JP2001096812A - Image-forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce banding and image noises and stabilize an image density by adopting a system of combining a minute matrix which decreases a resolution for multi-valued writing by one-dot modulation. SOLUTION: Dot data of two dots in line memories 1001 and 1002 are added by an adder 1005, and the added result is distributed according to an EVEN/ ODD table. At this time, a two-dot multi-valued γconversion table is used. When a total value of dot data of first dots and dot data of second dots exceeds a saturation value of dot data of single dots, the saturation value is distributed as the dot data of first dots, and a value obtained by subtracting the saturation value from the total value is distributed as the dot data of second dots.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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. More specifically, the present invention relates to a multi-valued writing by one-dot modulation combined with a fine matrix with little reduction in resolution. The present invention relates to an image forming apparatus that realizes high-quality image formation by reducing banding and image noise.

[0002]

2. Description of the Related Art For example, in a writing process in a digital copying machine, its resolution and gradation are important factors. Fine resolution and gradation that faithfully reproduces halftones are desired in copy processing for any document including characters and photographs.

Conventionally, there is an area gradation method using a dither matrix as a method for expressing gradation (see Japanese Patent Application Laid-Open No.
-144126, JP-A-56-17478, JP-A-57-76977, etc.).

However, in the area gradation method, since a pixel is composed of a plurality of dots and the density is expressed by the number of written dots, 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 is N and does not include the background white portion, but generally the resolution is 1 / N.

On the other hand, there has been proposed a one-dot multi-value writing method for realizing multiple gradations without lowering the resolution. 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 and a power modulation method for modulating the exposure intensity. The above-mentioned 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-level writing method is preferable.

[0007]

SUMMARY OF THE INVENTION However, the above 1)
The dot multi-value writing method has a disadvantage that banding is likely to occur. In a digital copying machine, banding occurring in a halftone area is caused by drive unevenness and vibration of a photoconductor, scan pitch unevenness of a writing optical system, and the like. The banding appears as a band-like density unevenness continuous in the main scanning direction. In particular, in the one-dot multi-level writing method, the scanning beam unevenness in the sub-scanning direction of the exposure laser diode causes the base of the exposure beam in the intermediate exposure area to overlap, thereby causing banding.

[0008] Further, due to the increase in resolution, accuracy for banding is also required. Also, in one-dot multi-level writing at about 400 dpi, which is currently widely used, in the current electrophotographic process, image noise due to density unevenness occurs in the halftone solid portion regardless of the modulation method, and halftone is generated. There is a problem that it is not reproduced smoothly.

The present invention has been made in view of the above, and employs a method of combining a multi-level writing by one-dot modulation with a minute matrix with a small decrease in resolution.
A first object is to form an image based on vertical lines by an inexpensive method by reducing banding and image noise to stabilize the image density, and to realize high-quality image formation. It is a second object of the present invention to form a continuous toner image in the main scanning direction to realize high quality image formation.

[0010]

According to the present invention, in order to achieve the above object, each dot forming image data obtained by reading a document is converted into writing data for every two adjacent dots.
In an image forming apparatus for forming an image based on the converted writing data, a first
When converting the dot data and the second dot adjacent to the first dot into write data, the total value of the dot data of the first dot and the dot data of the second dot is a single dot. If the saturation value of the dot data is exceeded, the saturation value is distributed as the dot data of the first dot, and the value obtained by subtracting the saturation value from the total value is used as the dot data of the second dot. It is characterized by having a distribution means for distributing.

Further, the distribution means distributes the total value as the dot data of the first dot when the total value does not exceed the saturation value of the dot data of a single dot. .

The image forming apparatus according to the present invention operates as follows. The image forming apparatus according to the present invention, when converting the first dot forming the image data and the second dot adjacent to the first dot into the write data, writes the dot data of the first dot and the second dot. If the total value of the dot data of the dots exceeds the saturation value of the dot data of a single dot, the saturation value is distributed as dot data of the first dot, and a value obtained by subtracting the saturation value from the total value Are allocated as dot data of the second dot.
Dot can be positioned as a stable dot, and the second dot can be positioned as an attached minute dot, whereby an intermediate level dot can be stably formed when performing area gradation in units of two dots. .

In the image forming apparatus according to the present invention, when the total value does not exceed the saturation value of the dot data of a single dot, the total value is allocated as the dot data of the first dot. The first dot can always be positioned as a stable dot.

