JPH01228849A - Image forming apparatus - Google Patents

Abstract

PURPOSE:To form halftone dot image having no light and shade irregularity and no interference irregularity by dividing color-separated image data to prede termined areas having different phases for respective colors to density-convert a pixel density in the area according to predetermined halftone dot information, and forming images of the number corresponding to the number of separated colors according to the binary signal responsive to the density. CONSTITUTION:After input video data Y, M, C, BK are used as they are or converted to halftone dots for respective colors to combine the part with a font pattern in a printer 2000, they are density=pulse width (PWM)-modulated. This PWM-modulated binary signal is formed in a horizontally (mainly) scanning beam at a high speed by a polygon mirror 2289 by driving a laser beam, and dot-exposed corresponding to the video data on the surface of a photosensitive drum 2900. In this case, a quiet Y component for its spectral luminous efficiency is set to a screen angle near 0 deg. so that a moire is scarcely observed, and a BK component is so corrected at its data that a further lower density area than the minimum values of the color components is not read as its density in a reader. Thus, even if a black color is set to an angle near 0 deg. or 90 deg., a moire with a pitch irregularity is scarcely observed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an image forming apparatus, and particularly to an image forming apparatus having a screen angle control function.

[Prior Art] Conventionally, in this type of device, if there is a discrepancy in speed synchronization between the photoreceptor drive mechanism and the laser irradiation mechanism, uneven shading of the image occurs.

For this reason, when an image was output, the halftone dots and uneven shading caused interference, resulting in image deterioration.

[Problems to be Solved by the Invention] The present invention eliminates the above-mentioned drawbacks of the prior art, and its purpose is to provide an image forming apparatus that forms a halftone image free from uneven shading and uneven interference. It's about doing.

[Means for Solving the Problems] In order to achieve the above object, the image forming apparatus of the present invention has the following features:
an image converting means that divides the color-separated image data into predetermined areas with different phases for each color and converts the pixel density in the area according to predetermined halftone information; The outline of the present invention is to include a binarization means for converting into a binarized signal according to the above-described binary conversion, and an image forming means for forming a number of images corresponding to the number of color separations according to the binary converted signal.

Preferably, the image conversion means calculates the output density of the pixel of interest V' by the following formula, VIJ' = nV + J-r (P IJ l)0≦v
,,'≦r Here, vl: pixel of interest input density n: number of pixels in a predetermined area r: maximum density PI,: weighting is based on the density conversion according to halftone information based on coefficients.

Preferably, the screen angle formed by the straight line connecting the centers of adjacent areas and the main scanning direction is selected within a range that is not substantially perpendicular to the sub-scanning direction for chromatic colors that are noticeable in terms of relative luminous efficiency. shall be.

Preferably, the screen angle is 20° to 70° for magenta or cyan, and 0° to 2° for yellow.
0°, and black is within the range of 70' to 90'.

[Operation] In this configuration, the image conversion means divides the color-separated image data into predetermined areas having different phases for each color, and converts the pixel density within the area in accordance with predetermined halftone information. Preferably, the image conversion means is based on the output density of the pixel of interest.
3. ' is the following formula, VIJ' :nV+4-r (P IJ
1) O≦■1. '≦r Here, V I J : Manual density of the pixel of interest n : Number of pixels in a predetermined area r : Maximum density P'l, : Weighting is density conversion according to halftone information based on coefficients. Preferably, the screen angle formed by the main scanning direction and a straight line connecting the centers of adjacent areas is selected within a range that is not substantially perpendicular to the sub-scanning direction for chromatic colors that are noticeable with respect to relative luminous efficiency.

Preferably, the screen angle is between 20° and 70° for magenta or cyan, and between 0° and 20° for yellow.
, in the range of 70' to 90'0 for black.

The binarization means converts the density-converted image data into a binarized signal corresponding to the density. Then, the image forming means forms a number of images corresponding to the number of color separations according to the binary converted signal.

[Description of Embodiments] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

<Description of Mechanism> FIG. 11 is a sectional view of the mechanism of the digital color printer according to the embodiment. Part of this printer 80 includes a league unit 100 that separates and reads a color original image, and a printer unit that creates a color reproduction image (copy image) or a printing original (color separation image) separated into each color version. It consists of 2000.

In the league unit 100, a document scanning unit 83 performs sub-scanning in six directions of arrows with an exposure lamp 85 turned on in order to read a document 84 placed on a document table. The reflected light from the original 84 is guided by a focusing rod lens array 86 and focused onto a contact type color CCD sensor section 87 . This CCD sensor chip has a resolution of 16pe, for example.
1 (62.5 μm) and consists of 1024 pixels. There are five sensor chips in total, which are arranged in a staggered manner in the main scanning direction.

Furthermore, each pixel of the sensor chip is 15.5μm x 62.5μm.
It is divided into three areas of m, and each area has cyan (C).
, green (G), and yellow (Y) color filters are attached. In this way, the optical image focused on the CCD sensor unit 87 is converted into C, G, '( electrical signals) and sent to the signal processing block 88.The signal processing block 88 converts these C, G, and Y electrical signals into Yellow (Y)
, converted into magenta (M), cyan (C), and black (BK) digital video data, and sent to the printer section 20 for each color.
Send to 00.

In the printer unit 2000, the input video data is processed as is, or after halftone processing, and in some cases, a font pattern is synthesized on a part of it, and then the density=
Pulse width (PWM) modulated. This PWM modulated 2
The value signal drives the laser beam ON10FF.

This laser beam is converted into a high-speed horizontal (main) scanning beam by a polygon mirror 2289 rotating at high speed. This main scanning beam is further reflected by a mirror 229o to expose dots corresponding to the video data on the surface of the photosensitive drum 2900. At this time, one main scanning length of the laser beam corresponds to one main scanning length of the video data. That is, the beam dot in the example has a resolution of 16pe1.

