JPH04348660A - Digital copying machine and its picture write method - Google Patents

Abstract

PURPOSE:To enhance the reproducibility of an intermediate tone by adopting a natural and regular dot pattern in a photograph mode. CONSTITUTION:Picture information of an original is read and converted into a picture signal and an A/D converter circuit is used to convert into a digital data of several gradation per one picture element and subject to MTF correction or smoothing processing or the like by a spatial filter circuit 16 after shading correction. Then a gamma conversion ROM 41 and a code conversion ROM 42 are used and the result is converted into a phase data to control a phase of pulse width modulation for each picture element in response to the position of the picture element to be written in and the gradation code data representing the density of the picture element in the photograph mode and a printer section writes the picture onto recording paper by using the data.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital copying machine and an image writing method therefor.

0002

2. Description of the Related Art In recent years, in digital copying machines and laser printers, advances in image writing technology have led to improvements in the quality of copied or printed images through single-dot multi-gradation expression.

[0003] In a digital copying machine that employs such a one-dot multi-gradation system, documents containing characters, lines, etc. can be clearly displayed without jagged edges in the characters using a character mode that emphasizes image resolution. Photo mode emphasizes reproduction of halftones for originals such as photographs,
It is necessary to copy the image so that it looks natural.

0004

[Problems to be Solved by the Invention] However, in conventional digital copying machines, the phase of each pixel in the photo mode is constant, so the dot pattern of the copied image becomes asymmetrical, which is difficult for the human eye to see. There was a problem that it gave an unnatural impression.

[0005] The present invention has been made in view of the above points, and it is possible to make dot patterns regular and natural when copying a photographic original using a digital copying machine.
The purpose is to improve the reproducibility of halftones. Also,
It also aims to improve resolution in text mode and improve midtone reproduction in photo mode.

[0006]

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a document reading section that reads image information of a document and converts it into an image signal, and an image sensor that performs input correction and output correction on the image signal. In a digital copying machine equipped with a processing section and a printer section that writes an image that has undergone input correction and output correction by the image processing section onto recording paper, the image signal is pulse width modulated for each pixel to write each dot multiple times. The present invention is provided with means for performing gradation, and means for controlling the phase of the pulse width modulation in accordance with the position of the pixel to be written in the photo mode.

[0007] In addition, in a digital copying machine equipped with an original reading section, an image processing section, and a printer section as described above, the image signal is pulse width modulated for each pixel to write each dot in multiple gradations. To provide an image writing method in which the phase of the pulse width modulation is controlled according to the density of left and right pixels adjacent to a pixel to be written in a character mode, and according to the position of a pixel to be written in a photo mode.

Furthermore, the phase of pulse width modulation in this image writing method is controlled in one of three types: a phase in which the dot writing position is shifted to the left side of the pixel, a phase in which it is shifted to the right side, and a phase in which the dot write position is shifted to the center. It is recommended that you select and do this.

[0009]

[Operation] In the digital copying machine according to the present invention, the image signal is pulse-width modulated for each pixel by the printer section, and each dot is written in multiple gradations, and in the photo mode, the phase of the pulse-width modulation is set for the pixel to be written. Since control is performed according to position, it is possible to create a regular and natural dot pattern by changing the phase of the dots for each pixel in photo mode.

Further, in the image writing method according to the present invention, the phase of pulse width modulation is controlled according to the density of the left and right pixels adjacent to the pixel to be written in the character mode, and according to the position of the pixel to be written in the photo mode. Therefore, in text mode, resolution can be increased by reducing the number of jagged lines and uneven units, and in photo mode, regular and natural dot patterns can be created to improve midtone reproducibility.

Furthermore, if the phase control of pulse width modulation is performed by selecting from three types of dot write positions, such as moving the dots to the left side, right side, or center of the pixel, when writing an image, the pixel It is possible to easily perform phase control according to the position of.

0012

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 2 is a schematic configuration diagram of a document reading device including a document reading section and an image processing section that constitute a digital copying machine that is an embodiment of the present invention, and FIG. 3 is a schematic configuration diagram of a laser printer that is the printer section of the digital copying machine. be. A general digital copying machine includes this document reading device 100 and a laser printer 2.
00 are often constructed as one unit, but here we will show an example in which the two are separable and function as a digital copying machine by electrically connecting them.