[0014]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing a configuration of a digital copying machine, a modulation system of a writing laser diode, image reading signal processing, image processing, and a two-dot multi-level circuit according to an embodiment of the present invention; It will be described in order.

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 the figure, 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 reflected by a mirror 113,
An image is formed on the light receiving surface of the CCD image sensor 117 via the lenses 114 and 115 and the lens 116. The light source 11
2 and the mirror 113, the traveling body 11 moving on the lower surface of the contact glass 111 in parallel with the contact glass 111.
8 is installed.

The main scanning is executed by the solid-state scanning of the CCD image sensor 117. The original image is read one-dimensionally by the CCD image sensor 117, and the entire original is scanned by moving (sub-scanning) the optical system. In this example, the density of the reading process is 40
0 dpi, A3 size (297 mm x 420 m
The configuration allows reading of documents up to m).

Next, a laser printer constituting the digital copying machine will be described. When the document reading device and the laser printer are integrally configured (this embodiment)
And the configuration may be separate and electrically connected only.

Each system such as a laser writing system, an image reproducing system, and a paper feeding system is integrally formed in the laser printer. As shown in FIGS. 1, 2 and 3, the laser writing system includes a laser output unit 219, an imaging lens group 120,
A mirror 121 is provided. 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 123 for uniformly charging the photosensitive drum 122, an eraser 124, and a formed electrostatic latent image are visualized. A developing unit 125; a transfer charger 126 for transferring the image of the photosensitive drum 122 to the transferred transfer paper; a separation charger 127 and a separation claw 128 for separating the transfer paper from the photosensitive drum 122; A cleaning unit 129 for cleaning the surface of the body drum 122 is provided.

A main scanning synchronization signal (PMSYN) is provided at a position near the photosensitive drum 122 where the laser beam is radiated.
A beam sensor 330 for generating C) is disposed (see FIG. 3). 131 is a conveyor belt, 132 is a fixing unit, 133 and 134 are paper feed cassettes, 135 and 136
Denotes a paper feed roller, and 137 denotes a registration roller.

The operation of the above-described arrangement will be described.
3 uniformly charges to a high potential. 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 according to the black / white of the recording pixel, the potential distribution corresponding to the recording image on the surface of the photosensitive drum 122 by the irradiation of the laser light,
That is, an electrostatic latent image is formed.

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 onto 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 transported by a transport belt 131, and a fixing unit 132 having a built-in heater.
And is discharged to a discharge tray (not shown).

In the digital copying machine shown in FIG.
The paper feeding system is composed of two systems. In one paper feed system,
A paper feed cassette 133 is provided, and a paper feed cassette 134 is provided in the other paper feed system. The recording paper in the paper supply cassette 133 is supplied by the paper supply roller 135.
The recording paper in the paper feed cassette 134 is fed by a paper feed roller 136.

The fed recording paper is fed to a registration roller 13.
7 and temporarily 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 pulse width modulation circuit used for pulse width modulation writing of a laser diode. In particular, in order to obtain a pulse width signal by a method using a delay line. It is composed of a plurality of delay elements 450 to 453, logic circuits (AND circuits 454 and 455, OR circuits 456 and 457) and a selector 458. The select signal from the selector is input to the LD drive circuit 459.

The generation of a pulse width signal by the pulse width modulation circuit in FIG. 4 will be described with reference to FIG. A write clock of about 50% duty is delayed by an arbitrary set time. 5
When a minute pulse width of 0% or less is obtained, AND of the write clock and its delay signal is obtained, and a pulse width inversely proportional to the delay amount is obtained (FIG. 5A). When a pulse width of 50% or more is obtained, the OR of the write clock and its delay signal is ORed to obtain a pulse width proportional to the delay amount (FIG. 5B). The selector 458 determines the pulse width based on the pixel data to be written.
And a pulse width signal continuous in the main scanning direction is obtained. As described above, by setting the output signal to an appropriate value using the plurality of delay elements 450 to 453, multi-tone pulse width modulation writing is executed.

In this embodiment, the pulse width modulation is executed by the method using the delay line. However, the pulse width modulation writing by the multi-tone laser diode may be executed by another method.

FIG. 6 shows the LD driving circuit 4 shown in FIG.
59 shows a circuit configuration of the circuit. The LD drive circuit 459 includes a D / A converter 655, a constant current source 652,
A transistor 656, a current / voltage conversion circuit 653,
It comprises an A / D converter 654 and a laser diode LD.

