JPH05284339A - Image forming device - Google Patents

Abstract

PURPOSE:To reproduce a picture with more fidelity with respect to an original by giving weighting to density based on adjacent read dot data, distributing the result to written dot data and concentrating the density to a specific dot. CONSTITUTION:Line memories 1101,1102 and latches 1103,1104 use plural dots of read data in the main scanning direction and in the sub scanning direction of read data in 6-bits comprising 2-bits in pairs inputted from a scanner to implement area gradation. An adder 1105 adds reflection density data of adjacent read dots for area gradation and a ROM 1106 implements density conversion by distributing the sum data to density data of picture elements so as to reach the saturation density by dots for area gradation thereby implementing density conversion. A demultiplexer circuit 1107 compares density data of adjacent read dots for area gradation and uses dots with high density in picture elements as density occurrence dots. Through the constitution above, banding is reduced, reproducibility with smooth intermediate tone is maintained, a picture error is reduced and a picture with more fidelity with respect to original information is reproduced.

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 image forming apparatus such as a digital copying machine, a laser printer, a facsimile apparatus, a display device, and the like, and more particularly, to a multi-gradation writing method. Density is weighted from the adjacent read dot data and distributed to the write dot data, and the density data of the pixel is distributed so that the saturation density is reached from a certain dot that performs area gradation, and the density of the pixel within the read original data is distributed. Generate density from high density dots,
The present invention relates to an image forming apparatus that reduces an image error that occurs in a two-dot multivalued line image and reproduces an image faithfully to a document.

[0002]

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

Conventionally, as a method of expressing gradation,
JP-A-54-144126, JP-A-56-174
There is an area gradation method using a dither matrix disclosed in Japanese Patent Laid-Open No. 78 and Japanese Patent Laid-Open No. 57-76977.
However, in the above-mentioned area gradation method, since a pixel is composed of a plurality of dots and density is expressed by the number of written dots, the resolution is lowered. In this case, in the binary writing method, assuming that the number of dots forming a pixel is N, the number of gradations represents N gradations without including a background white portion, but generally the resolution is It decreases to 1 / N.

On the other hand, there has been proposed a 1-dot multi-value writing system which realizes multi-gradation without lowering the resolution. This is to modulate the density of one dot to be written in, for example, electrophotographic laser beam writing.
The light modulation method of the laser diode for writing mainly includes a pulse width modulation method that modulates the exposure time and a power modulation method that modulates the exposure intensity. A specific technique is disclosed in Japanese Unexamined Patent Publication No. 62-49776 as the pulse width modulation method, and in Japanese Unexamined Patent Publication No. 64-1547 as the power modulation method.

Further, a two-dot multi-value writing method has been proposed in which multi-value writing by one-dot modulation is combined with a minute matrix that avoids deterioration of resolution as much as possible. The two-dot multi-value writing method regularly executes dot distribution of density data, as shown in FIGS. 15 (a) and 15 (b). The shaded area in the figure shows the density generation matrix, and the arrow direction shows the density data transfer. Image formation by this conventional method corresponds to the image of the conventional portion shown in FIG.

As one condition for improving the image quality of a digital copying machine, it is necessary to reproduce halftones with high precision, and to achieve both resolution and gradation, the above-mentioned 1-dot multi-value writing method is required. Is preferred.

[0007]

However, the above-mentioned 1
The dot multi-value writing method has a problem that banding easily occurs. For example, in a digital copying machine, banding that occurs in the halftone region is caused by uneven driving and vibration of the photoconductor, uneven scanning pitch of the writing optical system, and the like, and the banding is continuous band-shaped density unevenness in the main scanning direction. Appears as. Particularly, in the 1-dot multi-value writing method, banding occurs due to the overlapping of the skirts of the exposure beam in the intermediate exposure region due to the scanning pitch unevenness in the sub-scanning direction in the exposure laser diode.

[0008] Further, due to the higher resolution, the accuracy with respect to banding is being demanded. In the multi-value writing of 1 dot at about 400 dpi, which is widely used at present, in the current electrophotographic process, image noise due to uneven density occurs in the solid halftone portion regardless of the modulation method, and the halftone is smooth. There was a problem that it was not reproduced. Further, in the one-dot multi-value writing method of the electrophotographic method using the laser diode (LD), the toner binding force is weak in the halftone area, the image density becomes uneven, and the surface of the recording paper is rough. There will be an image.