On the other hand, the photosensitive drum 2900 is rotating at a constant speed in the direction of arrow B. Also, this photosensitive drum is equipped with a charger 2297 in advance.
Uniform charging is performed by this. By exposing the surface of this -like charged photoreceptor to a beam dot of video data, an electrostatic latent image of each color separation plate is formed. For example, electrostatic latent images are formed in the order of color plates Y, M, C, and BK each time the drum rotates once. The electrostatic latent image of each color plate is produced by a corresponding developing machine 22.
92 to 2295, and further transferred to the transfer drum 22.
The image is transferred onto a transfer material (paper, etc.) wound on the surface of the image forming apparatus 96.

The above description corresponds to one document exposure scan performed by the league unit 100. First, a Y component dot image is exposed on the photosensitive drum, this is developed by a Y developing machine, and the image is transferred to a transfer material.
Next, a dot image of the M component is exposed on the photosensitive drum, developed by an M developing machine, and transferred to a transfer material. Thereafter, exposure, development, and transfer of the C component and BK acid component are performed in the same manner.

Although not shown, the supply and operation of the transfer material are controlled at that time. That is, each color version data is superimposed on the same transfer paper and color composited to obtain a normal color copy. Alternatively, change the transfer material for each color plate, and change the number of color separations (Y, M
, C, BK, etc.) to create a color printing original plate.Although not shown, in the latter case, the number of colors to be developed is further controlled. That is, each color plate data is divided into corresponding Y, M
, C, and BK colors, but instead, each color plate data may be developed in any one of Y, M, C, and BK (for example, B
It is also possible to develop with only KI color.This makes it easy to compare and evaluate each original plate, and the color when printing in color is determined by the printing ink. Also, from this point of view, although the printer mechanism section in this embodiment is a color printer, it may be replaced with a normal BKI color printer mechanism section.

(Description of functional blocks) Figures 1 (A) and (B) show the digital color
Regarding the functional block diagram of the league printer, see Figure 1 (A
) is a functional block diagram of the league section 100, and FIG. 1(B) is a functional block diagram of the printer section 2000.

In FIG. 1(A), 10 is a control section, which performs main control of the league section 100. The control unit 1O is CPU10-1
, a ROM 10-2 storing, for example, the control program shown in FIG. 12 executed by the CPU 10-1, and a CPU
RAM10-3 used by 10-1 as work memory
Equipped with.

That is, the control unit IO controls the rotation of the motor 12 via the motor driver 13, and causes the document scanning unit 83 to read and scan the document image. Also, at that time, the constant voltage control circuit (C
The lighting of the exposure lamp 85 is controlled via VR. It also receives and receives print (start) command signals and other key operation signals from the operation unit 16, and sets various print operation modes. For example, the operation unit 16 is equipped with a print mode setting switch (not shown), and the control unit 10 uses this command to set the high resolution and halftone mode for characters, line drawings, etc. in the case of an expression mode command, for example. The operation mode is set so that photographic images are printed with high gradation.

Or, when issuing a copy mode command, each color separation signal is
The operation mode is set so that color composition is performed on a number of sheets of transfer paper, and when an original plate production mode command is issued, each color plate is formed on the number of transfer sheets corresponding to the number of color separations. There are various other directives. Then, the control section 10 transmits this print operation mode to the printer section 2000 via the communication line 24.

■ is a synchronization signal processing unit, whose main function is to generate various timing signals for the league side in synchronization with the BD double signal (printer horizontal synchronization signal) sent from the printer unit 2000 via line 22. It's about doing. 2
is a contact type color CCD sensor (87), which receives the league horizontal synchronization signal (RH3YNC) from the synchronization signal processing section 1.
A document image is read in synchronization with a signal), etc., and the read image signal 5 is output. The read image signal 5 is output for each pixel in the order of, for example, a C signal, a C signal, and a Y signal. In addition, in this embodiment, since the CCD sensor consists of 5 chips, there are actually 5 chips.
Signals for channels are generated simultaneously. A signal processing section 3 performs waveform shaping processing such as edge enhancement to prevent attenuation of high frequency components of the read image signal 5, for example.

6 is an image processing section, and the image processing section 6 includes an analog processing section 7, a connection memory 8, and an image processing unit (I
PU)9. The analog processing section 7 first separates the C, G, and Y signals for each pixel into C, C, and Y signals for each color.

Next, from the separated C, G, and Y signals, the red (R
), green (G), and blue (B) color signals. This formation is performed by the calculation process of (R)=(Y)-(G)(G)=(G)(B)=(C)-(G). The R and G obtained in this way.

Each B signal is a luminance signal, and its relationship with the output voltage is linear. This is further converted into density (LOG) and converted into 8-bit Y, M, and C density data (image data) by an A/D converter. This Y, M, and C image data is for five channels of the COD chip, and synchronization between each channel is not achieved. The connection memory 8 stores Y, M, and C image data for five channels so that they are all available. That is, 1024 pixels arranged in a staggered manner in the main scanning direction.
It is stored so that X5 pixels are substantially one straight line. Thereafter, a desired color signal is selected from the Y, M, and C image data in the connection memory 8 by the control section 10, and sent to the image processing unit (PU) 9 for each color. The r PU 9 performs, for example, shading correction processing to correct light distribution, masking processing to correct color tone, etc., and the video data of 8 bits per pixel as a result of the processing is sent from the I PU 9 to the printer unit 2000 via a signal line 11. Sent out.

In FIG. 1(B), a control section 2500 performs main control of the printer section 2000. The control section 2500
PU 2110, ROM 2502 that stores control programs executed by the CPU 2110, such as those shown in FIGS. It includes an A/D converter 2503 and the like that converts the detection signal into a digital signal. As a result, the control unit 2500 first controls the rotation of the drive motor 2285 to rotate the photosensitive drum 290°, the transfer drum 2296, etc. at a constant speed. In addition, the potential measurement unit 2 stores the amount of charge on the surface of the photoreceptor drum 2900 detected by the potential sensor 2600.
700 to A/D conversion and import it.