The document reading device 100 shown in FIG. 2 includes a contact glass 1 on which a document D to be read is placed;
From mirrors 3, 4, 5, 6, 7 and a lens 8, which image the reflected light (original image) from the light sources 2a and 2b, such as fluorescent lamps, and the original document D illuminated by the light sources, on the light receiving surface of the CCD image sensor 9. An optical system is provided.

The light sources 2a, 2b and the mirror 3 are attached to a traveling body 10 that moves parallel to the lower surface of the contact glass 1 in the sub-scanning direction (in the direction of the arrow in FIG. 2), and the mirrors 4 and 5 are attached to the traveling body 10. They are respectively mounted on traveling bodies 11 that move in the same direction at 1/2 the speed of the moving bodies 11, and sub-scanning is performed by these.

Main scanning is performed by the solid-state scanning of the CCD image sensor 9, and the image of the original D is read by the CCD image sensor 9. The entire original D is scanned by this main scanning and the sub-scanning by the above-mentioned optical system. and the image will be read. That is, these constitute an image reading section, and the image processing section that performs input correction and output correction on the read image signal will be described later.

In this embodiment, the reading density of the original is set to 16 pixels/mm in both main scanning and sub-scanning.
It is possible to read documents up to A3 size (297mm x 420mm).

On the other hand, the laser printer (printer section) 200 shown in FIG. It includes an image lens 22 and a mirror 23.

Inside the laser output unit 21,
A constant rotation polygon mirror (polygon mirror) rotated at high speed by a laser diode as a laser light source and a polygon motor is provided. Then, the laser beam output from this laser writing system 201 is transmitted to the image reproduction system 20.
The second photoreceptor drum 24 is irradiated with the light.

Further, a charger 25, an eraser 26, a developing unit 27, a separating claw 30, a cleaning unit 31, etc. are provided around the photosensitive drum 24. Other parts of the paper feeding system 203 and the image reproduction system 202 will be described later along with the operation of the laser printer 200. Note that a main scanning synchronization signal (MSYNC) is provided near one end of the photoreceptor drum 24 at a position where the laser beam is irradiated.
) is provided with a beam sensor (not shown) that generates the

FIG. 4 is a block diagram showing the configuration of the image processing section in the image reading apparatus 100 and the processing order of image signals (image data). 9 is a CCD image sensor, 12
1 is a sensor driver, 13 is an amplifier, 14 is an A/D conversion circuit, 15 is a shading correction circuit, 16 is a spatial filter circuit, 17 is an output modulation circuit, 18 is a processing/editing circuit, 1
9 is an output circuit.

By controlling the drive of the sensor driver 12, C
The image signal read by the CD image sensor 9 at a sampling density of 16 pixels/mm is first amplified to a predetermined voltage amplitude by the amplifier 13, and then amplified by the A/D conversion circuit 14.
It is converted into digital data with several gradations (for example, 256 gradations) per pixel.

After that, the shading correction circuit 15 corrects the uneven illuminance of the light sources 2a and 2b and the CCD image sensor 9.
The spatial filter circuit 16 performs shading correction to correct variations in sensitivity between each element, and further performs MTF correction to increase the resolution of characters and lines, smoothing processing to remove noise from photographs, etc.

Then, the output modulation circuit 17 performs γ correction in consideration of the γ characteristics of the laser printer 200, halftone expression processing in consideration of gradation reproducibility, and write signals generated by the printer section, in this embodiment, pulse width modulation. (PWM) signal is converted into code data (a code representing the pulse width and phase of PWM), and this is output from the output circuit 19 to the laser printer 200 to write an image on recording paper.

Note that the code data may be subjected to various processing and editing processes by the processing/editing circuit 18 and then output. In addition, the spatial filter circuit 1
6 and the output modulation circuits 17 and thereafter may be provided in the printer section.

In this embodiment, the output modulation circuit 17 modulates the pulse width of the image signal for each pixel according to its density, and also modulates the pulse width of the image signal in the character mode according to the density of the left and right pixels adjacent to the pixel to be written. Sometimes, the phase of pulse width modulation is controlled depending on the position of the pixel to be written, but the details will be described later.