Further, by combining the multi-value writing by the 1-dot modulation with the area gradation of the minute matrix that avoids the deterioration of the resolution as much as possible, the density of the banding and the halftone portion generated by the 1-dot multi-value writing. Image noise due to unevenness can be reduced, halftones can be reproduced smoothly, and gradation is stable, but with an image forming method using a 2-dot multi-valued line image that forms 200 lines with a writing density of 400 dpi, There is a problem that character and line cracks occur, image quality deteriorates, and 2-dot multi-valued 200 lines are visually resolved, and an image error occurs due to the regular density shift. (See the conventional portion of FIGS. 15 and 13).

The present invention has been made in view of the above, and reduces banding in the 2-dot multi-valued line image processing, maintains smooth reproducibility of halftone, and occurs in a 2-dot multi-valued line image. The objective is to improve image quality by reducing image errors and reproducing images that are more faithful to document information.

[0011]

In order to achieve the above object, the present invention provides a one-dot multi-gradation writing means and an area gradation means for performing area gradation with a plurality of dots in the main scanning or sub-scanning direction. And the reflection density data of adjacent read dots that perform area gradation are added, and the added data is distributed to the density data of pixels so that the saturation density is reached from a certain dot that performs area gradation, and density conversion is performed. Provided is an image forming apparatus comprising density conversion means and comparison means for comparing density data of adjacent read dots which perform area gradation, and which has a high density dot in a pixel of document read data as the density generation dot. To do.

Further, the 1-dot multi-gradation writing means, the area gradation means for performing area gradation with a plurality of dots in the main scanning or sub-scanning direction, and the density data difference between adjacent read dots for performing area gradation are calculated. Calculating means, adding means for adding reflection density data of adjacent read dots for performing area gradation, and adding the added data to pixel density data so as to reach saturation density for a certain dot for area gradation. When the density data difference between the adjacent read dots is within a certain reference value, the density generating dots are regularly arranged and the density data difference is greater than or equal to the reference value. In this case, the present invention provides an image forming apparatus that arranges dots with high density in pixels of the original read data.

Further, the 1-dot multi-gradation writing means, the area gradation means for performing area gradation with a plurality of dots in the main scanning or sub-scanning direction, and the density data difference between adjacent read dots for performing area gradation are calculated. Calculating means, adding means for adding reflection density data of adjacent read dots for performing area gradation, and adding the added data to pixel density data so as to reach saturation density for a certain dot for area gradation. An image forming apparatus having a density converting means for distributing and converting the density, comparing the high-order bit data of the density data of the adjacent read dots, and using a dot having a high density in a pixel of the original read data as a density generation dot. Is provided.

[0014]

The image forming apparatus according to the present invention weights the density of adjacent read dot data by the multi-gradation writing method, distributes the density to the write dot data, and further saturates from a certain dot which performs area gradation. The density data of the pixels is distributed so as to reach the density, and the density is generated from the dot having the high density in the pixel of the read original data.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the accompanying drawings. (1) Configuration of digital copying machine (2) Modulation method of write laser diode (3) Image reading signal processing (4) Image processing (5) 2-dot multi-valued algorithm (6) 2-dot multi-valued circuit (7) The effect of each embodiment will be described in detail in the order.

(1) Configuration of Digital Copying Machine FIG. 1 shows a digital copying machine comprising a laser printer to which a general laser writing means is applied and a document reading device. In FIG. The contact glass 111 for mounting is illuminated by the light source 112, and the reflected light from the image surface of the read document is reflected by the mirror 113,
An image is formed on the light receiving surface of the CCD image sensor 117 via 114, 115 and the lens 116. Also, the light source 11
2 and the mirror 113, the traveling body 11 that moves the lower surface of the contact glass 111 in parallel with the contact glass 111.
It is installed in 8.

Main scanning is performed by solid-state scanning of the CCD image sensor 117. The original image is one-dimensionally read by the CCD image sensor 117, and the entire surface of the original is scanned by moving (sub scanning) the optical system. In this example, the read processing density is 40 for both main and sub-scanning.
Set to 0 dpi, A3 size (297 mm x 420
(mm) documents can be read.

Next, a laser printer constituting the above digital copying machine will be described. When the document reading device and the laser printer are integrally configured (this embodiment)
In some cases, the configurations are separate and only electrically connected. In this laser printer, various systems such as a laser writing system, an image reproducing system, and a paper feeding system are integrally configured.

The above laser writing system is shown in FIG. 1, FIG. 2 and FIG.
As shown in FIG. 3, a laser output unit 219, an imaging lens group 120, and a mirror 121 are provided. A laser diode LD1 which is a laser light source is provided inside the laser output unit 219, and a polygon mirror (polygon mirror) 21 that rotates at a high speed and a constant speed by a motor is provided in the writing unit.
9a is equipped. Laser light output from the laser writing system is applied to the photoconductor drum 122 equipped in the image reproducing system via the polygonal mirror 219a and the mirror 121.