Furthermore, the image leading edge signal (IT
OP). In addition, signals such as humidity and temperature detected by other humidity sensor 2298 and temperature sensor 2299 are A/D converted and taken in, and are used for control such as correcting the development characteristics of the printer. In addition, the control unit 2500
4, various information is exchanged with the control section 10 of the league club.

2160 is a gradation control circuit whose main function is to synchronize the image clock signal (RCLK) of the league unit 100 and the image clock signal (VCLK) of the printer unit 2000, and to control the input voltage as necessary. Performing halftone processing on video data, converting input video data or video data after halftone processing according to the image output mode, and converting the gradation-converted video data into its density by PWM modulation. For example, converting the data into a corresponding binary signal. 2200 is a laser driver, and gradation control circuit 2
According to the PWM signal from 160, for example, the beam of the semiconductor laser 2223 is driven to 0N10FF.

FIG. 2 is a block diagram showing details of the gradation control circuit of the embodiment. In the figure, one of the input video data is a lookup table (LUT (1)) for halftone processing.
2101 and is converted into video data for halftone processing here. The LUT (1) of this embodiment is a ROM or RAM.
It consists of The LUT (1) is configured to input video data in advance so that a desired halftone effect can be obtained when the halftone video data is outputted by an electrophotographic process via the next halftone processing circuit 2102. This is a conversion table for converting . Details will be described later. A halftone processing circuit 2102 performs halftone processing, which will be described later, on the video data for halftone processing output from LUT (1). For example, image data is divided into predetermined areas, and the pixel density within the area is concentrated and represented by the density at the central pixel position, and converted into halftone video data.
Input to the A side terminal of 103.

The other side of the human video data is input to the B side terminal of the selector 2103 in case halftone processing is not performed.

The selector 2103 selects and outputs either post-halftone video data or pre-halftone video data in accordance with a select signal 2123 from the CPU 2210. For example, in a mode for creating a printing original, preferably the post-halftone video data is output. Select and output video data. Furthermore, when outputting a normal color copy, it is possible to select both post-halftone video data and pre-halftone video data. In short, various printing modes are considered, and various combinations of signal processing occur with each processing circuit described below according to these printing modes.

Next, the video data selected by the selector 2103 is input to the A side terminal of the selector 2104. Also selector 2
Font data from the font ROM 2108 is input to the B side terminal of 104 . This font data is used to synthesize (insert) a font pattern of characters or symbols into the selected portion of the video data.

As will be described later, the CPU 2110 sets the font code and the address at which the font code is to be synthesized, so that a desired font pattern can be synthesized at one or several locations in the image data of each color plate.

The 8-bit video data output from the selector 2104 is input to a buffer memory (FIFO) 2105 in synchronization with the RH3YNC signal and RCLK signal from the reader section 100. The stored video data is read out in synchronization with a horizontal synchronization signal (HSYNC signal) and a video clock signal (VCLK signal) from the printer side synchronization control circuit 2113. Thereby, speed matching between the league section 100 and the printer section 2000 is achieved.

The video data read from the buffer memory 2105 is stored in a look-up table (LUT) for printer characteristic correction.
(2)) Enter in 2106.

LUT (2) is video data that has been corrected in advance to match the input video data to the output characteristics of the printer (e.g. beam spot diameter, toner particle diameter, etc.) (to increase the gradation of the output density and make it linear). It is for creating. Details will be described later with reference to FIGS. 8(A) to 8(D).

LUT (2) Output video data is sent to the D/A converter 21
07, where it is converted into an analog video signal that changes in steps, and then sent to comparators 2117 and 211.
Input to one terminal of each of 8. Pattern signals (1) and (2) for binarizing (PWM modulation) the analog video signal according to its density are input to the other terminals of the comparators 2117 and 2118, respectively. The pattern signal (1) is for reproducing or generating, for example, a line image or a halftone image, and in this case, the resolution is an issue, so for example, the pattern signal (1) is a pattern with the same frequency as the video signal (for example, 400 lines). It is used as a signal. That is, one pattern signal is generated per pixel. The pattern signal (2) is
For example, it is for reproducing a halftone image, and in this case, it is necessary to increase the gradation, so for example, a pattern signal with half the frequency (for example, 200 lines) of the line drawing pattern signal is used. There is. In other words, one pattern signal is generated for every two pixels.

To explain according to the circuit, the crystal oscillator (XTAL) 21
Reference numeral 12 generates a clock signal having a frequency four times higher than that of the image clock signal. The synchronization control circuit 2113 receives the BD double signal I
A main scanning synchronization signal (H3YNC signal) and a basic clock signal (SCLK signal) are formed in synchronization with the TOP signal.

The frequency dividing circuit 2114 divides the 5CLK signal to generate pattern generation clock signals (TVCLK signal and PVCLK signal).
occurs. This TVCLK signal has, for example, twice the frequency of the video data signal and is a clock signal with a duty ratio of 50%. The pattern generation circuit 2115
An analog pattern signal (1) is generated according to the LK signal. In this embodiment, a triangular wave signal is used, for example. Comparator 2117 connects the analog video signal and pattern signal (
1) and compare the video density with pulse width modulation (PW
M modulation) PWM signal (1) is output.

Further, the PVCLK signal has a frequency 1/2 (or 2/3, etc.) times that of the video data signal, and is a clock signal with a duty ratio of 50%. The pattern generation circuit 2116 uses this PV
An analog pattern signal (2) is generated according to the CLK signal. In this embodiment, for example, this is also a triangular wave signal. Comparator 2118 compares the analog video signal and pattern signal (2) and outputs a PWM signal (2) in which the video density is pulse width modulated.

