = P S · (1 + ηδSA) + ηA · (1 + ηδSA) · I Becomes _S.

Placing the crossover frequency of the photoelectric negative feedback loop with f _0, step response of the optical output P _O can be expressed approximately as follows.

P _O = I _S / δS + {P _S −I _S / δS} · exp (−2πf ₀ t) P _S set by the second conversion means 441 is set to be equal to I _S / δT. but, for example, if P _S varies 5% by Drooping special, _{0 =} even a 40 MHz, the time required for the error of P _O is 0.4% or less becomes approximately 10 ns.

Also, the error of the total light quantity (integrated value of light output ∫P _OUT ) from immediately after changing the light output P _O 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 modulating the power of the laser diode LD1 using this method, 256 kinds of analog signals are input to the light emission level command signal.
Image output of 6 gradations is realized.

Next, a laser diode (LD) power modulation method 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 a total current of a plurality of constant current sources 641, 642, 643, and 644, and is switched by switches 645, 646, and 647 according to write data. . 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) Set in the laser diode light emission intensity range P _{0 to} P _max (however, 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) I1 = (1/7) · I max, I2 = (2/7) · I max, I3 =
(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 types of laser diode exposure outputs 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 separated into two systems of EVEN and ODD for high-speed transfer.
Each is amplified (signal amplified) in 3 and input to the switching IC 703 composed of an analog switch. Here, the signal is synthesized with a serial analog signal (signal synthesis). The analog signal synthesized by the switching IC 703 is amplified (variably amplified) by the amplifier 704 and the A / D converter 70
Enter 5 The image transfer speed of one pixel after synthesis is about 10MHz
Then, in synchronization with this, the A / D converter 705 converts the digital signal into a 6-bit digital signal of 64 gradations (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 original density
(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 Because a linear light source such as a fluorescent lamp is used and the 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 characteristics of the optical system using the CCD 117 (γ characteristics of the scanner) and the density reproduction characteristics of the laser printer using the electrophotographic method (γ characteristics of the printer) are not linear. Is not faithfully reproduced.
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 this 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 sub-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 the exposure power of the next dot is increased when the exposure power reaches the maximum value.

Density reproduction is executed using two dots in the main scanning or sub-scanning direction as a target pixel. 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, 51 for two dots in the main scanning and sub-scanning directions.
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
Matrix performs horizontal tone in a continuous intermediate density area and area gradation in the main scanning direction, 2 × in FIGS. 10 (b) and (d).
One matrix is based on a vertical line 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. 11 (a) is a block diagram of a two-dot multi-valued circuit, in which line memories 1101 and 1102 connected in series for inputting a 6-bit signal input from a scanner, and a latch 1103. , 1104, an adder 1105 connected to the line memories 1101, 1102 and latches 1103, 1104 via switches SW1 to SW4, respectively, and an RO connected to the adder 1105.
M1106. The output from 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 the ROM 1106, one table is composed of 265 bytes, the first 128 bytes of which are EVEN, and the latter 128 bytes are OD.
This is D data.

The first addition data is input to the address bus of the ROM 1106, and the 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 two-dot multi-valued circuit shown in FIG. 11A, the switches SW1 and EVEN / ODD are switched for each main scanning line, and the switches SW3 and SW4 are connected to the line memory 11.
The upper part is set so that the data from 01 and 1102 is selected.

FIG. 11B is an explanatory diagram showing a combination (1 × 2 matrix) with the area gradation in the sub-scanning direction. Two dots in the sub-scanning for reading correspond to two dots in the sub-scanning 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 switched 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 two-dot multi-valued circuit shown in FIG. 11 (a), the switches SW2 and EVEN / ODD are switched every one clock, and the switches SW3 and SW4 are the latches 1103, 11
Set the lower side so that the data from 04 is selected.

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 Convert the phase in writing and concentrate the dots
When forming an image of 100 lines, the switching cycle of EVEN and ODD is each performed by dividing the frequency by two. As described above, no loss of gradation information occurs in all modes. FIG. 12 shows an example of the γ conversion table used in the present apparatus.