As shown in FIG. 1, the photosensitive drum 12
Around the circumference of 2, a charging charger 123 that uniformly charges the surface of the photoconductor drum 122, a developing unit 125 that visualizes the formed electrostatic latent image, and a photoconductor drum on the conveyed recording paper. A transfer charger 126 that transfers the visible image formed on the surface 122, a separation charger 127 that separates the recording paper from the photosensitive drum 122, and a separation claw 128.
Further, a cleaning unit 129 for cleaning the surface of the photosensitive drum 122 after the transfer processing 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 light is emitted (see FIG. 3).

Further, 131 is a conveyor belt for conveying the recording paper after the transfer processing in a predetermined direction, and 132 is a conveyor belt 131.
The fixing unit for fixing the image on the recording paper conveyed by means of 133, 134 is a paper feed cassette for loading recording papers of different sizes, and 135 and 136 are for feeding the recording paper from the paper feed cassettes 133, 134. Paper feed roller, 1
A registration roller 37 conveys the recording paper to the transfer portion at a predetermined timing.

The operation of the above structure will be described. The surface of the photosensitive drum 122 is connected to the charging charger 12.
3 is uniformly charged to a high potential. When the surface of the photoconductor drum 122 is irradiated with laser light, the potential of the irradiated portion is lowered. Since the laser light is ON / OFF controlled according to the black / white of the recording pixel, the potential distribution corresponding to the recorded image on the surface of the photoconductor drum 122 by the laser light irradiation,
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 by the registration roller 137 to the portion where the toner image is formed at a predetermined timing and overlaps with the toner image. After this toner image is transferred onto the recording paper by the transfer charger 126, the recording paper is separated by the separation charger 127 and the separation claw 128 from the photosensitive drum 122.
Separated from. The separated recording paper is conveyed by the conveyor belt 131.
Fixing unit 13 that is transported by
After being heat-fixed by 2, the sheet is discharged to a sheet discharge tray (not shown). After the transfer process is completed, the surface of the photosensitive drum 122 is cleaned by the cleaning unit 129 to prepare for the next copying process.

In the digital copying machine shown in FIG. 1,
The paper feeding system consists of two systems, one of which is
The paper feeding cassette 133 is equipped, and the paper feeding cassette 134 is equipped in the other paper feeding system. The recording paper in the paper feed cassette 133 is fed by the paper feed roller 135, and the recording paper in the paper feed cassette 134 is fed by the paper feed roller 136. The fed recording paper is temporarily stopped while being in contact with the registration roller 137, and is conveyed to the transfer portion of the photosensitive drum 122 at a timing synchronized with the progress of the recording process. Although not shown, each paper feed system
It is equipped with a size detection sensor that detects the size of the recording paper in the cassette.

(2) Modulation Method of Writing Laser Diode FIG. 4 shows a laser diode (L) according to an embodiment of the present invention.
It is a block diagram explaining the power modulation system of D), and the light emission level command signal is input into the 1st electric current conversion means 440 and the 2nd electric current conversion means 441.

In the first current converting means 440, the light emitting level command signal is converted into a light emitting level command signal current (output current) I _S according to its strength. The output current I _S of the first current converter 440 is the input current (I _S −I _L ), which is the difference between the output current I _S and the photocurrent I _L proportional to the optical output P _O generated in the light receiving element 442 of the laser diode LD1. Then, it is input to the current amplifier 443. It said current amplifier 443, the input current (I _S -I _L) of A multiplied by the output current A (I _S -I _L)
Is output.

On the other hand, the light emission level command signal is converted by the second current converting means 441 into an output current I ₁ for causing the set light amount P _S to emit light. I, which is the sum of this output current I ₁ and the output current A (I _S −I _L ) of the current amplifier 443.
_{1 + A (I S -I L} ) is a forward current of the laser diode LD1.

In this way, the laser diode LD1
Obtains an optical output P _O determined by the forward current I ₁ + A (I _S −I _L ). That is, P _O = P {I ₁ + A (I _S -I _L )} P: The relational expression of the function representing the optical output of the LD 1-forward current characteristic is established.

Since I ₁ is set so that I _S ≈I _L , it can be approximated as follows.