The selector 2119 selects and outputs the PWM signal (1) of the A-side terminal in accordance with the control signal from the CPU 2110, for example, when a line drawing original is to be reproduced or halftone processing is to be output, and when a halftone image is to be reproduced, the PWM signal (1) of the B-side terminal is output. Terminal PWM signal (2)
Select and output.

Note that this selection is also free, and various combinations with other processing circuits can be considered.

Although not shown, this switching signal is not a switching signal from the CPU 2110, but is a known image area signal that identifies, for example, whether each pixel of a video signal belongs to a line drawing area or a halftone image area. A separation means may be provided and this image area separation may be used as a switching signal. In this way, even within one image, a faithful and high-quality image corresponding to the image tone of the original can be obtained. In this way, the selected PWM signal (1) or (
2) is further matched with the movement of the transfer material by the gate circuit 2120, and input to the laser driver 2200, which drives the semiconductor laser 2223 with a constant current for a time corresponding to the pulse width of the PWM signal, and drives the photoreceptor drum. 2900
Forms an electrostatic latent image on the surface.

FIG. 3(A) is a timing chart of main signals in the printer section. The figure shows examples of a horizontal synchronization signal BD, a blanking signal, a reference clock signal 5CLK, a pattern generation clock signal TVCLK, PVCLK, a video clock signal VCLK, etc.

FIG. 3(B) is a block configuration diagram showing details of the synchronous control circuit section. In the figure, a crystal oscillator 2112' causes a synchronization circuit 2128 to generate a clock signal with a frequency four times or more higher than the image clock signal. The synchronization circuit 2128 outputs the HSYNC signal, the VCLK signal, and the 5CLK signal at timing synchronized with the external BD double signal ITOP signal. The frequency dividing circuit 2114 inputs the 5CLK signal, which has the same period as the VCLK signal and a duty ratio of 50% (
7) Output the TVCLK signal and the PVCLK signal with a period twice (or three times, etc.) that of the VCLK signal and a duty ratio of 50%. Although not shown, the blanking signal is BD
It is formed by a counter that measures a time shorter than the BD signal period, which is reset at the falling edge of the double signal.

Here, the PVCLK' signal in FIG. 3(A) will be explained. This PVCLK' signal is useful when performing screen angle control on video data without halftone processing (during normal halftone image reproduction). This P
The VCLK' signal is a clock signal having a phase delay of, for example, 1.5 pixels from the HSYNC signal. When this is compared with the PVCLK signal of normal phase, it is delayed by one pixel. In this embodiment, for example, during normal halftone image reproduction, the PVCLK signal and the PVCLK signal are used for the H3YNC signal.
'The signal is switched and used every line or every few lines in the sub-scanning direction. For example, if you switch every line, 4
This means that the screen angle is controlled by 5 degrees.

In FIG. 3(B), the HSYNC signal is input to shift register 2130 and is shifted by the 5CLK signal. The output of each stage of the shift register 213O is connected to the input terminal of the selector 2131. On the other hand, after the counter circuit 2132 is reset by the ITOP signal,
Count program information is set in advance by the CPU 2110. The count program information is, for example, count sequence information such as repeating 2 to 5 as a count value output, or repeating 5 to 6 as a count value output. Counter circuit 2132 counts H3YNC signals according to this information. For example, H
Each time the 3YNC signal is generated, it counts as 3-4-5-3-4-5.

This count value is input manually to the selection terminal of the selector 2131. On the other hand, the selector 2131 selects and outputs the signal at input terminal 3 when the count value is 3, and selects and outputs the signal at input terminal 4 when the count value is 4. The output of the selector 2131 is input to the frequency division start terminal of the frequency divider circuit 2114. On the other hand, the frequency dividing circuit 2114 is
It has been reset in advance by the H3YNC signal and the counting function has been stopped. When a signal from the selector 2131 is input there, the frequency division operation starts from that point. In this way, the PVCLK signal, TVCL, which has a different phase for each line.
Can generate K signal. Regarding the relationship with the screen angle,
Now, when the screen angle θ is defined as θ=jan-'b/a, the value of a is determined by the count value of the counter circuit 2132, and the value of b is determined by the count sequence.

These are values that can be freely set by the CPU 2110.

Description of halftone processing> The halftone processing described below consists of halftone processing of the density of image data divided into predetermined areas (for example, concentration of pixel density at the central pixel position, representation) and optimization. This allows screen angle control to be performed all at once in real time.

First, a look-up table (LUT) for halftone correction (
The details of 1)) will be explained.

FIG. 7 is a diagram illustrating the conversion characteristics of LUT (1) of the embodiment. In the figure, there is a linear relationship between the Y, M, and C video data input to the printer section and the ink (or toner) density. However, when halftone processing, which will be described later, is performed, a linear relationship cannot be maintained. Therefore, the pre-input Y, M, C
Add density correction to video data. The first quadrant of the figure shows the relationship between the corrected injection force level and the ink density,
There is a linear relationship. The ink density on the vertical axis is the ink density when printing was performed using the color separation plates output by the apparatus of this embodiment. The second quadrant shows the relationship between ink density and halftone output density level. The third quadrant shows the relationship between the halftone output density level and the corrected input level. Further, the fourth quadrant shows the relationship between the corrected input level and the corrected previous input level, which gives the conversion characteristics of LUT (1).

Incidentally, if the color separation plates of the embodiment can constitute ideal halftone dots, the halftone dot output densities in the third and fourth quadrants may be set as the halftone dot density (%).

Actual table information is obtained, for example, by actual measurement. For example, the corrected pre-human level e. When obtaining the ink density , , the halftone output density d is such that the ink density is , . The corrected input level E is such that the zero-order halftone dot output density dn is obtained. seek. As a result, LUT (1) is set to the pre-correction input level e. It is sufficient to create the corrected output level En for the corrected output level En. In this way, the input level OOH
Find all conversion levels corresponding to ~FFH. When this conversion characteristic differs for each color, LUT (1)
are also created for each color.

