JP2002252763A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device that controls a gradation characteristic caused by a dot gain so as to improve the gradation characteristic and increases number of effective gradations. SOLUTION: An output value after quantization for error arithmetic operations is set to a data correction section 27 by an instruction from a data selection/switching section 28. For example, in the case of four-valued error diffusion processing, an initial value 85 of dot data with a small size is set to (revised into) 102 and an initial value 170 of dot data with a medium size is set to (revised into) 204.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an image forming apparatus applied to a laser printer, a digital copying machine, a color laser printer, a digital color copying machine and the like.

[0002]

2. Description of the Related Art Normally, a single dot shape in an image forming apparatus is circular or elliptical. By laying out such dots, the density of the image becomes the maximum density. When this maximum density is formed, a dot diameter larger than the dot pitch interval of image formation, so-called dot gain, occurs. Due to this dot gain, the rate of increase in density with respect to the image input data increases, and a density saturation phenomenon occurs in which all areas are filled with dots before the data becomes maximum.

As an apparatus of this type, for example, a gradation recording apparatus for an image in which an output level is corrected in accordance with a dot gain corresponding to an output pattern of a plurality of pixels around a pixel of interest to improve halftone reproduction characteristics. (See Japanese Patent Application Laid-Open No. 3-80766) and correcting the error data using the difference between the actual dot size of the recording system and the theoretical dot size to obtain the sum of the density of the input signal and the output. Error diffusion processing device with equal sum of signal densities (JP-A-4-150)
272).

[0004]

The above-mentioned dot gain has a higher rate of occurrence in distributed dither or error diffusion processing for distributing and arranging dots than dot concentrated dither, and has a higher density than image input data. Saturation is fast. In an electrophotographic printer, this density saturation is further accelerated due to the accuracy of the engine and the stability of dot reproduction. For example, an electrophotographic printer with a writing density of 600 dpi is more likely to be used in an electrophotographic printer with a 1200 dpi high density writing system. Becomes stronger.

[0005] Further, when the output data is 4 in an electrophotographic printer.
In image formation quantized by the 2-bit error diffusion method, the writing exposure energy is small, the image density becomes considerable at the stage where the minimum dot is arranged on the entire surface and the image is formed, and the density saturation occurs before the maximum dot is generated. , And the number of gradations is significantly reduced.

Further, according to the sensitivity of the human eye, the gradation characteristic corresponds to the lightness, and the gradation characteristic with a linear lightness is excellent. The gamma correction unit selects the gradation data that can be taken from the image data. However, when the gradation curve by the error diffusion process is steep (steep), the number of gradations actually used is reduced. .

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to control a gradation characteristic generated by a dot gain in an error diffusion process so that the gradation characteristic is good and effective. An object of the present invention is to provide an image forming apparatus with an increased number of tones.

[0008]

SUMMARY OF THE INVENTION According to the present invention, the gradation characteristic is varied in the four-value (2-bit) error diffusion processing in which the gradation number is increased and the number of gradations is increased. Improve. In addition, by varying the numerical data of the dot for which the error calculation is performed, the gradation characteristic of the image data is controlled, and by changing the dot numerical data other than the full dot for performing the error calculation, the maximum density of the image is improved. In a guaranteed state, the gradation characteristics for the image data are controlled. Further, by making the numerical value data of the dots variable for each dot for which the error calculation is performed, the gradation characteristics for the image data can be freely controlled. Further, in the binary error diffusion processing, the tone characteristics for the image data are controlled.
Further, in the present invention, the numerical data of the dot on which the error calculation is performed is changed to a larger value, and the effective number of gradations is increased. Further, the number of gradations is increased by extending the numerical data of the dot for which the error calculation is performed on both the highlight and dark sides of the image. Also, by inputting the numerical data of the dot for which the error calculation is performed by a specific input means, the gradation characteristics for the image data are controlled. Further, a specific person sets the above-mentioned numerical data to optimally control the gradation characteristics of the image forming apparatus.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be specifically described below with reference to the drawings. FIG. 7 shows an example of an image forming apparatus including a digital copying machine to which the present invention is applied. The digital copying machine includes a scanner 400 as an image reading device, an image recording device 411 including a laser printer as an image forming unit, and a circuit described later. The scanner 400 illuminates an original such as a bookbinding original placed on a flat original platen 403 with an illumination lamp 502 and reflects the reflected light image on a mirror group 503 to 505 and a lens 50.
6, the image is formed on a reading sensor 507, and the document is scanned by moving the illumination lamp 502 and the mirror groups 503 to 505 to read the image information of the document and convert it into an electric image signal. An image signal obtained by the reading sensor 507 is transmitted to a printer 41 via a circuit described later.
Sent to 1.