[0030]

[Equation 1]

Letting the radiation sensitivity S of the light receiving element and the coupling efficiency with the laser diode LD1 be α, it can be expressed as P _O = P _S + η · A · (I _S −P _O · S · α)

[0032]

[Equation 2] Becomes

Assuming that the crossover frequency of the photoelectric negative period loop is f ₀ , the step response of the optical output P _O is: P _O = I _S / αS + {P _S −I _S / αS} · exp (−2πf ₀ It can be expressed approximately as t).

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

Further, the integral value of the total light (light output up to the time tau ₀ which is set immediately after changing the optical output P _O ∫P
_OUT ) If the crossover frequency f ₀ for making the error 0.4% or less is τ ₀ = 50 ns, it suffices that f ₀ ≧ 40 MHz, and a crossover frequency of this level can be easily realized.

As described above, this system can realize a laser diode control system of high speed, high accuracy, and high resolution. Furthermore, a laser diode LD using this method
By modulating the power of 1, 256 analog signals are input to the light emission level command signal, and image output of 256 gradations of 1 dot is realized in the laser printer.

Next, a power modulation method of a laser diode (LD) according to another embodiment using a plurality of constant current power supplies will be described. The drive control method of the laser diode in this embodiment utilizes the relationship (IL characteristic) between the forward current (I) and the emission intensity (L) of the laser diode shown in FIG. The IL characteristic of this laser diode is
The forward current that is equal to or higher than the threshold current (Ith) is substantially linear, and the differential quantum efficiency (n) at that time is treated as constant.

As shown in FIG. 6, the control method is such that the forward current is driven by the total current of a plurality of constant current sources 641, 642, 643, 644, and the switch 6 is driven by the write data.
It switches at 45, 646 and 647. A bias current larger than the threshold current is supplied by the constant current source 641, and the laser diode drive current is 3 bits 8 by the constant current sources 642, 643, 644 weighted so that the current value becomes 1: 2: 4. To control the value. The current values at that time are I ₁ , I ₂ , and I ₃ , respectively.
The minimum bias current that does not drive 646 and 647 is I
It is ₀ . Therefore, the light emission intensity (light quantity) by each current I _{0 to} I ₃ is as shown in FIG. 5, and the light quantity by all combinations of the currents I _{0 to} I ₃ is 8 from L _{0 to} L _7, and the light quantity difference is equal. can get.

The above setting procedure is executed as follows. The LD emission intensity range is set to P _{0 to} P _max (however,
P ₀ ≈0). LD minimum emission intensity P ₀ ← LD forward current I ₀ is determined. LD maximum emission intensity P _max ← LD forward current I ₀ + I
Determine I _max by _max . I1 = (1/7) · I _max , I2 = (2/7) · I
_max , I3 = (4/7) · I _max . From the 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 by 8-bit emission data, 256 laser diode exposure outputs can be obtained. Is obtained.

(3) 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 5000 pixels, 400
It is possible to perform dpi reading processing, and simultaneously reads the reflected light in the main scanning direction of the document. The optical data accumulated by the CCD 117 is converted into an electric signal (photoelectric conversion), waveform correction such as clamping, amplification, and A / D conversion are executed, and output as a 6-bit digital signal to the IPU (image processing device).

More specifically, the CCD 117
After being photoelectrically converted, the analog data output of is output separately for two systems of EVEN and ODD for high-speed transfer, amplified by amplifiers 702 and 703 (signal amplification) respectively, and sent to a switching IC 703 composed of analog switches. input. Here, it is combined with a serial analog signal (signal combination). 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 rate of one pixel after composition is about 10 MHz, and in synchronism with this, the A / D converter 705 converts it to a digital signal of 6-bit 64 gradations (signal digitization). Further, the (variable) amplifier 704 reads a reference white plate (not shown) before scanning a document and controls the amplification degree to an appropriate value in order to correct the light amount variation of the exposure fluorescent lamp.

(4) Image Processing A digital signal for each pixel indicating the density of a document is input to the IPU (image processing unit) 800 and subjected to image processing. IP
The flow of image processing by U800 is shown in FIG. IPU8
00 is composed of a plurality of LSIs, and executes the following control based on it in addition to image processing.

Shading correction processing Since a linear light source of a fluorescent lamp is used and light is condensed by a lens, the light amount becomes maximum at the central portion of the CCD 117 and decreases at the end portions. Further, the CCD 117 has variations in sensitivity among individual elements. In both of the above, the document data is corrected based on the reference white plate read data for each pixel.

MTF Correction Processing In an optical system using a lens or the like, the read output by the CCD 117 is read as blunt because the peripheral pixel information is affected by the performance of the lens and the like. Therefore, when one pixel data is obtained, correction is performed based on the peripheral pixel level to obtain an image with high reproducibility.