FIG. 12 (a) is a γ conversion table used for a 1 × 2 matrix, and FIG. 12 (b) is a γ conversion table used for a 2 × 1 matrix. Both tables are set so that the copy density is substantially equal to the document density.

When one of the EVEN dots reaches the maximum value up to the intermediate density, the exposure intensity of the ODD dot is increased. This allows
The dots are concentrated while maintaining the density information of two dots.

Further, γ can be freely controlled by using the γ conversion 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 RAM is used for the conversion table and the γ conversion data is rewritten.

Generally, the inverse conversion of the printer's gamma characteristic expressed by the print density with respect to the writing exposure light amount is converted into a table value, whereby the gamma characteristic of the printer alone can be made linear.

The two-dot multi-valued circuit in FIG. 11A is configured in the IPU 800, converts image data for each dot from the scanner, and outputs the converted data to the writing system.

As a result, a writing process of 512 gradations is realized using two dots in the main scanning and sub-scanning directions as one pixel.

 FIG. 13 shows a part of the operation unit of the present invention.

When the image processing key 1 is pressed, the following modes are sequentially shifted, and the indications 2, 3, and 4 of each mode are turned on. (A) Combination with area gradation in main scanning direction (2 × 1 matrix), (B) Combination with area gradation in sub-scanning direction (1 × 2 matrix), and (C) Some of the above methods (A) and (B) automatically switch the processing method depending on the direction and size of the original.

The standard mode is the automatic mode (C), which can be changed by pressing the image processing key 1 in accordance with the document. On the display of the operation unit, mode (A) is a vertical line, mode (B) is a horizontal line,
The mode (C) is an automatic mode. Operation unit CP
U (not shown) executes the mode input control, and sends the mode signal to the IPU 800 that executes the write processing.
Based on this signal, the IPU 800 executes data processing and data switching processing of the two-dot multi-level circuit.

Next, the automatic selection mode of this embodiment will be described.

In a method in which a small matrix is combined, the resolution in the direction in which the area gradation is performed is reduced, and fine lines and character breaks may be visualized. Therefore, 1 ×
Switch between 2 and 2 × 1 matrices to make character breaks less noticeable.

In particular, Japanese kanji (Minchotai) have thin horizontal lines and image degradation due to horizontal thin line cracks occurs. However, the direction of area gradation is automatically switched according to the direction and size of the original to reduce the resolution degradation. An image with little influence can be obtained.

On the other hand, the character direction often depends on the direction and size of the document. For example, in the case of office documents, many small-sized originals use a vertically long sheet of paper. In an A4 landscape original, characters are oriented horizontally in the transport direction, and lines in the sub-scanning direction are thin. As a result, fine line cracks are easily conspicuous, and a 1 × 2 matrix is used to prevent cracks in the main scanning direction. Conversely, for A4 portrait documents, it is better to use a 2 × 1 matrix.

A3 manuscript drawn with fine characters is A4 with two-page spread
In many cases, it is a document, and a 1 × 2 matrix is suitable. The automatic selection mode is set according to the original size and direction detected or determined based on the transfer paper size copying magnification. Main document size and orientation settings
This is shown in the table in Fig. 14 (standard setting for automatic selection mode). Of course, not all manuscripts apply to the above table, so manual setting is also possible.

Further, in the embodiment of the present invention, an A3 portrait feed copying system is taken as an example, but a similar process may be executed according to the transport direction.

The halftone area of the two-dot multi-tone image has less density unevenness and is expressed more smoothly than the one-dot multi-tone image. In addition, density reduction, halftone trailing edge white spots, and the like that have occurred at the boundary of gradation are improved. The one-dot multi-tone image did not show regularity in the way of toner application, which caused the halftone image to appear noisy. Is clearly reproduced, which looks visually smooth.

Further, in the processing of a plurality of dots, for example, three dots, banding is eliminated, the density is stabilized, and the halftone is smoothly reproduced.

The multi-tone writing method using a plurality of dots according to the present method is an image processing method that can be used regardless of the laser diode modulation method such as power modulation or pulse width modulation in addition to the present embodiment.

FIG. 15 is a block diagram of a two-dot multi-valued circuit according to a second embodiment of the present invention, and the same parts as FIG.
The same reference numerals are used and the description is omitted.