In the printer 411, a writing device 508 comprising a writing optical unit as an exposure means
Converts an image signal into an optical signal and exposes it to an image carrier made of a photoconductor, for example, a photoconductor drum 509, and performs optical writing corresponding to a document image to form an electrostatic latent image. The writing optical unit 508 drives the semiconductor laser by the light emission drive control unit based on the image signal, emits a laser beam intensity-modulated by the image signal, and deflects and scans the laser beam by the rotary polygon mirror 510 to perform f / θ. Irradiate the photosensitive drum 509 via the lens and the reflection mirror 511.

The photosensitive drum 509 is rotated by a driving unit, rotates clockwise as shown by an arrow, and is uniformly charged by a charger 512 as a charging means. A latent image is formed. The electrostatic latent image on the photosensitive drum 509 is developed into a toner image by a developing device 513, and a transfer material made of transfer paper is transferred to one of a plurality of paper feed units 514 to 518 and a manual paper feed unit 519. Is supplied to the registration roller 520 from the printer.

The registration roller 520 is connected to the photosensitive drum 50.
A transfer bias is applied from a transfer power supply to the transfer belt 521 to convey the transfer paper to the toner image on the photosensitive drum 50.
9 to transfer the toner image on the transfer paper. The transfer paper is conveyed by a conveyance belt 521, the toner image is fixed by a fixing unit 522, and is discharged to a discharge tray 523 as a copy. After the transfer of the toner image, the photosensitive drum 509 is cleaned by the cleaning device 524 and is gradually charged by the electric charger 525 to prepare for the next image forming operation.

FIG. 8 shows the configuration of the image processing section of the digital copying machine. The image processing unit includes a scanner processing unit that corrects read data, a digital image processing unit that processes and corrects a digital image, and a writing processing unit that modulates a writing LD. The data level of 600 dpi analog data read by the CCD 1 is adjusted by the AGC 2. Next, the analog data of each pixel is converted into a digital value of 8 bits per pixel by the AD conversion 3, and the shading correction 4 corrects the variation of the pixels of the read CCD and the illuminance.

Subsequently, a filtering process 5 is performed. Specifically, MT for correcting the amplitude of an image generated by reading
An F correction and a smoothing process for expressing a halftone image smoothly are performed. Then, a scaling process 6 in the main scanning direction is performed according to the copy magnification, and a γ correction 7 for converting into a writing density is performed. Finally, halftone processing 8 is performed, and the data is converted into data of several bits per dot and transmitted. Image writing is normally output at 600 dpi, but 120
Output at 0 dpi. In the case of outputting at 1200 dpi, an input image of 600 dpi is double-density-converted in the main scanning and sub-scanning directions before the halftone processing.
In addition, processes (not shown) such as a background removal process, a flare removal process, a scanner γ process, and image editing are performed.

An error diffusion process for quantizing image data according to the present invention is provided in the halftone processing unit 8.

FIG. 9 shows a configuration of a conventional error diffusion process. The error diffusion process compares the multi-tone image data 20 with a threshold value 22 to determine an output value 23. And its output value 2
The difference 24 between 3 and the image data 20 is stored as an error 25 of the pixel. In the next pixel, the product of the peripheral pixel (a to l) and the error matrix coefficient 26 corresponding to the pixel is added to the image data of the pixel of interest (marked with *) 21, and the output value 23 To determine. By repeating the above for each pixel, error diffusion processing in which the density of the image is stored is performed.

