JP4148443B2 - Image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
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]
[Prior art]
Conventionally, as a halftone reproduction method in an image forming apparatus, a dither method, a density pattern method, and an error diffusion method are generally used. In the dither method, a color is expressed by a combination of gradations in a plurality of pixels and in a color image. The dither method used in general printing is excellent in graininess, and expresses a halftone image smoothly. In the so-called area gradation method typified by the dither method, the resolution deteriorates in order to obtain gradation. Further, in the dither method for generating a periodic image with respect to a printed image such as a halftone dot, moire is likely to occur.
[0003]
There is an error diffusion method as a method of expressing gradation while maintaining resolution. The error diffusion method can obtain a resolution that is faithful to the original image and is suitable for the reproduction of character images, but halftone images such as photographic parts are arranged with isolated dots dispersed or irregularly connected, The graininess is poor and a peculiar texture may occur. In particular, in an electrophotographic printer, an image is unstable because it is formed with isolated dots, and granularity deterioration and banding are likely to occur due to density unevenness.
[0004]
In addition, the error diffusion process is complicated because it performs a product-sum operation for the diffusion of quantization errors in the surrounding pixels, and requires a very long processing time. In particular, as the image output density increases, the number of pixels per unit area increases and the amount of calculation increases. Specifically, if the pixel density is changed from 600 dpi to 1200 dpi, the number of pixels increases 4 times, and if it is 2400 dpi, 16 times, which increases in proportion to the square of the resolution, the processing speed is increased to obtain the same productivity. It is necessary to plan.
[0005]
Therefore, error diffusion processing is performed in a state where the resolution of the input image data is low, and a method of outputting high-resolution binary image data is used, so that error diffusion processing is required compared to error diffusion of a high-resolution image as it is. There has been proposed a technique that enables high-speed processing by reducing the operation and its circuit.
[0006]
For example, there is a device that converts input image data to a high resolution by converting the magnification, reduces the number of gradations by multi-value error diffusion, and binarizes the result to a higher resolution by a density pattern method or dither (Japanese Patent Laid-Open No. Hei 7). -295-295).
[0007]
There is also a method of converting binary data obtained by error diffusion at low resolution (600 dpi) into binary image data at high resolution (1200 dpi) by pattern matching (see Japanese Patent Laid-Open No. 11-1555064).
[0008]
[Problems to be solved by the invention]
The above-described former device prevents a reduction in calculation time and an increase in circuit scale due to a buffer memory, obtains a binary image signal having sufficient gradation at a high speed, and eliminates a moiré and a rosetta pattern so as to eliminate a multi-value error. A gradation process for reducing the number of gradations per pixel by diffusion and a gradation process called a density pattern method are performed in duplicate based on the result. However, with regard to the arrangement of dots, with a simple density pattern method or dither method arrangement, dot reproducibility worsens with higher density writing such as 1200 dpi, especially with a printer using electrophotography. Further, the arrangement by the density pattern method or the dither method has a periodicity in the image and may cause moire.
[0009]
In the latter method, the highlight graininess is greatly improved with less buffer memory and less processing, but the change in the image is small compared with the error diffusion processing of 600 dpi, and the improvement in the image quality is remarkable. I can't.
[0010]
An object of the present invention is to provide an image forming apparatus which realizes error diffusion processing at high speed and improves image quality such as image graininess and resolution in a high-density writing type electrophotographic printer. is there.
[0011]
[Means for Solving the Problems]
In the present invention, multi-level error diffusion processing is performed using a dither threshold corresponding to the degree of edge of an image, thereby converting low-resolution input multi-level data into high-resolution binary data at high speed.
[0012]
In the present invention, an image having excellent granularity and stability is formed by using a dot concentration type dither threshold and controlling the output dot position so that dots are concentrated in a halftone dot pattern.
[0013]
In the present invention, an optimum dither threshold matrix for a specified output mode is used, and the amplitude of the dither threshold matrix is controlled in accordance with the edge degree of the image, thereby forming an image having both graininess and sharpness. To do.