Main scanning direction scaling processing In this embodiment, the image reading and writing resolutions are the same 400 dpi, but the reading pixel frequency is about 1.
Since 0 MHz and the writing pixel frequency differ by about 12 MHz, frequency conversion is executed. The clock conversion is realized by reading and writing in the two-line memory, and the main scanning magnification is calculated by calculation using peripheral pixel data in the main scanning direction.

Γ-correction processing 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, and remain as they are. The original density is not reproduced faithfully. The above may be corrected individually, but in the present image forming apparatus, conversion processing is performed in consideration of both. Also, when adjusting the density manually, the density can be adjusted by changing this value.

Other Processes In addition to the above, the IPU (image processing device) 800 uses the AGC (A
Controls such as auto gain control (auto gain control), image conversion such as masking, trimming, mirroring, black and white reversal, document size and density detection, image detection of markers and the like are also executed.

According to the present invention, one dot 256 gradation output by power modulation of the laser diode is combined with a matrix of 2 dots in the main scanning direction and the sub scanning direction. FIG. 9A is a 1 × 2 matrix for performing area gradation with two adjacent dots in the sub-scanning direction, and FIG. 9B is a 2 × 1 matrix for performing area gradation with two adjacent dots in the main scanning direction. Indicates the embedded method. In the low density portion, when the exposure power is increased from one dot to reach the maximum value (255), the exposure power of the next dot is increased. Further, in the above, density reproduction is executed by using two dots in the main scanning or sub-scanning direction as the target pixel. The read 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 reflection density of the original, and the density data of 2 dots is added to the digital value, and γ conversion is executed for the added value to convert it to the writing density data by the above method. To do. As a result, 512 gradations can be realized with 2 dots in the main scanning direction and the sub scanning direction.

FIG. 10 is an explanatory diagram showing the operation of generating the halftone density area. In FIG. 10, the density is filled from the EVEN dots. The 1 × 2 matrix shown in FIGS. 10A and 10C, which executes the area gradation in the sub-scanning direction, has a horizontal line tone in a continuous intermediate density region, and the area gradation in the main scanning direction is shown in FIG. The 2 × 1 matrix in (d) has a vertical line tone in a continuous intermediate density region.

FIGS. 10C and 10D are respectively shown in FIG.
The writing phases of (a) and (b) are alternately changed. Two dot lines are formed in the main scanning and the sub-scanning.
An image of 0 line is formed. As a result, the number of gradations does not change, but the lines are concentrated and the apparent resolution is reduced by half.

(5) 2-dot multi-valued algorithm The density is reproduced by using 2 dots in the main scanning or sub-scanning direction as the target pixel. The reading voltage of the CCD 117 is proportional to the amount of received light. The amount of light received by the CCD 117 is linear with respect to the amount of reflected light of the document, and the light amount data of 2 dots is added as a digital value. By adding the reflected light amount data, the total reflection amount for two dots can be known, and data in which two dots are one pixel can be obtained. Then, γ conversion is performed on the added value to convert it into writing density data. The density data distribution and gradation correction are converted into the writing density data of one pixel formed by two dots so as to be equal to the read density of one pixel formed by two dots.

The density conversion algorithm of 2-dot multi-value writing is F (Dm + Dm _{+ 1} ) = Pm + Pm _{+ 1} However, when Pm + Pm _{+ 1} <255, when Pm _{+ 1} = 0 Pm + Pm _{+ 1} ≧ 255, Pm = 255 It is expressed by the formula.

Here, Dm is the main scanning or sub scanning direction m
The read density data of the dot, Pm is the write density data of the m dot in the main scanning or sub-scanning direction, and F is the density conversion function including γ correction. When the density conversion is linear, the density conversion function F is a constant and is represented by 4.

The conventional two-dot multi-value writing dot formation method based on the above method is shown in FIG. 15 in order to regularly distribute the density data dots. The shaded area indicates the density generation dots, and the arrow direction indicates the transfer of density data. 1 × 2 matrix of FIG. 15 (a), FIG.
In the 2 × 1 matrix image of 5 (b), the weighted distribution of 2 dots is in the same direction, and horizontal and vertical line images are formed.

In the first embodiment of the present invention, weighted distribution is performed from dots having high density in the document information, and while maintaining the density of pixels, when Dm ≧ Dm _{+ 1} and Pm + Pm _{+ 1} <255, When Pm ₊₁ = 0 Pm + Pm ₊₁ ≧ 255, when Pm = 255 Dm <Dm _{+ 1} , when Pm + Pm _{+ 1} <255, when Pm = 0 Pm + Pm _{+ 1} ≧ 255, Pm _{+ 1} = 255 Perform the conversion.