The error diffusion processing includes a multi-level error diffusion having a plurality of thresholds and outputting quantized data of a plurality of gradations, and a binary error diffusion formed by ON or OFF dots. In multi-level error diffusion, various types of data are output for each dot, and an image is formed with dots of different sizes.

For example, in the error diffusion method in which output data is quaternary (2 bits), three levels of quantization thresholds are provided, and data obtained by adding target pixel data and error data of peripheral pixels is compared with each threshold. The output dot size of the target pixel is determined.

In the embodiment of the present invention, the initial value is
The quantization thresholds of the three levels are set to 42, 127, and 212, and the values of the output data are set to 0, 85, 170, and 255. 0 is dot off, 85 is an output value (density value) corresponding to a small size dot, 170 is an output value (density value) corresponding to a medium size dot, and 255 is an output value (density value) corresponding to a large size dot. Value).

According to the output data value, as described above, the exposure amount of the writing laser diode is pulse-width-modulated or power-modulated to perform multi-level writing. Then, the difference between the output data value and the data obtained by adding the error data of the target pixel data and the peripheral pixels is distributed to the peripheral pixels as an error value.

In binary error diffusion, a single quantization threshold is set to 1
27, and the output data value is 0 (dot off) or 25
5 (dot on) to control the appearance of dots.

FIG. 1 shows the configuration of the error diffusion processing according to the present invention. In the present invention, a data correction unit 27 and a data selection / switching unit 28 are further provided in the configuration of the conventional error diffusion processing. Hereinafter, as an error diffusion process, an output value is determined by comparing the multi-tone image data with a threshold value.
bit) The present invention will be described using an error diffusion method as an example.

The data correction unit 27 converts four output data values according to the dot output size. The data correction unit 27 performs data selection / operation for performing operation input control, for example.
The switching unit 28 receives an instruction to change the calculation data. That is, the data correction unit 27 variably sets the output data values 0, 85, 170, and 255 for performing the error calculation, which is the initial value, according to an instruction from the data selection / switching unit 28. Note that output values 0, 85, 170, and 25 for output 23 for performing multi-level writing by laser.
5 itself is fixed as in the prior art.

In the error calculation processing, the difference between the output value of the data correction unit 27 and the data obtained by adding the error data of the target pixel data and the error data of the peripheral pixels is used as the error value of the pixel.
When the output value of the data correction unit 27 is changed, the error value of the pixel changes, affecting the dot density of peripheral pixels, and controlling the gradation characteristics.

FIG. 2 shows a guidance display on the LCD in the operation input unit of the image forming apparatus. The data selection / switching unit 28 is, for example, an operation input unit, and sets a data selection / switching mode by key input. That is, to set the input mode (SP mode), open the rear panel of the image forming apparatus main body and input a dip switch of the control panel, or simultaneously press a plurality of keys of the operation input unit for several seconds. Operable. After setting the SP mode, the data selection / switching unit 28 changes the setting by inputting the SP mode number 4-101 or 4-102 with the ten keys.

FIG. 2A shows SP mode number 4-101.
4 shows a guidance display when "" is input. (A) is an example in which intermediate dot data of an output dot is selected at two levels, and the initial value “8” of dot data 1 of a small size dot is selected.
To change “5” to “102”, use the cursor keys to
Move 02 to the selection side and confirm with the Enter key. Similarly, the dot data 2 of the medium-size dot has the initial value “17”.
"0" or "204" can be selected.

FIG. 2B shows the SP mode number 4-102.
4 shows a guidance display when "" is input. (B) is an example in which the intermediate dot data of the output dot is changed to an arbitrary value. When changing the dot data 1 of the small size dot, continuously press ↑ or ↓ with the cursor key to change the numerical value. Up and down. Similarly, the dot data 2 of the medium size dot can be arbitrarily set.