In the above configuration, the amount of light emitted by the laser diode is determined in advance by the constant current source 652 of the forward current of the laser diode, and the pulse width modulation switching is executed by switching with the pulse width signal.

Image Read Signal Processing FIG. 7 shows a detailed block diagram of the image read signal processing. CC
D (charge-coupled device) 117 has about 5,000 pixels and 400 pixels.
It can read dpi, and simultaneously reads the 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), and waveform correction such as clamping is performed.
Amplification and A / D conversion are performed, and output to an IPU (image processing apparatus) 800 as a 6-bit digital signal.

More specifically, the analog data output of the CCD 117 is EVEN, OD for high-speed transfer.
D is output separately to two systems, and amplified (signal amplified) by amplifiers 702 and 703, respectively, and input to a 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 input to the A / D converter 705. The image transfer speed of one pixel after the synthesis is about 10 MHz, and in synchronization with this, the A / D converter 705 converts the digital signal into a 6-bit 64 gradation digital signal (signal digitization).

The (variable) amplifier 704 reads the 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 A digital signal for each pixel indicating the document density is input to an IPU (image processing apparatus) 800 and subjected to image processing. IP
FIG. 8 shows the flow of image processing by U800. The IPU 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 light is condensed by a lens, the amount of light 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. In an optical system using an MTF correction 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 and was distorted. 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. In this embodiment, the image reading and writing resolutions are the same 400 dpi, but the reading pixel frequency is about 10M.
Hz and the writing pixel frequency are different at about 12 MHz, so the 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. gamma correction In addition to the above, the IPU (image processing device) controls AGC and the like,
Image conversion such as masking, trimming, mirroring, black-and-white reversal, document size and density detection, and marker and other image detection are also performed.

In this embodiment, a matrix of two dots in the main scanning direction and the sub-scanning direction is combined with the output of 256 gradations per dot by pulse width modulation.

FIG. 9A shows an optical writing system of a 1 × 2 matrix, and FIG. 9B shows an optical writing system of a 2 × 1 matrix. In the low-density portion, the exposure time is longer than that of one dot, and when the maximum exposure time is reached, the exposure time of the next dot is increased.

In the case where density reproduction is performed with two consecutive dots in the main scanning direction, the writing of each dot may be performed with a pulse width that grows from the center thereof. When the density, that is, one dot density is saturated, the two pulse widths become continuous, the effect of dot concentration is further increased, and the gradation becomes continuous. However, when the line concentration is further performed as described later, it is preferable to alternately form the pulse width every two dots so that the generated pulse width is continuous.

On the other hand, when the density reproduction is performed with two dots in the sub-scanning direction, the writing of each dot is performed with a pulse width that grows from the center of the dot so that the dot does not shift in the main scanning direction due to the pulse width at each density. Good.

The density reproduction is performed using two dots in the main scanning or sub-scanning direction as a pixel of interest. The reading density of the CCD 117 is proportional to the amount of received light. Accordingly, the amount of light received by the CCD 117 is linear with respect to the original reflection density, and the density data of two dots is added as a digital value. After that, the added value is subjected to γ conversion, and converted into writing density data by the above method. As a result, two scans in the main scanning and sub-scanning directions
512 gradations are realized by dots.

FIG. 10 is a block diagram of a two-dot multi-valued circuit. Line memories 1001 and 1002 connected in series to input a 6-bit signal input from a scanner, and latches 1003 and 1004 and the line memories 1001, 10
02 and the latches 1003 and 1004 have switches SW respectively.
An adder 1005 connected via 1 to SW4;
And a ROM 1006 connected to the adder 1005. The output of the ROM is output to the printer as an 8-bit data signal.

Hereinafter, i. A 1 × 2 matrix, ii. 2x1
A matrix, iii. This will be described in detail with respect to dot concentration. i. 1 × 2 matrix When area gradation is performed with two dots in the sub-scanning direction (1 × 2 matrix)
2 matrix) is composed of two line memories 1001, 10
02, the read data for two main scanning lines is delayed. Thereafter, the two 6-bit data are added to the adder 1005
And the 7-bit data is stored in a ROM for γ conversion.
1006. In the ROM 1006, one table is composed of 265 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 1006, and the EVEN data indicated by the address is output as write data. The same data is added in the next line, and ODD data is output from the data bus as write data. Switching between EVEN and ODD is performed 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-level circuit shown in FIG. 10, the switches SW1 and EVEN / ODD are switched for each main scanning line, and the switches SW3 and SW4 select data from the line memories 1001 and 1002. Up.