Further, in the second embodiment of the present invention,
The density difference of the read dots is compared with the threshold value T, and when | Dm−Dm ₊₁ | <T or Dm ≧ Dm ₊₁ and Pm + Pm ₊₁ <255, when Pm ₊₁ = 0 Pm + Pm ₊₁ ≧ 255, Pm = 255 | Dm−Dm _{+ 1} | ≧ T and Dm <Dm _{+ 1} , when Pm + Pm _{+ 1} <255, when Pm = 0 Pm + Pm _{+ 1} ≧ 255, the conversion is executed under the condition of Pm _{+ 1} = 255. To do.

(6) Two-dot multi-valued circuit FIG. 11 is a block diagram showing the configuration of a two-dot multi-valued circuit for executing weighting processing of two-dot multi-valued and dot phase. Line memory (FIFO) 110 connected in series for inputting signals
1, 1102 and a latch (DF / F) 1103, 11
04, the adders 1105 connected to the line memories 1101 and 1102 and the latches 1103 and 1104 via switches SW1 to SW4, and the adder 110, respectively.
5 is composed of a ROM 1106 (LUT: lookup table) connected to the memory 5, and a separation circuit 1107 composed of a data subtraction unit and a comparison unit. The RO
The output from M1106 is output to the printer as an 8-bit data signal.

In the above configuration, 2 dots each 6 bits
2-dot multi-valued write data is created from the read data of, and the density generation dot is switched for each pixel. Further, a delay unit for the data signal is provided so that the data signal and the switching signal (SEL) are synchronized. Hereafter, 1 × 2 matrix,
The 2 × 1 matrix and generation of density dots will be described in detail.

1 × 2 Matrix When area gradation is executed with 2 dots in the sub-scanning direction (1 × 2
2 matrices) are two line memories (FIFO) 1
Using 101 and 1102, the read data for two main scanning lines is delayed. SW1 selected from SW3 and 4,
Switching is performed for each main scanning line. After that, 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. ROM
In 1106, one table is composed of 256 bytes, the first half 128 bytes of which is EVEN and the latter half 128 of which.
The bytes are ODD data.

First, the first addition data is the ROM 1106.
Data and switching signal (SE
The EVEN data indicated by the address L) is output as the write data. The same data is added on the next line, and the data and the write data indicated by the switching signal (SEL) are output from the data bus. Switching between EVEN (L) and ODD (H) is selected by a switching signal (SEL), and data of density generation dots is designated by EVEN.

In the block diagram of the 2-dot multi-valued circuit shown in FIG. 11, the switch SW1 and EVEN / ODD are switched every main scanning line, and the switches SW3 and SW4 select the data from the line memories 1101 and 1102. Set it to the upper side.

2 × 1 matrix When area gradation is performed with 2 dots in the main scanning direction (2 × 1 matrix), two latches (D-F / F) 1103 are used.
1104 is used to delay the read data of 2 dots in the main scanning direction. Thereafter, as in the case of the 1 × 2 matrix, addition and γ conversion are performed and write data is output. Then, the process shifts to the next 2 dots and the processing is repeated. In this case, it is selected by SW3, 4 (lower side in the figure), and SW2
Switches every writing clock. As described above, gradation information is not lost in all modes.

Generation of Density Dots The separation circuit 1107 is composed of a data subtraction unit and a comparison unit. The subtraction of 6-bit data is executed by taking the complement and adding, and when it is determined that the target dot data is larger than the other data (the reading density is high), the switching signal (SEL) indicates the density generation side indicated by EVEN. The data of.

Further, in FIG. 11, the subtraction data to be obtained is 0 to 63, and the value is compared with the threshold value (TH). The threshold value can be input from 0 to 63, and the comparison is determined by overflow due to addition of the complement of the subtraction data and the threshold data. As a result of the comparison, in the pixel in which the overflow occurs, the density difference of 2 dots becomes larger than the set threshold value. Therefore, when it is determined that the target dot data is larger than the other data as described above, the switching signal (SEL)
Indicates the data of the dot on the density generation side indicated by EVEN. On the other hand, when the density difference of two dots is smaller than the threshold value, the EVEN and ODD dots are regularly generated as shown in FIGS. 15A and 15B regardless of the size of the dot data. As described above, the write data is generated by discriminating in real time from the image data.