The above input example is a 2-bit output of four values.
This is the case where error diffusion is used, and the output data value 0 when no dot is output and the output data value 255 when the maximum dot is output can be similarly changed. When binary error diffusion is used, it is also possible to change the data value 0 when not outputting a dot and the data value 255 when outputting a dot, and in any case, the gradation characteristic is controlled. You.

FIG. 3 is a graph of the image density ID with respect to input image data showing the gradation characteristics when an image is formed by the conventional error diffusion process in an electrophotographic printer having a writing density of 600 dpi or more. The solid line in FIG. 3 is obtained by quantizing the four-valued output data by a two-bit error diffusion method, and the broken line is a characteristic example of the binary error diffusion.

Both of the characteristics of the electrophotographic engine and the effect of the dot gain show a characteristic of drawing an S-shaped curve. Particularly, in a dark part, the rate of density increase with respect to image input data increases, and A density saturation phenomenon occurs in which all areas are filled with dots before reaching the maximum. This tendency is more pronounced for high-density writing electrophotographic printers of 1200 dpi or more. Also, in image formation in which the output data is quantized by the 2-bit error diffusion method with an electrophotographic printer, the reproducibility of isolated dots in the image highlight area is poor. Occurs, and the number of gradations is significantly reduced.

FIG. 4 shows a first example of gradation characteristics when an image is formed by the 2-bit error diffusion processing according to the present invention, compared with gradation characteristics (solid line) when an image is formed by the conventional 2-bit error diffusion processing. (Broken line) is shown.

The dashed line in FIG. 4 changes the initial value 85 of the dot data 1 of the small size dot shown in FIG. 2A to 102, and changes the initial value 170 of the dot data 2 of the medium size dot.
Is a gradation characteristic when is changed to 204. In the characteristics indicated by the broken line, it can be seen that the gradation characteristics have spread to the high density side, and the number of gradations that can be taken for the input image data has increased.

FIG. 5 shows a second example of the gradation characteristic when an image is formed by the 2-bit error diffusion processing according to the present invention, compared with the gradation characteristic (solid line) when the image is formed by the conventional 2-bit error diffusion processing. (Broken line) is shown.

The dashed line in FIG. 5 indicates that the initial value 170 of the medium-size dot data 2 is 204, while the dot data 1 of the small-size dot shown in FIG.
This is the gradation characteristic when changing to. In the characteristics indicated by the broken line, the same gradation characteristics as before are shown up to the middle density region,
When a medium-size dot starts to be generated, an increase in dot density is suppressed, and the gradation characteristic is expanded to a higher density side.

FIG. 6 shows a third example of the gradation characteristic when an image is formed by the 2-bit error diffusion processing according to the present invention, compared with the gradation characteristic (solid line) when the image is formed by the conventional 2-bit error diffusion processing. (Broken line) is shown.

As shown in FIG. 2B, the broken line in FIG. 6 indicates that each dot data (computed value) is an arbitrary value, and the initial value 85 of the dot data 1 of the small size dot is 6
8 shows the gradation characteristics when the initial value 170 of the dot data 2 of the medium-size dot is increased to 204.
In the characteristics shown by the broken line, the low density side of the image has a high rate of increase in dot density and the rise of density is fast, while the high density side of the image has a low rate of increase in dot density and saturation of density is low. Be late. Therefore, the gradation change area is spread over the entire area of the image, and the brightness can be set to a more linear gradation characteristic.

When the setting of the gradation characteristic of the error diffusion processing is changed as described above, the table value of the γ correction is set automatically or manually, and in the case of the copying system, the original density and the copying density become close. Set as follows.

FIG. 10 shows the structure of an electrophotographic color printer. The image forming system of the color printer is a four-drum tandem engine type image forming apparatus that forms an image of four colors in an independent image forming system and combines the images.