Ii. 2 × 1 matrix When performing area gradation with two dots in the main scanning direction (2 × 1 matrix)
One matrix) uses two latches 1003 and 1004 to delay read data for two dots in the main scanning direction. Hereinafter, similarly to the case of the 1 × 2 matrix, the addition processing and the γ conversion processing are executed to output the write data. EV
Switching between EN and ODD is performed based on the write dot cycle (WRITEC).
LK). 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. 10, the switches SW2 and EVEN / ODD are switched every writing clock, and the switches SW3 and SW4 select data from the latches 1003 and 1004. To the bottom.

Iii. Concentration of dots Convert the phase in writing and concentrate the dots 10
In the case of forming a zero-line image, the switching cycle of EVEN and ODD is performed by dividing the frequency by two. As described above, no loss of gradation information occurs in all modes.

FIG. 11 shows an example of a gamma conversion table used in the present apparatus. The γ conversion table shown in FIG. 11 outputs a copy density substantially equal to the original density (A) and linearly outputs input data for executing a printer tone check. (B).
When one of the EVEN dots reaches the maximum value up to the intermediate density, 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 determined by the γ conversion table.
Can be controlled, 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 gamma characteristic represented by the print density with respect to the writing exposure light amount is converted into a table value, so that the gamma characteristic of the printer alone can be made linear.

The two-dot multi-valued circuit of FIG.
0, and converts the image data for each dot from the scanner and outputs it 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.

Using the two-dot multi-value writing method according to the present invention, images at each density were output, and the effect of banding (band-like density unevenness in the sub-scanning direction) was confirmed. The banding of the 2-dot multi-tone image was greatly reduced as compared with the tone image.

Next, a second embodiment of the present invention will be described. The description that overlaps with the first embodiment is omitted for simplification.
FIG. 12 is a block diagram of a power modulation system of a laser diode (LD) according to a second embodiment of the present invention. The light emission level command signal is transmitted from a first current conversion unit 1200 and a second current conversion unit 1201. Is input to

In the first current converting means 1200, the light emission level command signal is converted into a light emission level command signal current (output current) I _S according to the strength. The output current I _S of the first current conversion means 1200 becomes an input current I _S −I _L which is a difference from the photovoltaic current I _L proportional to the light output P _O generated in the light receiving element 1202 of the laser diode LD1. Input to the current amplifier 1203.

The current amplifier 1203 has an input current I _S −
The I _L and outputs the A multiplied by the output current A (I _S -I _L). On the other hand, the light emission level command signal is converted by the second current conversion means 1201 into an output current I ₁ for emitting the set light amount P _S. I ₁ + A (I _S −I) which is the sum of the output current I ₁ and the output current A (I _S −I _L ) of the current amplifier 1203.
I _L ) is the forward current of the laser diode LD1.

Thus, the laser diode LD1
Obtains a light output P _O determined by the forward current I ₁ + A (I _S −I _L ). That is, the following relational expression holds. P _O = P {I ₁ + A (I _S −I _L )} P: Function representing light output-forward current characteristic of laser diode LD1

Here, since I ₁ is set so that I _S ≒ I _L , 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 a [delta] the coupling efficiency with, expressed as _{P O = P S + η ·} a · (I S -P O · S · δ), P O = {P S / (1 + η · δ · S · a) ｝ ＋ I _S ｛η ・ A
/ (1 + η · δ · S · A)｝.

Assuming that the crossover frequency of the photoelectric negative period loop is f _O , the step response of the optical output P _O can be approximately expressed as follows. P _O = I _S / δ · S + {P _S −I _S / δ · S} · exp (−
2π · f _O t)

The P _S set by the second conversion means 1201 is set to be equal to I _S / δS. For example, if P _S fluctuates by 5% due to the droop characteristic, f _O = 40 MHz Even if it is, the time required for the error of P _O to become 0.4% or less is about 10 ns.

Further, the total light quantity (integral value of light output の) from immediately after changing light output P _O to a set time τ _O.
The crossover frequency f _O for which the error (P _OUT ) is 0.4% or less is sufficient if f _O ≧ 40 MHz when τ _O = 50 ns. Such a cross frequency can be easily realized. As described above, this method can realize a high-speed, high-precision, high-resolution laser diode control method.