The decision of switching between 1 × 2 and 2 × 1 matrices is based on the data of the adder 1105a or 1105b of the 2-dot multi-valued circuit.
The data is input to a determination circuit 1500 having a comparator and configured by logic, data to be input to the ROM 1106 is selected from the detected value, and the density conversion table for each matrix is switched by an upper address to execute the processing.

Using the two-dot multi-valued circuit of this embodiment, images at each density were output, and the effect of banding (band-like density unevenness in the main scanning direction) was confirmed. For image output, banding occurs by adding 1% rotation speed unevenness to the photosensitive drum at a pitch of 2 mm. In the sensory evaluation, as shown in FIG. 16, the banding of the image of two dots and multiple gradations is greatly reduced as compared to the image of one dot and multiple gradations by the power modulation of the laser diode.

From FIG. 16, even in the 1 × 2 and 2 × 1 area gradations, the occurrence of banding differs depending on the density of the halftone.

Therefore, in this embodiment, the area gray scale in the main scanning direction and the area gray scale in the sub-scanning direction are switched according to the writing density, the low density area performs the area gray scale in the sub scanning direction, and the high density area performs the area gray scale in the main scanning direction. By performing area gradation, banding is reduced at all halftone densities, and a high-quality image can be obtained.

Specifically, when the area gradation of the two-dot matrix is executed, when only one of the dots is exposed, the 1 × 2 matrix is suitable for preventing the overlapping of the exposure beams, and the second dot is also exposed. In this case, the 2 × 1 matrix has the first dot at the saturation density and the banding is inconspicuous.

Therefore, the criteria for switching is 50% reflectance
, Or near the intermediate density (ID = 0.3). As a result, banding is reduced at all densities, and a high-quality image is obtained.

The criterion for performing the switching depends on the exposure beam diameter and the scanning pitch unevenness, but may be about ID = 0.6 from the table in FIG.

Further, in the present embodiment, since each of the output density characteristics with respect to the input data is different, independent density conversion is executed, and all the processes are set so as to faithfully reproduce the original density. Therefore, the switching density point is determined from the read data. However, both have 2 dots (primary, secondary 4
(Dots), and when the added value of two dots in the sub-scanning direction is equal to or less than the reference density, processing is performed using a 1 × 2 matrix. Similarly, when performing the above-mentioned dot concentration, processing is performed in units of two dots.

In the above example, the switching process is performed within the document. However, it is also possible to make a determination based on the average or maximum density of the document and execute one copy process in the same process.

〔The invention's effect〕

As described above, according to the image forming apparatus of the present invention, the following effects can be obtained.

First, in the method of combining with a minute matrix, the resolution in the direction in which the area gray scale is executed is reduced, and fine lines and broken characters may be visualized. For this reason, a selection means by key input is provided to switch between the 1 × 2 and 2 × 1 matrices according to the original, so that high quality image formation with less noticeable character breaks can be realized. In particular, Japanese kanji (Mincho) have thin horizontal lines and image degradation due to horizontal thin line cracks occurs. However, since the direction in which area gradation is executed can be switched depending on the character direction, resolution degradation An image with little influence is obtained.

Second, since the 1 × 2 and 2 × 1 matrices are switched and used according to the original, high-quality image formation with less noticeable character breaks can be realized. In particular, Japanese kanji (Minchotai) have thin horizontal lines, and image degradation due to horizontal thin line cracks may occur. However, the direction in which area gradation is automatically executed can be switched according to the direction and size of the original. Therefore, an image with less influence of resolution degradation can be obtained.

Thirdly, there is provided a means for switching between the area gradation in the main scanning direction and the area gradation in the sub-scanning direction. Since the area gradation is performed with a plurality of dots, banding is reduced at all halftone densities, and high-quality image formation is realized.

Fourth, when the area gradation of the two-dot matrix is executed, overlapping of the exposure beams is prevented by the 1 × 2 matrix when only one dot is exposed, and 2 × 1 when the second dot is also exposed. Since the banding is inconspicuous at the saturation density of the first dot due to the matrix (the reference density at which the switching is performed is an intermediate density (ID = 0.
3) or the vicinity thereof is preferable), banding is reduced at all densities, and high-quality image formation is realized.