Next, the image forming process in laser printer 200, which is the printer section shown in FIG. 3, will be briefly described. First, the circumferential surface of the photosensitive drum 24 is charged to a uniform high potential by the charging charger 25, and when the circumferential surface is irradiated with laser light by the laser writing system 201, the potential of the exposed portion decreases. The laser light is controlled on/off according to the black/white of the image, and is pulse width modulated (PWM) according to the gradation level of the image.
), the lighting pulse width of the laser diode is changed to control the irradiation area.

By the irradiation or irradiation area of this laser light, a potential distribution corresponding to the gradation level of the recorded image, that is, an electrostatic latent image, is formed on the circumferential surface of the photoreceptor drum 24.
When the portion on which the electrostatic latent image is formed passes through the developing unit 27, toner is attached depending on the level of the potential, and the electrostatic latent image becomes a visualized toner image. Therefore, the recording sheet 32a or 32b is fed from the cassette 33a or 33b to the area where the toner image is formed at a predetermined timing.
A toner image is superimposed on the surface of the recording sheet 32a or 32b.

When the toner image is transferred onto the recording sheet by the transfer charger 28, the recording sheet is separated from the photosensitive drum 24 by the separation charger 29 and the separation claw 30, and transferred to the fixing roller 35 with a built-in heater by the conveyance belt 34. The transferred image is heated and fixed there, and then discharged onto the paper discharge tray 36.

In this embodiment, the paper feed system 203 of the laser printer 200 has two systems, and the recording sheet 32a in the upper paper feed cassette 33a, which is one paper feed system, is fed by a paper feed roller 37a. The recording sheet 32b in the lower paper feeding cassette 33b, which is the other paper feeding system, is fed by a paper feeding roller 37b.

[0030] Then, the recording sheet 32a or 32b fed by either paper feeding roller 37a or 37b
The recording medium temporarily stops with its leading end in contact with the registration roller 38, and is sent to a transfer position where it contacts the photosensitive drum 24 at a timing synchronized with the progress of the recording process. In addition,
Although not shown, each paper feeding system is provided with a recording sheet size sensor for detecting the size of the recording sheets 32a, 32b stored in the cassettes 33a, 33b.

Next, image signal processing according to the present invention will be explained. This digital copying machine uses a PWM method that modulates the pulse width of the pulse signal that lights up the laser diode of the laser printer 200, which is the printer section, in order to achieve several gradations per dot (for example, 64 gradations) with the printer output signal. It has two image playback modes: a text mode that plays back text and lines at high resolution, and a photo mode that plays back photos with many halftones with an emphasis on gradation. . In addition to the above two modes, a mode may be provided in which the text area and photographic area of the document are discriminated and optimal processing is performed for each area.

In the case of character mode, after the MTF correction in the spatial filter 16 shown in FIG. The data is converted to output data of 64 gradations per dot, and data of left and right pixels is further monitored, and if there is a density difference, phase control processing is performed to shift the lighting pulse to the dark side of the left and right.

In the case of photo mode, the smoothness of the gradation as a whole is more important than the resolution, so rather than expressing the gradation in units of one dot, it is preferable to express the gradation using multiple dots, such as by dithering. In many cases, it is better to express gradations through combinations. Therefore, instead of MTF correction, smoothing processing is performed for the purpose of noise removal, and then data is converted to data with 64 gradations per dot, and as a result, the ratio of black dots in the area of multiple dots is controlled. control the pulse width and phase.

FIGS. 5 and 6 are explanatory diagrams each showing an example of gradation expression using a combination of a plurality of dots. Figure 5 is 3
This is an explanatory diagram of the case where gradation is expressed using 9 pixels of ×3, and the pulse width and phase are changed so that the black dot area starts from the center pixel, gradually extends up and down, and then the black dot area is expanded to the left and right. control.