YMCK (yellow, magenta, cyan,
Each black (black) imaging system has a small diameter OP that forms a latent image.
A charging roller, a developing unit, a cleaning and charge elimination unit are arranged from the upstream side of the image forming so as to surround the C drum and surround it. A toner bottle unit for supplying YMCK toner is arranged beside each block.
Independent optical writing units are arranged, LD light source, optical components such as collimating lens and fθ lens,
The writing optical path is guided to the charging roller of the OPC drum via a polygon mirror and a return mirror. A series of transfer belt units are arranged on the right side of the apparatus in contact with each OPC drum, and transfer the toner image onto the transfer paper while holding the transfer paper. The imaging system is YM from the bottom of the figure.
The developing machines are arranged in the order of CK (yellow, magenta, cyan, black), and optical writing is performed based on image data corresponding to each color.

At the lower side of the apparatus, paper is fed in a horizontal direction from a paper feeding tray for storing the transfer paper, and the transfer paper is transported vertically in the image forming system direction. I have. Further, the sheet after image formation is conveyed to a fixing unit on the upper side of the apparatus, where the toner is fixed on the transfer sheet by heat and pressure, and discharged to a discharge tray above the color printer with the image side down.

The present invention is applied to the above-described four-drum tandem engine type color image forming apparatus. A color image in which an image forming engine is one system and sequentially synthesizes four-color images on a transfer drum or on transfer paper. The same can be applied to a forming apparatus.

In a color copying machine for printing a read image, a scanner having the same configuration as that shown in FIG. 7 is connected to the printer shown in FIG. 10 to copy a document image. The scanner is composed of a color scanner. The color components of each pixel of the original are separated into RGB by a color CCD and read.
And the image is formed with the YMCK toner as described above.

In a color printer and a color copying machine,
Since a color is expressed by a combination of each of the yellow, magenta, cyan, and black gradations, the gradation reproduction range and the number of gradations are important. Therefore, it is effective to increase the number of gradations of each color as in the present invention.

Further, the gradation discrimination ability differs depending on the toner color. In general, yellow has less change in lightness than magenta and cyan, and the actual number of gradations may be small. Therefore,
It is also possible to make the dot calculation data value different for each color and select the gradation characteristic to be used. Specifically, for magenta, cyan, and black, it is effective to set the gradation characteristics close to linear with respect to brightness by the processing of the present invention.

As described above, when the setting of the gradation characteristic of the error diffusion processing is changed for each color or for a single color, the γ correction value and the color conversion coefficient value for each color are automatically or manually set.

In the above-described embodiment, the gradation is controlled by changing the dot calculation data (the output value after quantization for performing the error calculation) of the error diffusion process. However, the present invention is not limited to this. The control may be performed by a method of adding or subtracting a specific value from the error value of the error calculation. That is, the dot density is suppressed by adding the error value, and the dot density is increased by subtracting the error value. In this method, it is difficult to control the gradation correction area, but different gradation characteristics can be obtained as in the above-described embodiment.

[0047]

As described above, according to the present invention, the following effects can be obtained. (1) In the error diffusion process, the gradation characteristics generated by the dot gain can be easily controlled, the gradation characteristics are good, and the number of effective gradations increases. (2) By changing the numerical data of the dot for which the error calculation is performed, the gradation characteristics for the image data can be easily changed. (3) By making the dot numerical data other than the maximum dot for which the error calculation is performed variable, it is possible to change the gradation characteristic of the image data while guaranteeing the maximum density of the image. (4) By making the dot numerical data variable for each dot for which the error calculation is performed, the gradation characteristics for the image data can be freely changed. As a result, it is also possible to set the brightness gradation characteristics of the image data to be nearly linear. In color image formation, the number of color reproductions can be increased. (5) It is possible to easily change the gradation characteristics for image data by the binary error diffusion process having relatively good gradation characteristics. (6) By making the numerical data of the dot for which the error calculation is performed variable on the larger side, the increase rate of the dot density with respect to the data is suppressed, and the tone reproduction area on the high density side is expanded. (7) It is possible to increase the effective number of tones by shifting the tone reproduction range of the numerical data of the dot for which the error calculation is performed on both the image highlight and the dark side. (8) The gradation characteristics for the image data can be changed by means (SP mode) for inputting the numerical data of the dot for which the error calculation is performed. (9) When changing the dot calculation data value according to the present invention, γ correction and color correction are performed in accordance with the change. If not performed, density deviation and color mismatch will occur, preventing correct image formation. In the present invention, since a means for changing the numerical data of the dot on which the specific user or the like performs the error calculation is provided, without causing a density deviation or the like,
Data values can be set easily.