By modulating the power of the laser diode LD1 using this method, 25
Six types of analog signals are input, and an image output of 256 gradations per dot is realized in the laser printer.

Next, a power modulation method for a laser diode (LD) according to a second embodiment using a plurality of constant current power supplies will be described. The drive control method of the laser diode in the present embodiment uses the relationship (IL characteristic) between the forward current (I) and the light emission intensity (L) of the laser diode shown in FIG.

The IL characteristic of this laser diode is almost linear for a forward current equal to or higher than the threshold current (Ith), and the differential quantum efficiency (n) at that time is treated as being constant.
In the control method, as shown in FIG. 14, the forward current is driven by the total current of a plurality of constant current sources 1441, 1442, 1443, and 1444, and the switch 14 is driven by the write data to switch 14.
Switching is performed at 45, 1446, and 1447. A bias current larger than the threshold current is supplied by the constant current source 1441, and the constant current sources 1442, 1443, and 1444 weighted so as to have a current value of 1: 2: 4 reduce the driving current of the laser diode to 3 bits 8 bits. Control to a value. The current values at that time are I ₁ , I ₂ , and I ₃ , respectively, and the minimum bias current that does not drive the switches 1445, 1446, and 1447 is I ₀ . Therefore, the light emission intensity (light amount) by each of the currents I _{0 to} I ₃ is as shown in FIG. 13, and the light amount by all combinations of the currents I _{0 to} I ₃ is equal to the light amount difference in eight cases from L _{0 to} L _7. Can be

The setting procedure at that time is executed as follows. (A) Set in the laser diode emission intensity range 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) ·
Let I _max , I ₃ = (4/7) · I _max . From the above,
Assuming that the number of constant current sources is ^n, a light emission intensity of 2 ⁿ is 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 can be obtained. .

Next, image processing in the second embodiment will be described. In the present embodiment, a matrix of two dots in the main scanning and sub-scanning directions is combined with the output of 256 gradations per dot by laser diode power modulation.

FIG. 15 shows a 1 × 2 matrix optical writing system. In the low density portion, the exposure power is increased from that of the preceding one dot, and the exposure power of the next succeeding dot which has the maximum value is increased.

The density reproduction is executed using two dots in the sub-scanning direction as a pixel of interest. The reading density of the CCD 117 is proportional to the amount of received light. Accordingly, the amount of light received by the CCD 117 is linear with respect to the document reflection density, the density data of two dots is added to the digital value, the added value is subjected to γ conversion, and converted into write density data by the above method. As a result, 512 gradations are realized by two dots in the main scanning direction.

The chart of the formed halftone density area is generated as shown in FIG. In the figure, the density is filled from the EVEN dots. The 1 × 2 matrix in FIGS. 16A and 16B in which the area gradation is executed in the sub-scanning direction is based on the horizontal line.

FIG. 16 (b) is a diagram in which the writing phase of FIG. 16 (a) is alternately changed, and two dot lines are formed in the main scanning direction to form an image of 100 lines. As a result, although the number of gradations does not change, the lines are concentrated, and the apparent resolution is reduced by half.

Next, a two-dot multi-valued circuit according to the second embodiment will be described. FIG. 17 is a block diagram of a two-dot multi-valued circuit, in which a line memory 170 connected in series for inputting a 6-bit signal input from a scanner.
1 and 1702, an adder 1703 connected via a switch SW1, and a ROM 1704 connected to the adder 1703. The ROM 17
The output of 04 is output to the printer as an 8-bit data signal.

Hereinafter, i. A 1 × 2 matrix, ii. This will be described in detail with respect to dot concentration. i. 1 × 2 matrix When area gradation is performed with two dots in the sub-scanning direction (1 × 2 matrix)
2 matrix) are two line memories 1701, 17
02, the read data for two main scanning lines is delayed. Then, the two 6-bit data are added to the adder 1703.
And the 7-bit data is stored in a ROM for γ conversion.
Input to 1704. In the ROM 1704, one table is composed of 265 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 1704, and the EVEN data indicated by the address is output as write data. The same data is added in the next line, and ODD data is output from the data bus as write data. Switching between EVEN and ODD is performed 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. 17, the switch SW1 and EVEN / ODD are switched every main scanning line.

Ii. Concentration of dots Convert the phase in writing and concentrate the dots 10
In the case of forming an image of line 0, the switching cycle of EVEN and ODD is performed by dividing the line cycle (PMSYNC) by two.
As described above, no loss of gradation information occurs in all modes.