LU in ROM 1106 used for this device
FIG. 12 shows an example of the γ conversion table of T (lookup table). The copy density is set to be substantially equal to the original density. In FIG. 12, when one of the EVEN dots is exposed up to the intermediate density and is increased with respect to the input data, and the EVEN dot reaches approximately the maximum value,
Increase the exposure intensity of ODD dots. Thereby, the dots are concentrated while maintaining the density information of 2 dots.
In general, the inverse conversion of the γ characteristic of the printer represented by the print density with respect to the writing exposure light amount is made into a table value, so that the γ characteristic of the printer alone can be made linear.

The 2-dot multi-valued circuit shown in FIG.
It is configured in the IPU 800 and converts the image data for each dot from the scanner and outputs it to the writing system. As a result of the above, writing of 512 gradations is realized with a unit of 2 dots in the main scanning direction and the sub scanning direction as one pixel.

FIGS. 13A and 13B are examples of image output according to the conventional method and the method according to the present invention. FIG. 13A is a vertical line image, FIG. 13B is a character “A”, and FIG. 13C is a graphic pattern.
Is the state of the original, is the output example of the conventional method, and is the output example of the present invention. Also, this is
In the method using the 2 × 1 matrix in (b), the data output is represented by dots and the density is represented by the size of the dots in order to simplify the explanation. Both form one pixel in units of 2 dots, but in the present invention, the image is reproduced at a position more faithful to the original. In particular, FIG.
It can be seen that the output image shown in (a) is a line image with an equal pitch, in which the pitch error is reduced and line breakage is also reduced.

FIG. 14 is a block diagram showing the configuration of the 2-dot multi-valued / dot phase weighting circuit according to the second embodiment of the present invention. The separation circuit 1107 of the 2-dot multi-valued circuit shown in FIG. Instead, the comparison circuit 1401 is provided. The other symbols are the same as those in FIG.
It is the same as that of the reference numeral.

In the block diagram of the 2-dot multi-valued circuit shown in FIG. 11, comparison is made with read data of 6 bits each for 2 dots,
The density generation dot was switched for each pixel. Moreover, a threshold for comparison was set, and when the data difference was within the threshold, the density generation dots were arranged regularly. On the other hand, FIG.
In the 2-dot multi-valued circuit shown in (1), the upper 2 bits of the 6-bit read data are input to the comparison circuit 1401 as comparison data, and the comparison result causes the density to be generated from the dot having the high read density as described above. As a result, the variation of the lower 4 bits of the read data is ignored and compared,
The same effect as when the threshold for comparison is provided can be obtained.

In the circuit example of FIG. 14, the upper 2 bits of the read data of 6 bits are used as the comparison data. However, in accordance with the image forming system, several bits of the read data (from 1 to the read data bit number -1). ) Can be fixed or arbitrarily set. If the number of bits of the comparison data is reduced, the tendency of the line image as shown in FIG. 15 becomes stronger, as in the case where the comparison threshold value is increased in the above example.
Excellent in high-quality reproduction of photographic images. On the other hand, increasing the number of bits of the comparison data is effective in improving the resolution of images of characters and lines.

In the present invention, in the processing of a plurality of dots, for example, 3 dots, banding is eliminated, density stability is improved, and halftones are reproduced smoothly. Further, in that case, the effect of the writing method of the present invention is further increased, and the information on the resolution of the original is maintained.

The multi-gradation writing method using a plurality of dots according to this method can be used for image processing irrespective of the laser diode (LD) modulation method such as power modulation or pulse width modulation in addition to this embodiment. It is a method.

(7) Effects of Each Embodiment The effects of each of the above embodiments will be summarized and described below. First, by weighting the densities of adjacent read dot data, distributing them to the write dot data, and concentrating the densities on specific dots, banding and density unevenness in the halftone part caused by multi-value writing of one dot. The image noise due to is reduced. Further, by performing the density shift based on the document information, image errors can be reduced, more faithful image reproduction with respect to the document information can be achieved, and high image quality with excellent resolution can be realized. Therefore, this method can successfully reproduce mixed images of characters and photographs.

Second, from the adjacent read dot data,
By weighting the densities, distributing them to the write dot data, and concentrating the densities on specific dots, it is possible to reduce banding and image noise due to density unevenness in the halftone portion that occur due to multi-value writing of one dot. Furthermore, when the difference in the density data of adjacent dots is within a certain reference value, the density generating dots can be regularly arranged to smoothly reproduce the halftone uniform density portion that is often found in the photograph part. Further, when the density data difference is equal to or larger than a reference value, dots having a high density in the pixels of the original read data are arranged, and an image faithful to the original information can be reproduced. In particular, this is effective when the read data has an error or noise component, and is also effective for halftone dot originals. Further, it is possible to prevent erroneous detection due to variations in the sensitivity of the CCD 117.