[0014]
In the present invention, by switching the quantization threshold of the edge amount according to the white background determination result, it is easy to take the character region as the maximum edge level and to make the high density halftone dot as the maximum edge level.
[0015]
Furthermore, in the present invention, by performing expansion / contraction of the edge level, it is possible to prevent an area having a large edge level other than the halftone image from spreading more than necessary.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.
FIG. 14 shows an example of an image forming apparatus comprising a digital copying machine to which the present invention is applied. The image processing unit of the digital copying machine is roughly divided into a scanner system process for correcting read data, a digital image process for processing and correcting a digital image, and a writing system process for modulating a writing LD.
[0017]
The data level of 600 dpi analog data read by the CCD is adjusted by AGC. In AD conversion, analog data for each pixel is converted into a digital value of 8 bits per pixel (0 (white) to 255 (black)), and shading correction corrects variations in pixels and illuminance of the read CCD. Next, in the filter processing, MTF correction for correcting the amplitude of an image generated by reading and smoothing processing for smoothly expressing a halftone image are performed. Subsequently, in the main scanning scaling, a scaling process in the main scanning direction is performed in accordance with the copying magnification, and γ correction for converting to writing density is performed. Finally, halftone processing is performed and converted into 1-bit or 2-bit data per dot and transmitted. In addition, processing (not shown) such as background removal processing, flare removal processing, scanner γ, and image editing is added.
[0018]
(Example 1)
As Example 1 of the present invention, a 5-value error diffusion process using a dither threshold is performed on 600 dpi input data (256 gradations), and the quantization result is converted to 1200 dpi dot on / off (binary image data). An example of output as
[0019]
FIG. 1 shows the configuration of Embodiment 1 of the present invention. The input level of 600 dpi is subjected to filter processing / magnification processing by the filter / magnification processing unit 1, and then an edge level is calculated by the edge level calculation unit 2. Although details of the edge level calculation unit 2 will be described later, the edge level calculation unit 2 determines a plurality of edge levels for each pixel of 600 dpi. In this embodiment, there are two levels of edge level 0 or 1. However, it is assumed that the edge level 1 has a higher edge degree of the image.
[0020]
The calculated edge level is subjected to expansion processing in the edge level expansion unit 3. Specifically, this is processing for setting the maximum value of the edge level in a predetermined area (for example, a 5 × 5 area centered on the target pixel) to the edge level of the target pixel. The edge level after the expansion process is contracted in the edge level contracting unit 4. Specifically, this is processing for setting the minimum value of the edge level in a predetermined area (for example, a 5 × 5 area centered on the target pixel) to the edge level of the target pixel. The contracted edge level finally becomes the edge level of the pixel.
[0021]
This edge level is used for switching between the γ table in the γ correction processing unit 5 and the dither threshold matrix in the dither threshold selection unit 6. The γ correction processing unit 5 performs γ correction processing by switching between two γ tables according to the edge level.
[0022]
FIG. 2 shows an example of the γ table. When the edge level = 0, correction is performed with γ having characteristics close to linear, and when the edge level = 1, correction is performed with γ having substantially S-shaped characteristics.
[0023]
The dither threshold value selection unit 6 selects four dither threshold value matrices (a) to (d) shown in FIG. 3 when the edge level = 0, and sets the position value corresponding to the target pixel position of each matrix to the threshold value ( thr1 to thr4). When the edge level = 1, all of thr1 to thr4 are set to 128 (fixed threshold).
[0024]
The quantizing unit 7 quantizes the data after the γ correction processing into five values by performing multi-level error diffusion processing using the threshold value determined by the dither threshold value selecting unit 6.
[0025]
In the error diffusion process, the adder 11 for adding the quantization error of the processed pixel to the multivalued image data, and the output data of the adder 11 are converted into four quantization threshold values Thr1, Thr2, Thr3, Thr4 (Thr1 <Thr2 < Quantization unit 7 for performing five-value quantization by Thr3 <Thr4), subtractor 9 for calculating a quantization error from the input and output of this quantization unit 7, and the quantization error calculated by this subtractor 9 The error calculation unit 10 calculates an error amount to be added to a pixel to be processed next in accordance with a predetermined error matrix and supplies the calculated error amount to the adder 11.