Using the two-dot multi-value writing method according to the present embodiment, images at each density were output, and the effect of banding (band-like density unevenness in the sub-scanning direction) was confirmed.

For image output, banding is forcibly generated by adding 1% rotation speed unevenness at a pitch of 2 mm of the photosensitive drum. In the sensory evaluation, as shown in FIG. 13, banding was greatly reduced in the two-dot multi-tone image as compared with the one-dot multi-tone image by power modulation of the laser diode.

Next, a third embodiment of the present invention will be described.
Note that, for simplification, the same portions as those in the above-described embodiment will not be described. In the present embodiment, two gradations in the main scanning and sub-scanning directions are output to 256 gradation outputs per dot by pulse width modulation.
It is a combination of dot matrices.

FIG. 9A shows a 1 × 2 matrix optical writing system, and FIG. 9B shows a 2 × 1 matrix optical writing system. In the low density portion, the exposure time is longer than that of one dot, and when the maximum exposure time is reached, the exposure time of the next dot is increased.

When density reproduction is performed with two consecutive dots in the main scanning direction, each dot may be written with a pulse width that grows from the center thereof. When the density, that is, one dot density is saturated, the two pulse widths become continuous, the effect of dot concentration is further increased, and the gradation becomes continuous. However, when the line concentration is further performed, it is preferable that the pulse width is formed alternately on the left and right every two dots so that the generated pulse width is continuous.

FIG. 18 shows a write pulse generating circuit in this case. In FIG. 18, a saw-tooth wave generating circuit 1801 composed of a constant current source and a charge / discharge device, a D / A converter 1802 for converting multi-valued write data into analog data,
It comprises a comparator 1803 for comparing the output from the sawtooth wave generating circuit 1801 with the output from the D / A converter 1802.

The generation of the pulse signal by the write pulse generation circuit shown in FIG. 18 is shown by the signals at the output points A, B and C in FIG. By the write pulse generating circuit shown in FIG. 18, a pulse signal for each dot is generated from the left side of the dot write position. That is, in the continuous output of the two-dot processing, the pulse width signal is continuous, and one pixel is formed by two dots in the main scanning direction. FIG. 20 shows the state of dot formation.

On the other hand, when density reproduction is performed with two dots in the sub-scanning direction, writing of each dot is performed at a pulse width that grows from the center of the dot so that there is no deviation in the main scanning direction due to the pulse width at each density. Is better. The method of generating the pulse width signal in this case is executed using the delay line shown in FIG.

Further, it can also be executed by the pulse generating circuit shown in FIG. 21 is configured by replacing the sawtooth wave generation circuit 1801 shown in FIG. 18 with a triangular wave generation circuit 2101. This causes the writing dot to grow from its center as shown in FIG.

Further, when the phase is converted as described above, the dots at both ends of the pixel, that is, the dots for which the density conversion within one dot is executed by the pulse width modulation, are subjected to the pulse conversion so that the pulse signal becomes continuous. Generated from the center side. FIG. 23 shows the state of pixel formation. Figure 1 shows the pulse generation circuit.
8 is performed by alternately selecting the saw-tooth wave generation circuit 1801 shown in FIG. Accordingly, the pulse width located on the right side of the pixel is formed on the left of the dot, and the pulse width located on the left side of the pixel is formed on the right of the dot, so that the pulse width signal is continuous.

FIG. 23 shows the state of the pulse width signal.
With two dots in the main scanning or sub-scanning direction as the pixel of interest,
Perform density reproduction. The reading density of the CCD 117 is proportional to the amount of received light. Accordingly, the amount of light received by the CCD 117 is linear with respect to the original reflection density, and the density data of two dots is added as a digital value. After that, the added value is subjected to γ conversion, and converted into writing density data by the above method. As a result, 5 in 2 dots in the main scanning and sub-scanning directions.
Twelve gradations are realized.

The chart of the formed halftone density area is generated as shown in FIG. In the figure, the density is filled from the EVEN dots. The 1 × 2 matrix shown in FIGS. 24A and 24C for executing the area gradation in the sub-scanning direction is based on a horizontal line,
24 (b) and (d) of FIG.
The × 1 matrix is based on vertical lines.

FIGS. 24C and 24D respectively show FIGS.
The writing phases of (a) and (b) are alternately changed.
Two-dot lines are formed in the main scanning and sub-scanning to form an image of 100 lines. As a result, although the number of gradations does not change, the lines are concentrated, and the apparent resolution is reduced by half.