The following margins in FIGS. 4(A) to 4(D) relate to diagrams for explaining the halftone processing pattern of the embodiment, and FIG. 4(A) shows an example for C data. In the figure, 500 is one pixel, and each pixel is shown as an array from the starting address (0, 0) of data for one image. 600 is a basic cell, which halftones the density within the bold line area (within a predetermined area) in the figure (concentrates and represents the pixel density to, for example, the density at the center pixel position)
This is a block unit for A basic cell of C data consists of, for example, 13 pixels. The numbers (1 to 13) attached to each pixel in the basic cell indicate the priority order.
The priority level becomes lower towards 3. For data of the same color, the same priority is given to other basic cells. .

It should be noted that the illustrated priority order is an example that is adapted to the printer characteristics of the embodiment, and is not limited to this, and various other modifications are possible.

Halftone processing of pixel density within a basic cell is performed according to the following equation (halftone calculation formula).

That is, (attention pixel output data) = (attention pixel input data) × (number of pixels in basic cell) - (priority - 1) This is performed by sequentially moving the pixel of interest in the scanning and sub-scanning directions. For example, when the pixel of interest is at priority level 11, (output density) = (input density) x 13 - (11-1) -1) XFFH) increases, and the density at this pixel position becomes relatively low.

Further, as a result, when (output density) is 01, the output density is clamped to "OOH". Conversely, (output density)>F
When it is FHl, the output density is clamped to "FFH".

Similarly, when the pixel of interest is located at the priority level l, (output density) = (input density) x 13 - (1-1)XFFH. Since the priority is 1, the density to be subtracted is zero. In this way, the pixel density is concentrated toward the central pixel position within the basic cell and represented. The printing original plate formed with halftone dots in this way has good ink adhesion and is stable.

A matrix 700 indicates a block unit in which the illustrated halftone processing pattern is repeatedly used in the main scanning and sub-scanning directions. The matrix size of the C data is, for example, (13×13) pixels. As is clear from the figure, document images of any size can be processed by connecting a plurality of these matrices in the main scanning and sub-scanning directions. In this embodiment, this periodicity is utilized to store this matrix pattern in a memory, repeat the pattern, and use it in real time to save pattern memory and enable high-speed calculation.

Further, the triangles in the figure are for indicating the screen angle θ, and this screen angle θ represents the inclination of the arrangement of the basic cells 600. In the figure, when a and b are determined,
The screen angle θ is determined by θ=jan−′b/a. For example, the screen angle of C data is θ=5
6.3° is given.

FIG. 4(B) shows an example of a halftone processing pattern for M data. In the figure, a basic cell 600 consists of 13 pixels and has the same shape as in FIG. 4(A). Further, the screen angle is given as θ=33.7°. By the way, when comparing FIG. 4(B) with FIG. 4(A), the basic cell 600 of M data starts from address (0,0) (
phase angle) are different. For this reason, the center pixel positions of both do not overlap. That is, the main concentration information does not overlap.

As a result, neither C ink nor M ink is crushed during printing, and high quality and stable printing can be achieved.

FIG. 4(C) shows an example of a halftone processing pattern for BK data. In addition, BK data is C, M.

It is generated from Y data using a known method. In the figure, the basic cell 600 consists of 10 pixels, and its shape is also 4th.
This is different from those in Figures (A) and (B). Although not limited to this shape, it is suitable for providing a screen angle of θ=71.6°, for example. Furthermore, the phase angle from address (0,0) is also different.

FIG. 4(D) shows an example of a halftone processing pattern for Y data. In the figure, a basic cell 600 consists of 10 pixels, and is suitable for providing a screen angle of θ=18.4°, for example, although the shape is not limited to this. Furthermore, the phase angle from address (0,0) is also different.

FIG. 5 is a block diagram of the halftone processing circuit according to the embodiment. In the figure, the halftone video data output from LUT (1) is latched into a D-type flip-flop (D-F/F) 2301 in synchronization with the RVCLK signal. On the other hand, the counter 2304 counts the RVCLK signal after being reset by the RH3YNC signal. That is, Fig. 4 (
A) to (D) main scanning direction addresses are formed. Further, after the counter 2305 is reset by the ITOP signal, the R
Count HSYNC signals. That is, FIG. 4(A)~
(D) A sub-scanning direction address is formed.

Although not shown, count initialization data corresponding to the processing color is set in the counters 2304 and 2305 by the CPU 2110, and each counter repeats a counting operation with a count value corresponding to the initialization data. For example, when processing C data or M data, the count value O~
Repeat with 12. Further, when processing BK data or Y data, the process is repeated with count values 0 to 9, respectively.

2306 is a pattern memory;
) halftone processing patterns (priority order data) are stored. In this way, the color selection signal (Y
, M, C, BK), and as main and sub-scanning progresses, the priority data of any one of the matrices shown in FIGS. 4(A) to 4(D) is sequentially read out. A table memory 2302 receives input data of the pixel of interest and priority data corresponding thereto, and outputs output data of the pixel of interest according to the above halftone calculation formula. At this time, in the same way as described above, the table for the case where the number of pixels in the basic cell is 10 or 13 is used according to the color selection signal from the CPO 2110. The output data of the pixel of interest read out in this way is synchronized with the RVCLK signal.
2303 and output to the next stage circuit.

Note that the memory 2302 and the memory 2306 mentioned above may be ROM or R.
AM may be used, and instead of employing a look-up table method using memory, a hardware arithmetic circuit may be used.

FIG. 6(A) is a diagram showing an example of screen angle distribution in the embodiment, FIG. 6(B) is a diagram showing an example of screen angle distribution used in the conventional printing field, and FIG. 6(C) to (H) is a diagram showing an example of moire fringes.