Fifth, when one-dot multi-gradation writing is combined with area gradation in the main scanning direction or area gradation in the sub-scanning direction, output density characteristics for input data are different from each other, and independent density conversion is performed. In addition, since the density conversion is performed so that the density is continuous even in the portion where the writing is switched, gradation reproduction is performed smoothly and continuously, and high-quality image formation with less banding is realized.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing the structure of a digital copying machine to which the image forming apparatus according to the present invention is applied, and FIGS.
FIG. 4 is an explanatory diagram showing a configuration of a laser writing system in the digital copying machine shown in FIG. 4. FIG. 4 is a block diagram showing a power modulation system of a laser diode (LD) used in the digital copying machine shown in FIG. FIG. 5 is a graph showing the relationship (IL characteristic) between the forward current (I) and the emission intensity (L) of the laser diode, FIG. 6 is a circuit diagram showing a control method of the laser diode, and FIG. 8 is a block diagram showing a flow of image processing performed by the image processing apparatus, and FIGS. 9A and 9B are 1 × 2 matrix and 2 × 1 matrix, respectively. FIGS. 10 (a), (b), (c) and (d) are charts showing a halftone density region to be formed, and FIG. 11 (a) is the present invention. Block diagram of a two-dot multilevel circuit used in an image forming apparatus according to FIG. 11 (b) is an explanatory diagram showing a combination with an area gradation in the sub-scanning direction, FIG. 11 (c) is an explanatory diagram showing a combination with an area gradation in the main scanning direction,
12 (a) and 12 (b) are tables each showing 2-dot multi-value γ conversion, FIG. 13 is an explanatory diagram showing a part of the operation unit, FIG. 14 is a table showing automatic selection mode standard setting, and FIG. FIG. 16 is a block diagram showing a second embodiment of a two-dot multi-valued circuit, and FIG. 16 is a table showing banding ranks. 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 element 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 memories 1103, 1104… Latch 1105… Adder 1106… ROM 1500 …… Delay circuit

Claims (5)

(57) [Claims] A first means for executing a one-dot multi-tone writing process for a plurality of dots arranged only in the main scanning direction which constitute one pixel during printing; and forming one pixel during printing. A second means for executing a one-dot multi-tone writing process for a plurality of dots arranged only in the sub-scanning direction, and a switching input means for switching between the first means and the second means. An image forming apparatus comprising: 2. A first means for executing a one-dot multi-tone writing process on a plurality of dots arranged only in the main scanning direction which constitute one pixel at the time of printing, and comprising one pixel at the time of printing A second means for executing a one-dot multi-gradation writing process for a plurality of dots arranged only in the sub-scanning direction, a switching input means for switching between the first means and the second means, Detecting means for detecting a direction and a size, wherein the switching input means is configured to detect the first and second means based on a direction and a size of the document detected by the detecting means.
An image forming apparatus characterized by switching means. 3. A first means for executing a one-dot multi-tone writing process for a plurality of dots arranged only in the main scanning direction which constitute one pixel at the time of printing, and comprises one pixel at the time of printing. A second means for executing a one-dot multi-gradation writing process for a plurality of dots arranged only in the sub-scanning direction, and a switching input means for switching between the first means and the second means. The image forming apparatus, wherein the switching input unit switches to the second unit when the density is lower than the specific reference density, and switches to the first unit when the density is equal to or higher than the specific reference density. 4. The method according to claim 3, wherein the specific reference density is an intermediate density (ID = 0.
3) An image forming apparatus characterized by being at or near the same. 5. Density conversion data storage means for storing density conversion data for equalizing the image density of a document and the image density printed on recording paper, and only in the main scanning direction which constitutes one pixel during printing. A first means for executing a one-dot multi-gradation writing process based on the density conversion data stored in the density conversion data storage means for a plurality of dots arranged in a plurality of dots, and constituting one pixel during printing. Second means for executing a one-dot multi-tone writing process based on density conversion data stored in the density conversion data storage means for a plurality of dots arranged only in the sub-scanning direction; An image forming apparatus comprising: a switching input unit that switches between a unit and a second unit.