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. In the 1-dot multi-tone image, no regularity was observed in the way of toner application, and this caused the halftone image to appear noisy. It clearly reproduces, 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 halftones are 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 and pulse width modulation in addition to the present embodiment.

[0095]

As described above, according to the present invention, when the first dot forming the image data and the second dot adjacent to the first dot are converted into the writing data, the first dot is formed. When the total value of the dot data of the dot and the dot data of the second dot exceeds the saturation value of the dot data of the single dot, the saturation value is distributed as the dot data of the first dot, and Since the value obtained by subtracting the saturation value from the value is allocated as dot data of the second dot, the first dot can be positioned as a stable dot, and the second dot can be positioned as an attached micro dot. Thus, when performing area gradation in units of two dots, dots at an intermediate level can be formed stably.

According to the present invention, when the total value does not exceed the saturation value of the dot data of a single dot, the total value is allocated as the dot data of the first dot. The first dot can be positioned as a stable dot.

[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 configuration of a pulse width modulation circuit used in the digital copying machine shown in FIG.

FIG. 5 is an explanatory diagram showing generation of a pulse width signal by the pulse width modulation circuit shown in FIG.

FIG. 6 is a circuit diagram showing a drive circuit 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.

FIG. 9 is an explanatory diagram showing a 1 × 2 matrix and a 2 × 1 matrix optical writing system;

FIG. 10 is a block diagram of a two-dot multilevel circuit used in the image forming apparatus according to the present invention.

FIG. 11 is a table showing a two-dot multi-value γ conversion.

FIG. 12 is a block diagram showing a power modulation system of a laser diode (LD) used in the digital copying machine shown in FIG.

FIG. 13 is a graph showing a relationship (IL characteristic) between a forward current (I) and a light emission intensity (L) of a laser diode.

FIG. 14 is a circuit diagram showing a control method of a laser diode.

FIG. 15 is an explanatory diagram showing a 1 × 2 matrix optical writing system.

FIG. 16 is a chart showing a halftone density region to be formed.

FIG. 17 is a block diagram of a two-dot multi-level circuit according to the second embodiment.

FIG. 18 is a block diagram illustrating a configuration of a pulse signal generation circuit.

FIG. 19 is an explanatory diagram showing signal waveforms at various parts of the pulse signal generation circuit shown in FIG.

20 is an explanatory diagram showing a state of dot formation by the pulse signal generation circuit shown in FIG.

FIG. 21 is a block diagram showing a configuration of another pulse signal generation circuit.

FIG. 22 is an explanatory diagram showing a state of dot formation by the pulse signal generation circuit shown in FIG. 21;

FIG. 23 is an explanatory diagram showing a state of pixel formation.

FIG. 24 is a chart showing a halftone density region to be formed.

[Explanation of symbols]

117 CCD image sensor 122 Photoconductor drum 219 Laser output unit 330 Beam sensor 450-453 Delay element 454,455 AND circuit 456,457 OR circuit 458 Selector 459 LD drive circuit 652 Constant current source 702,704 Amplifier 703 Switching IC 705 A / D converter 800 IPU (image processing device) 1001, 1002, 1701, 1702 Line memory 1003, 1004 Latch 1005, 1703 Adder 1006, 1704 ROM 1200 First current converting means 1201 Second current converting means 1202 Light receiving element 1203 Current Amplifiers 1441 to 1444 Constant current source 1445 to 1447 Switch 1801 Sawtooth wave generating circuit 1802 D / A converter 1803 Comparator 2101 Triangular wave generating circuit Road

Claims (2)

[Claims] 1. Each dot forming image data obtained by reading a document is converted into write data every two adjacent dots,
An image forming apparatus that forms an image based on the converted writing data, wherein when converting a first dot forming the image data and a second dot adjacent to the first dot into writing data, The dot data of the first dot and the second dot data
If the total value of the dot data of the dot data exceeds the saturation value of the dot data of a single dot, the saturation value is distributed as the dot data of the first dot, and the saturation value is calculated from the total value. An image forming apparatus, comprising: a distribution unit that distributes a value obtained by subtracting as a dot data of the second dot. 2. The method according to claim 1, wherein the distributing means distributes the total value as dot data of the first dot when the total value does not exceed the saturation value of dot data of a single dot. The image forming apparatus according to claim 1.