Thirdly, the number of bits for data comparison is reduced,
The same effect as described above can be obtained by comparing the high-order bit data. In this case, since it is not necessary to calculate the difference and the number of data bits is small, the hardware configuration is simple and can be realized at low cost.

[0076]

As described above, according to the image forming apparatus of the present invention, the weighted distribution is executed for the write dot data based on the adjacent read dot data, and further the area gradation is executed. The density data of pixels is distributed so as to reach the saturation density from a certain dot to generate a density higher than that of a dot in a read document, so that banding reduction and halftone in a 2-dot multi-value line image processing are performed. It is possible to improve the image quality by maintaining the smooth reproducibility of the above, reducing the image error occurring in the 2-dot multi-valued line image, and reproducing the image reproduction more faithful to the document information.

[Brief description of 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 (No. 1) of a laser writing system in the digital copying machine according to the present invention.

FIG. 3 is an explanatory diagram showing a configuration (No. 2) of a laser writing system in the digital copying machine according to the present invention.

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 is a graph showing the relationship (IL characteristic) between the forward current (I) and the emission intensity (L) of the laser diode according to the present invention.

FIG. 6 is a circuit diagram showing a control system of a laser diode according to the present invention.

FIG. 7 is a block diagram showing each unit that executes image reading signal processing according to the present invention.

FIG. 8 is a block diagram showing a flow of image processing by the image processing apparatus according to the present invention.

FIG. 9 is an explanatory diagram showing a 1 × 2 matrix and a 2 × 1 matrix light amount system according to the present invention.

FIG. 10 is a chart showing a halftone region according to the present invention.

FIG. 11 is a block diagram showing a configuration of a 2-dot multi-valued circuit according to the present invention.

FIG. 12 is an explanatory diagram showing a 2-dot multi-valued γ conversion table according to the present invention.

FIG. 13 is an explanatory diagram showing an example of image output according to the present invention and the conventional method.

FIG. 14 is a block diagram showing the configuration of another 2-dot multi-valued circuit according to the present invention.

FIG. 15 is an explanatory diagram showing an example of image output by a conventional 2-dot multi-value writing method.

[Explanation of symbols]

117 CCD image sensor 122 photoconductor drum 219 laser output unit 330 beam sensor 440 first current conversion means 441 second current conversion means 442 light receiving elements 641 to 644
Constant current source 645-647 switch 702 704
Amplifier 703 Switching IC 705 A / D converter 800 IPU (Image processing device) 1101 1102 Line memory (FIFO) 1103 1104 Latch (D-F / F) 1105 Adder 1106 ROM
(LUT) 1107 Separation circuit 1401 Comparison circuit

Claims (3)

[Claims] 1. A 1-dot multi-gradation writing means, an area gradation means for performing area gradation with a plurality of dots in the main scanning or sub-scanning direction, and reflection density data of adjacent read dots for performing area gradation are added. Then, the addition data is distributed to the density data of the pixels so that the saturation density is reached from a certain dot that performs the area gradation, and the density conversion means that performs density conversion, and the density data of the adjacent read dots that perform the area gradation. An image forming apparatus, comprising: a comparing unit for comparing the above-described items, wherein a dot having a high density in a pixel of document reading data is used as the density generating dot. 2. A 1-dot multi-gradation writing means, an area gradation means for performing area gradation with a plurality of dots in the main scanning or sub-scanning direction, and a density data difference between adjacent read dots for performing area gradation is calculated. Calculating means, adding means for adding reflection density data of adjacent read dots for performing area gradation, and adding the added data to pixel density data so as to reach saturation density for a certain dot for area gradation. When the density data difference between the adjacent read dots is within a certain reference value, the density generating dots are regularly arranged and the density data difference is greater than or equal to the reference value. In this case, the image forming apparatus is characterized in that dots are arranged in a pixel of the document read data with a high density. 3. A one-dot multi-gradation writing means, an area gradation means for performing area gradation with a plurality of dots in the main scanning or sub-scanning direction, and a density data difference between adjacent read dots for performing area gradation is calculated. Calculating means, adding means for adding reflection density data of adjacent read dots for performing area gradation, and adding the added data to pixel density data so as to reach saturation density for a certain dot for area gradation. A density conversion means for allocating and converting the density, and comparing the high-order bit data of the density data of the adjacent read dots, and making a dot having a high density in a pixel of the original read data a density generation dot. Image forming apparatus.