[Brief description of the drawings]

FIG. 1 shows a configuration of an error diffusion process according to the present invention.

FIGS. 2A and 2B show a guidance display on an LCD in an operation input unit of the image forming apparatus.

FIG. 3 shows gradation characteristics when an image is formed by a conventional error diffusion process.

FIG. 4 shows a 2-bit image according to the present invention, compared with a gradation characteristic (solid line) when an image is formed by a conventional 2-bit error diffusion process.
A first example (broken line) of gradation characteristics when an image is formed by error diffusion processing is shown.

FIG. 5 shows a 2-bit image according to the present invention compared with a gradation characteristic (solid line) when an image is formed by a conventional 2-bit error diffusion process.
7 shows a second example (broken line) of gradation characteristics when an image is formed by the error diffusion process.

FIG. 6 shows a 2-bit image according to the present invention in contrast to a gradation characteristic (solid line) when an image is formed by a conventional 2-bit error diffusion process.
A third example (broken line) of gradation characteristics when an image is formed by error diffusion processing is shown.

FIG. 7 illustrates an example of an image forming apparatus including a digital copying machine to which the present invention is applied.

FIG. 8 shows a configuration of an image processing unit of the digital copying machine.

FIG. 9 shows a configuration of a conventional error diffusion process.

FIG. 10 shows the structure of an electrophotographic color printer.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 CCD 2 AGC 3 AD conversion part 4 Shading correction part 5 Filter processing part 6 Main scanning magnification change part 7 γ correction part 8 Halftone processing part 9 LD 20 Input data 21 Adder 22 Quantization part 23 Output data 24 Error calculator 25 error buffer 26 error diffusion matrix 27 data correction unit 28 data selection / switching unit

 ──────────────────────────────────────────────────続 き Continued on front page F term (reference) 2C087 AA15 AA16 AC08 BA02 BA03 BA07 BB10 BD05 BD36 2C187 AC07 2C262 AA24 AA26 AA27 AB07 AB13 BB01 BB08 BB10 BB15 BC07 BC10 EA02 5C077 LL04 MP08 NN12 PP33 RR08 TT03

Claims (7)

[Claims] 1. A quantizing means for quantizing multi-gradation image data into N (≧ 2) values by an error diffusion method, an image forming means for forming an image using the quantized output value, An image forming apparatus comprising: variable control means for variably controlling an output value after quantization for performing an error operation. 2. The image forming apparatus according to claim 1, wherein the variable control unit variably controls an output value other than an output value corresponding to a dot having a maximum size. 3. The image forming apparatus according to claim 1, wherein the variable control unit controls an output value corresponding to a small size dot and an output value corresponding to a medium size dot independently. 4. The image forming apparatus according to claim 1, wherein said variable control means variably controls an output value corresponding to a medium-sized dot to a large value. 5. The variable control means variably controls an output value corresponding to a small-sized dot to a small value and variably controls an output value corresponding to a medium-sized dot to a large value. The image forming apparatus according to claim 1, wherein the number of gradations is increased. 6. A quantizing means for quantizing multi-gradation image data into N (≧ 2) values by an error diffusion method, an image forming means for forming an image using the quantized output value, An image forming apparatus comprising: variable control means for variably controlling an output value after quantization for performing an error operation; and input means for giving a predetermined instruction to the variable control means. 7. The image forming apparatus according to claim 6, wherein the input unit is operated by a service technician or a specific user.