In other words, in this method, there are three types of filling for one pixel: 1/3 width, 2/3 width, and full filling,
There are three types of phases: left-aligned, right-aligned, and center. First, in the first to third stages from the lightest density, as shown in (a) to (c), the width is 1/3,
2/3 width, fully filled, and in the 4th and 5th stages (d),
As shown in (e), the pixels located above and below the center pixel are filled with 1/3 width and 2/3 width, respectively, using the center phase.

[0036] Also, in the sixth stage, as shown in (f),
The pixels located to the left of the center pixel are filled with a right-aligned phase, and the pixels located on the right are filled with a left-aligned phase, each with a width of 1/3, and in the seventh step, the pixels located to the left of the center pixel are filled, as shown in (g). Pixels are filled with a right-aligned phase and pixels located on the right are filled with 2/3 width each with a left-aligned phase.
The pixels located above and below the center pixel are completely filled.

Next, in the eighth step, as shown in (h),
The pixels located on the left and right of the center pixel are completely filled, and the phase of the pixels located diagonally above, below, and to the left of the center pixel is aligned to the right, and the phase of pixels located diagonally above, below, and to the right of the center pixel is aligned to the left, and the phase is adjusted by 1/1, respectively. In the 9th stage, as shown in (i), pixels in the upper, lower, left, right, and diagonal directions of the center pixel are filled in with 2/3 width while keeping the phase unchanged, and in the 10th stage, which is the darkest, 9 pixels of 3 x 3 are filled. The entire area is painted black.

Although this embodiment shows an example of a 10-level pattern, if the resolution of PWM is 64 levels, it is possible to obtain 9×64+1=557 levels of gradation, so from among these, It is preferable to select the necessary level by considering the γ characteristics of the laser printer.

FIG. 6 is an explanatory diagram of the case where gradation is expressed using 16 pixels of 4×4, and in this case, the unit of the number of gradations is 2×2.
The 4×4 unit controls the blackening pattern of multiple pixels that determines the tone of the entire image. Each of the 4 x 2 x 4 pixels (hereinafter referred to as a "block") obtained by dividing the 4 x 4 x 16 pixels into 4 parts increases the black area by the same area each time the gradation level is raised. Make it.

For example, in the first stage, as shown in (a) of the same figure, the pixels located at the upper left of each of the upper and lower blocks on the left side are aligned to the right, and the pixels located at the lower left of each of the upper and lower blocks on the right side are aligned to the right. (for example, 1/
8 width), and in the second stage, as shown in (b),
Furthermore, the pixels located at the upper right of each of the upper and lower blocks on the left are thinned with a left-aligned phase, and the pixels located at the lower right of each of the upper and lower right blocks are thinned with a left-aligned phase (for example, 1/8
width) and in the third step fill them even thicker as shown in (c).

Next, in the fourth step, as shown in (d),
Furthermore, the pixels located at the bottom left of each of the upper and lower blocks on the left side have a right-aligned phase, the pixels located at the lower right of each of them have a left-aligned phase, and the pixels located at the upper left of each of the upper and lower blocks on the right side have a right-aligned phase, The pixels located at the top right of each of them are filled in thinly with a left-aligned phase, and in the 5th and 6th stages, they are gradually filled in thicker as shown in (e) and (f), respectively, and in the 7th stage, as shown in (g). 4×
Fill in the 16 pixels of 4 so that two thick vertical lines are located at the center of the two blocks on the right and the two blocks on the left.

Furthermore, in the eighth step, as shown in (h), the two pixels located at the bottom of each block are completely filled, and in the ninth step, as shown in (i), the pixels located at the upper left of each block are completely filled. All pixels are also filled in. Then, in the final 10th step, all 16 4×4 pixels are filled in, as shown in (j).

In this way, the positions where the black dots are filled differ depending on the pixel position, and when the density is low, the black dots are distributed diagonally, and as the level increases, the vertical stripes are gradually distributed. As the color changes, I control the distribution of relatively large black parts to be diagonal, just like when the color is light, and as the level increases further, vertical stripes appear, and then the remaining white parts become distributed vertically and horizontally. to control.

Therefore, if the pulse width and phase are controlled in this way, when the black area is small, the diagonal pattern will be less noticeable to the human eye and give a more natural impression; As the size increases, the texture becomes more noticeable and the white distribution becomes more balanced.