In the field of printing technology, for example (13×13) pieces of fiberglass can be bundled together, so that it is easy to keep the distributed screen angle accurate during marking. On the other hand, since a laser beam printer is used in this embodiment, the polygon mirror 2289 and the photosensitive drum 2900
rotational unevenness must be taken into account. In other words, the combination of both rotational unevenness causes unevenness in the amount of laser irradiation per hour, and this irradiation unevenness affects the formation of a latent image on the photoreceptor drum and even the development of the image, which causes shading in the output image. This appears as unevenness (pitch unevenness).

This pitch unevenness is considered to be a high frequency component having an angle of 00 or 90' when considered in relation to an image subjected to halftone processing. Generally, moiré appears in color plates with a small angular difference from pitch unevenness. For this reason, when an image is formed at the same angle as in the printing method, the M and C components are easily visible as moiré due to pitch unevenness.

This is equivalent to the "moiré by line screen + halftone dots" shown in FIG. 6(H). Therefore, in this embodiment, the Y component, which is inconspicuous with respect to the relative luminous efficiency, is set at a screen angle close to Oo to make the moiré visible. Furthermore, although BK is originally a color that is easy to see, the data of the BK acid component in this example, although not shown, is corrected so that the lowest value of each color component does not have a lower density area as the density in the league part loO. There is. Therefore, as described above, pitch unevenness is due to unevenness in light intensity, so it has a characteristic that light density is easier to see than high density. For this reason, pitch unevenness and moire are difficult to see in black even when the angle is Oo or close to 90°.

8(A) to (D) are L for the printer output characteristics of the embodiment.
It is a figure explaining the conversion characteristic of UT (2). The printer output image needs to have linear characteristics between the input data level and the printer output density in accordance with the characteristics of the printer used. By the way, for example, if the toner particle diameter is not sufficiently small compared to the beam spot diameter, even if the output beam has 256 gradations, the toner particles will be at a maximum of 32 gradations.
There are cases where only one item is attached. This is inconvenient because only 32 gradations can actually be expressed. Therefore, if we set an area of (2 x 2) dots as the unit of printer output, or (n
It is possible to express linear gradation down to the gradation level.

When doing this, in the main scanning direction, for example, image data for m pixels may be subjected to PWM conversion using a pattern signal (triangular wave) having a period m times the pixel period. By the way, in the sub-scanning direction as well, it is desired to obtain the same effect as in the main scanning for n-line 2 minutes.

However, the same effect cannot be obtained by repeating the same main scan in the sub-scanning direction as in this embodiment. Therefore, in the sub-scanning direction, multiple types of gradation conversion tables are provided.
By switching and using the table in a predetermined sequence, the same effect as in the main scanning direction can be obtained.

LUT (2) is a table for this purpose, and takes into consideration the overall output characteristics of the laser beam printer of the embodiment. The output characteristics of a laser beam printer include the relationship between the beam pulse width and the photosensitive drum surface potential (EV characteristics) and the relationship between the photosensitive drum surface potential and output image density (VD characteristics).
is possible. Since the former EV characteristic has a substantially linear characteristic, it will be described here as a table for correcting the latter VD characteristic. The VD characteristics vary depending on whether halftone processing is performed on the image data, the frequency of the PWM modulation signal (pattern signal), the developer used, and the like. For this reason, in this embodiment, a plurality of tables are prepared in advance according to the VD characteristics, and the CPU 211
0 is selected and used.

Here, halftone processing is not performed, and the pattern signal frequency to the comparator is 1/2 or 1/2 of the video signal frequency.
Case 3 will be explained.

FIG. 8(A) is a diagram showing the VD characteristics of the example. In the figure, the drum surface potential on the horizontal axis indicates the difference potential (contrast potential) between the surface potential of the photosensitive drum and the developing bias potential. FIG. 8(B) is a diagram showing an example of a characteristic for linearly converting the VD characteristic of FIG. 8(A). That is, this objective can be achieved by replacing the horizontal and vertical axes in FIG. 8(A), and the characteristic table shown in FIG. 8(B) can be obtained. However, in this embodiment, it is desired to further improve the gradation of the output image (particularly the gradation of the highlight portion). Therefore, by changing the conversion table every line or every few lines in accordance with 1/2 to 1/3 times the pattern signal frequency and at a period of 2 lines, 3 lines, etc. in the sub-scanning direction, The aim is to make the gradation linear and to concentrate the halftone dots.

FIG. 8(C) is a diagram showing correction table characteristics of an embodiment used when the pattern signal frequency is 1/2 of the video signal frequency. In the figure, in the table of characteristic ■, the output level initially rises at a slope twice that of the curve in Figure 8 (B) until it reaches the level FFH, and then the input level increases to FFH.
It remains constant until . Also, the table of characteristics ■ is
level OO until the table output reaches level FFH.
It is created so that it maintains H and thereafter rises to level FFH at a slope twice as high as that of the curve in FIG. 8(B).

In this example, since the pattern signal frequency is 1/2, one output density dot is formed by two pixels of the video signal. On the other hand, in the sub-scanning direction as well, the tables ``■'' and ``■'' in the figure are switched and used every line with a period of two lines. As a result, the ■ table gives a dark color, and the ■ table makes a light color. As a result, the effect of forming one output density dot in two lines in the sub-scanning direction is obtained.

Note that the table characteristics are not limited to those of ■ and ■.

FIG. 8(D) is a diagram showing correction table characteristics of an embodiment used when the pattern signal frequency is 1/3 of the video signal frequency. Incidentally, as described above, the <VD characteristic depends on the pattern signal frequency.