Next, after performing the image signal processing according to the characteristics of the printer as described above, the image data is sent to the printer section, and the data format at the time of sending will be explained. If the lighting pulse width for fully lighting the laser diode with one dot completely black is set to 75 nsec,
In order to realize 16 gradations including no lighting (that is, 0 lighting), the pulse is divided equally into every 5 nsec, and one code is assigned to each of the 16 pulses, resulting in 4-bit code data.

Furthermore, as shown in FIG. 7, select one of three types: phase ■ that shifts the lighting pulse to the left, phase ■ that shifts it to the right, and phase ■ that positions the lighting pulse in the center and creates equal pulses on the left and right sides. When performing phase control using a 2-bit code, a 2-bit code is required to indicate the phase. For example, as shown in FIG. 8, 16 types of code data can be prepared with a total of 4 bits by combining the gradation signal and the phase signal.

In addition, in the case of the above-mentioned 64-gradation embodiment, 1
Assuming that the lighting pulse width when fully lighting the laser diode with all black dots is 63 ns, in order to achieve 64 gradations including no lighting (0 lighting), 64 pulses are divided evenly every 1 ns. It is necessary to allocate one code to each to create 6-bit code data.

Furthermore, this code data requires a 2-bit code to instruct the above-mentioned three types of phase control of the lighting pulse, and when the 6-bit gradation signal and the 2-bit phase signal are combined, the total is 8 bits. becomes a signal. Note that a signal like a code may be output in which one type of pulse is assigned to one code without separating the phase and gradation.

The laser printer 200, which is a printer section that receives such a signal, translates it into an actual lighting signal and sends it to the laser diode drive section. The settings should be agreed between the printer department and the printer department.

Here, if we focus only on the phase data when the phase and gradation are separated, in character mode it is determined by the density difference between the left and right pixels, and in photo mode it is determined by the position of the pixel (x, y address), the phase data may not be created by the image processing unit but may be created by another functional unit after the gradation data is determined and the pixel address is determined.

For example, only the gradation data is sent to the printer section and the printer section creates the phase data, or the translation section translates it into laser diode (LD) lighting pulses without taking the trouble of creating the phase data. The phase may also be controlled by. Therefore, in this way, it is possible to omit the transfer of phase data to the printer section, so depending on the physical connections involved in data transfer, it is possible to omit electric wires or simplify the pattern of the printer board.

Next, the circuit configuration of the output modulation circuit in this embodiment will be explained. FIG. 1 is a block diagram showing an example of an output modulation circuit in which γ conversion and code conversion are configured using separate look-up table (LUT) ROMs. This circuit includes a γ conversion ROM 41, a code conversion ROM 42,
It consists of an X counter 43 and a Y counter 44, and inputs the image data from the spatial filter circuit 16 shown in FIG. Performs density control, etc. as a copying machine.

[0053] In other words, when the operator uses the density adjustment function of the digital copying machine to make the density darker or lighter,
The table in the γ conversion ROM 41 is switched in accordance with the density data. Code conversion into gradation code data and phase data is carried out by the code conversion ROM 42.
This is performed regardless of the presence or absence of phase data, depending on the position (address) of the pixel to be written, which is determined by the counter values of the counter (main scanning counter) 43 and the Y counter (sub-scanning counter) 44.

FIG. 9 is a block diagram showing an example of an output modulation circuit in which γ conversion and code conversion are implemented in one look-up table ROM. That is, the γ conversion + code conversion ROM 45 has the conversion function of the γ conversion ROM 41 and code conversion ROM 42 shown in FIG.
The process is similar to that described for the example. Note that this lookup table memory may be configured with a RAM.

Next, FIG. 10 is a block diagram showing an example of a circuit that translates gradation data into a laser diode (LD) drive signal. This circuit consists of a speed conversion buffer 46, a translation circuit 47, a delay circuit 48, an The gradation data (which may include phase data) sent to the image processing section is sent to the translation circuit 47 after absorbing the difference between the pixel clock in the image processing section and the writing pixel clock.