However, for convenience of explanation, in Figure 0, which will be explained using the same VD characteristic, the table of characteristic ■ initially rises to level FFH at a slope three times that of the curve in Figure 8 (B), After that, the input level remains constant until it reaches FFH. In addition, the table with characteristic ■ maintains the level OOH until the table output of characteristic ■ reaches level FFH.
After that, it rises to level FFH at a slope 1.5 times that of the curve in FIG. 8(B). In this example, since the pattern signal frequency is 1/3, one output density dot is formed by three pixels of the video signal.

On the other hand, in the sub-scanning direction as well, the tables ``■'' and ``■'' in the figure are switched every line at a period of three lines. For example ■
Switch like -■→■.

As a result, an effect is obtained in which shading is applied to each output line, and one output density dot is formed in three lines in the sub-scanning direction.

Note that the table characteristics are not limited to those of ■ and ■.

Incidentally, since the VD characteristics actually differ depending on the pattern signal frequency, the values shown in Fig. 8 (
Create the tables C) and (D). In addition, when creating the above table, the pattern signal frequency is 1/1/1 of the video signal frequency.
It is not limited to the case of 2.1/3. It can be created in the same manner for other frequencies.

FIG. 9 is a block diagram showing details of the font control circuit of the embodiment. CPU2110 is font ROM21
Data is given to the terminal S of 08, and the font to be printed is selected. Furthermore, the main scanning address data to be printed is latched in a latch circuit 2142, and the sub-scanning address data is latched in a latch circuit 2148. The address data of the latch circuit 2142 is the Q of the comparator 2141.
The address data of the latch circuit 2148 is input to the Q terminal of the comparator 2147. On the other hand, the counter 2140 is reset by the RH3YNC signal and the RV
Count CLK signals. That is, the number of pixels in the main scanning direction is counted.

Further, the counter 2146 is reset by the ITOP signal,
Count the RH3YNC signal. That is, the number of lines in the sub-scanning direction is counted.

The number of pixels of the counter 2140 is determined by the comparator 214.
1, and the comparator 2141 is input to the P terminal of
If =Q is satisfied, a logic level is output to the terminal (P=Q). This is the character output position in the main scanning direction. Furthermore, this logic level is input to the J terminal of F/F2143, and the next RVCLK signal causes F/F2143 (7) HE
The NB signal becomes a logic level. On the other hand, counter 2145
starts counting the RVCLK signal in synchronization with the logic level of the HENB signal, and stores the count output in the font ROM.
2108 main scanning address. Also counter 2
144 is also synchronized with the logic level of the HENB signal.
Start counting the K signal, and when the predetermined number of counts is reached, the R
Outputs logic level to C terminal. This logical level is F/
It is input to the terminal of F2143, and the next (7) RVCL
The K signal causes the F/F 2143 (7) HENB signal to go to logic O level. This causes counters 2144 and 21
45 stops counting and its output is reset. From the above, the HENB signal is a signal that is turned ON every line at the relevant character position in the main scanning direction.

On the other hand, the number of lines in the counter 2146 is determined by the comparator 21
47, and when the comparator 2147 satisfies P=Q, it outputs a logic level to the terminal (P=Q). This is the character output position in the sub-scanning direction. Furthermore,
This logic level is input to the J terminal of F/F2149, and the following RH3YNC signal causes F/F2149's ■E
The NB signal becomes a logic level. On the other hand, counter 2151
starts counting the RHSYNC signal in synchronization with the logic level of the VENB signal, and sends the count output to the font RO.
Provided as the sub-scanning address of M2108. In addition, the counter 2150 also synchronizes with the logic level of the VENB signal.
It starts counting the YNC signal, and when a predetermined number of counts is reached, a logic level is output to the RC terminal.

This logic level is input to the terminal of F/F2149, and the VE of F/F2149 is input by the next RH3YNC signal.
The NB signal becomes a logic O level.

As a result, counters 2150 and 2151 stop counting, and their outputs are reset. From the above, VE
The NB signal is a signal that turns ON at the relevant character position in the sub-scanning direction. HENB signal and VENB signal are AND gate 2
153 and forms the SEL signal at its output.

In this way, a font pattern can be synthesized at any position in the output image. The CPU 2110 has a latch circuit 2142.
2148 and font selection data can be changed as appropriate, so a plurality of different fonts can be synthesized at any position in the image.

FIG. 10 is a diagram showing an example of an output image synthesized with fonts by the font control circuit of the embodiment. In the figure, “#” is a registration mark for positioning, and “M, C, Y
, Bk'' are color information marks for identifying each color version.The rest are color separation versions of the original image read by league lOo.

<Flowchart> FIG. 12 is a flowchart showing the operation of the control unit 10 of the league club. This control program is built in ROM10-2. In the figure, when power is turned on to the league section 100, an initial display routine is executed in step S1. This routine, for example, checks each I10,
These processes include checking the indicator display, initializing RAM10-3, and moving the document scanning unit to its scanning starting point. In step S2, the printer waits for connection to the printer control unit 25000 via the communication line 24. Communication line 24 is not connected or printer unit 2
If the power is not turned on to 000, it is not connected. After checking the connection status in step S2, step S3
, and turn the 1-color print (copy) switch on the operation panel to O.
Wait for N. When the print switch is turned on, the process advances to step S4, and a print ON command is output to the printer section 2000 together with print mode information. This print mode information includes whether or not it is the color separation plate output mode, and is output according to instructions to the operation unit 16. In step S5, an ITOP signal from the printer section 2000 is waited for. When the ITOP signal is input in step S5, the process advances to step S6, where the document image is scanned and video data is output to the printer section 2000. At that time, selections such as print mode are given to the control section 1o from a scanning section (not shown).

The control unit 10 transfers this information to the control unit elements and the printer 2000.
The information is transmitted to the control unit 2500 of.

FIG. 13(A) is a flowchart showing the operation of the control section 2500 of the printer section. In the figure, a reader section 10
When a print ON command is received from 0, step 52200
Enter. In step 52201, it is checked whether or not the color separation plate (original plate for printing) output mode is selected. If it is not the 0 color separation plate output mode, the process proceeds to step 52202, in which an image such as a color copy is output using a normal print sequence, or a color separation plate output mode is selected. Step 5 when in disassembled version output mode
2203 is the RAM for printer output characteristic correction (LUT2
) 2106 is a lookup table for the 400 line output during the above halftone processing as Y and M.

Set each color in C and . Step 52204
Now select the 400 line (A side) input of the selector 2119 in FIG. In step 52205, the RAM for halftone processing is
Lookup tables are set in (LUT (1)) 2101 for each color of Y, M, C, and so on. In step 52206, the A side input of the selector 2103 in FIG. 2 is selected. In step 52207, an image of the Y separated version is outputted to the first transfer material according to the processing procedure described later, and in step 52208, an image of the M separated version is outputted to the second transferred material in the same way.

In step 52209, a C-separated image is outputted onto the third transfer material in the same manner. In step 52210, a BK separated image is outputted onto the fourth transfer material in the same manner.

FIG. 13(B) is a flowchart showing details of the Y-separated image output procedure of the embodiment. In the figure, step 5
At 2212, the Y phase correction table of LUT2 is selected.

In step S2213, the Y halftone dot processing table of LUT (1) is selected. In step S2214, the halftone processing circuit 21
Set Y month initialization data for 02. In step 52115, necessary data is set in the font control circuit 2109. Necessary data includes, for example, a registration mark "#" added for the purpose of registration, a color information mark "Y" indicating that it is a Y-separated version, and their output addresses.

In step 52216, the image data of the Y separation plate is developed by the black (BK) developer 2295, and the image of the Y separation plate is output. B with a corner and registration mark 1#” and color information mark “M”C”Bk”
The K-phenomen 2295 develops the image, and outputs an image of each color separation.

In Figure 10, the image itself of each color separation is black (B
K) is printed. However, these can be easily distinguished by the character marks "'Y, M, C, Bk" printed at the same time.Also, by aligning each registration mark "#", accurate positioning can be performed.

[Effects of the Invention] As described above, according to the present invention, color-separated image data is divided into predetermined areas with different phases for each color, and the pixel density in the area is density-converted according to predetermined halftone information. Therefore, the colors of each color plate do not overlap, allowing stable and high-quality color printing.

In addition, since the distribution of screen angles is taken into consideration between chromatic colors that stand out and those that do not, it is possible to create stable and high-quality color printing plates even if there are problems with the accuracy of the mechanism or changes over time. .

[Brief explanation of the drawing]

Figures 1 (A) and (B) show the digital color image of the embodiment.
A functional block diagram of the reader/printer, FIG. 2 is a block configuration diagram showing details of the gradation control circuit of the embodiment, FIG. 3(A) is a timing chart of main signals in the printer section, FIG. 3(B) is a A block configuration diagram showing details of the synchronization control circuit section, FIGS. 4(A) to (D) are diagrams explaining the halftone processing pattern of the embodiment, and FIG. 5 is a block configuration of the halftone processing circuit of the embodiment. Figure 6 (A) is a diagram showing an example of screen angle distribution in the embodiment; Figure 6 (B) is a diagram showing an example of screen angle distribution used in the conventional printing field; ) to (H) are diagrams showing examples of moiré fringes, Figure 7 is a diagram explaining the conversion characteristics of LUT (1) in the example, and Figures 8 (A) to (D) are diagrams illustrating the conversion characteristics of LUT (2) in the example. ), FIG. 9 is a block configuration diagram showing details of the font control circuit of the embodiment, FIG. 10 is a diagram showing an example of an output image synthesized with fonts by the font control circuit of the embodiment, FIG. 11 is a sectional view of the mechanism of the digital color league printer of the embodiment. FIG. 12 is a flowchart showing the operation of the control section 10 of the lever section. FIG. Flowchart Showing Operation FIG. 13(B) is a flowchart showing details of the Y-separated image output procedure of the embodiment. In the figure, 84...document, 83...document scanning unit,
86...Rod lens array, 87...Color CC
D sensor section, 88... signal processing block, 2289...
...Polygon mirror, 2290...Mirror, 2900
. . . Photosensitive drum, 2292-2295 . . . Developing device, 2296 . . . Transfer drum. Figure 3 (B) (A) Cyan (e = 56.3°)
(B) Malenta (olI33.7°](
C) BLock (e”71.6°)
(D) YeLLow (elI18.4°) Fig. 4 Fig. 70 Fig. S (A) Fig. 8 (B) Fig. 8 (C)
Figure 8 CD) Figure 10

Claims (4)

[Claims] (1) an image conversion means for dividing the color-separated image data into predetermined areas with different phases for each color and converting the pixel density in the area according to predetermined halftone information; Binarization means for converting image data into a binary signal according to the density of the image data, and image forming means for forming a number of images corresponding to the number of color separations according to the binary converted signal. An image forming apparatus characterized by: (2) The image conversion means calculates the output density of the pixel of interest V_i_j' by the following formula, V_i_j'=nV_i_j-r(P_i_j-1)0
≦V_i_j′≦r where, V_i_j: pixel of interest input density n: number of pixels in a predetermined area r: maximum density P_i_j: density conversion according to halftone information based on a weighting coefficient. The image forming apparatus described in . (3) The screen angle formed by the straight line connecting the centers of adjacent areas and the main scanning direction is selected within a range that is not approximately perpendicular to the sub-scanning direction for chromatic colors that are noticeable in terms of relative luminous efficiency. The image forming apparatus according to claim 1. (4) Screen angle is 2 for magenta or cyan.
4. The image forming apparatus according to claim 3, wherein the angle is within the range of 0° to 70°, 0° to 20° for yellow, and 70° to 90° for black.