Next, from the delay circuit 48 to the translation circuit 47
A delayed clock is sent out by delaying the write basic clock in steps of pulse width modulation (PWM), and is sent to the speed conversion buffer 46 according to the values (addresses indicating pixel positions) sent from the X counter 43 and Y counter 44. A desired LD drive signal is generated by appropriately combining the gradation data and phase data sent from the LD drive signal.

In this way, in the text mode, the jaggedness and unevenness of the lines can be reduced, so the edges of the image can be reproduced neatly, and in the photo mode, the edges of the image can be clearly reproduced.
Since a symmetrical and regular dot pattern can be created in units of x3 pixels, natural gradation can be expressed.

[0058]

[Effects of the Invention] As explained above, according to the digital copying machine and image writing method according to the present invention,
In photo mode, it is possible to create a regular and natural dot pattern to improve the reproducibility of midtones.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an example of an output modulation circuit 17 shown in FIG. 4.

FIG. 2 is a schematic configuration diagram of a document reading device constituting a digital copying machine that is an embodiment of the present invention.

FIG. 3 is a schematic configuration diagram of a laser printer, which is also a printer section.

4 is a block diagram of an image processing section in the image reading device 100 shown in FIG. 2. FIG.

FIG. 5 is an explanatory diagram when gradation is expressed using 9 pixels of 3×3.

FIG. 6 is an explanatory diagram when gradation is expressed using 16 pixels of 4×4.

FIG. 7 is a timing chart showing examples of types of PWM phases of a write clock and a lighting signal.

FIG. 8 is a diagram showing an example of 4-bit code data for a combination of pulse width and phase of a grayscale signal.

9 is a block diagram showing another configuration example of the output modulation circuit 17 in FIG. 4. FIG.

FIG. 10 is a block diagram showing an example of a circuit that translates gradation data into an LD drive signal.

[Explanation of symbols]

1 Contact glass 2a
, 2b Light source 3, 4, 5, 6, 7 Mirror
8 Lens 9 CCD image sensor
10, 11 Traveling object 12 Sensor driver 13 Amplifier 14 A/
D conversion circuit 15 Shading correction circuit 16 Spatial filter circuit 17
Output modulation circuit 18 Processing and editing circuit
19 Output circuit 21 Laser output unit 22 Imaging lens 23 Mirror 24
Photosensitive drum 25 Charger
26 Eraser 27 Developing unit
28 Transfer charger 29 Separation charger 31
Cleaning units 32a, 32b Recording sheets 33a, 3
3b Paper feed cassette 34 Conveyor belt
35 Fixing roller 36 Paper output tray
37a, 37b Paper feed roller 38 Registration roller 41
γ conversion ROM42 code conversion ROM
43 X counter 44 Y counter
45 γ conversion + code conversion ROM 46 Speed conversion buffer 47
Translation circuit 48 delay circuit
100 Original reading device 200 Laser printer (printer section) 201
laser writing system

Claims (3)

[Claims] 1. A document reading section that reads image information of a document and converts it into an image signal; an image processing section that performs input correction and output correction on the image signal; and an image processing section that performs input correction and output correction by the image processing section. In a digital copying machine, the digital copying machine is equipped with a printer unit that writes the image on recording paper, and a means for writing each dot in multiple gradations by pulse width modulating the image signal for each pixel; 1. A digital copying machine comprising means for controlling the phase of width modulation according to the position of a pixel to be written. 2. A document reading section that reads image information of a document and converts it into an image signal, an image processing section that performs input correction and output correction on the image signal, and an input correction and output correction performed by the image processing section. In a digital copying machine, the image signal is pulse width modulated for each pixel to write each dot in multiple gradations, and the phase of the pulse width modulation is An image writing method characterized in that in a character mode, the image writing method is controlled according to the density of the left and right pixels adjacent to the pixel to be written, and in a photo mode, it is controlled according to the position of the pixel to be written. 3. The image writing method in a digital copying machine according to claim 2, wherein the phase of the pulse width modulation is controlled by a phase in which the dot writing position is shifted to the left side of the pixel, a phase in which it is shifted to the right side of the pixel, and a phase in which the dot write position is shifted to the center of the pixel. An image writing method characterized in that the image writing method is performed by selecting one of three types of phases.