[0026]
The relationship between the input value and the output value of the quantization unit 7 is as follows: output value = 0 when input value <Thr1, output value = 64 when Thr1 ≦ input value <Thr2, and output when Thr2 ≦ input value <Thr3. When value = 128, Thr3 ≦ input value <Thr4, output value = 192, and when Thr4 ≦ input value, output value = 255.
[0027]
The dither threshold matrix of 6 × 6 (3 × 3 at 600 dpi) as shown in FIG. 4 is arranged by spirally arranging the thresholds increasing in step width 3 from 74 to 179 from the smallest value in 1200 dpi. This is a dot concentration type that forms a 200-line halftone dot. In FIG. 4, four threshold values inside the solid grid correspond to one pixel of 600 dpi. As shown in FIG. 4, the threshold values (74, 77, 80, 83) from the smallest to the fourth in this dither threshold matrix are arranged at different pixel positions of 600 dpi. That is, in the case of 2 × 2 pixels (110, 107, 113, and 80 pixels) in the upper left in FIG. 4, the lowest 80 is assigned to the upper left pixel in FIG. (B) The next lowest 110 is assigned to FIG. 3C, and the highest 113 is assigned to FIG. 3D.
[0028]
The dither threshold selection unit 6 outputs four thresholds corresponding to one pixel of 600 dpi in the dither threshold matrix as quantization thresholds corresponding to the pixel. For example, at the upper left pixel position, among the four threshold values (80, 107, 110, 113) at that position, the minimum 80 is the quantization threshold Thr1, the next smaller 107 is the childization threshold Thr2, and the next smaller 110 is the quantum. The quantization threshold Thr3 and the largest 113 are output as the quantization threshold Thr4.
[0029]
The 3 × 3 dither threshold matrix shown in FIG. 3 is obtained by rearranging the thresholds in the dither threshold matrix shown in FIG. 4 at 600 dpi pixel positions for each quantization threshold. 3A corresponds to the quantization threshold Thr1, (b) corresponds to the quantization threshold Thr2, (c) corresponds to the quantization threshold Thr3, and (d) corresponds to the quantization threshold Thr4. To do.
[0030]
The dot output position control unit 8 converts 600 dpi 5-value data obtained by the error diffusion processing into 1200 dpi 2 × 2 pixel dot on / off (binary image data).
[0031]
The dot output position control unit 8 first calculates the number of output dots. In other words, the number of dot-on pixels in 2 × 2 pixels on 1200 dpi binary image data corresponding to each pixel on 600 dpi multi-value image data is determined. Specifically, when the quantized data value (pentarization result) is 0, 0 is quantized, when the quantized data value is 64, 1 is quantized, and when the quantized data value is 128, 2 is quantized. When the data value is 192, 3 is output as the number of dots, and when the quantized data value is 255, 4 is output as the number of dots.
[0032]
Next, the dot output position is determined. The dot output position determines how the determined number of dots are arranged in a 1200 × 2 2 × 2 pixel according to the corresponding position on the dither threshold matrix (FIG. 4) of the pixel to be processed.
[0033]
More specifically, for example, when the pixel at the upper left pixel position in the dither threshold matrix shown in FIG. 4 is to be processed, the first dot is placed at the position having the smallest value among the 2 × 2 threshold values at the upper left. The second dot is arranged at the next largest threshold position, the third dot is arranged at the next largest threshold position, and the last dot is arranged at the largest threshold position. In this case, if the number of dots is 2, as shown in FIG. 5A, dots are output to the right two pixel positions (thresholds 80 and 107) in 2 × 2 pixels (the two pixels are dot-on). Pixel). If the number of dots is 1, a dot is output at the lower right pixel position, and if the number of dots is 3, a dot is also output at the upper left pixel position. Similarly, when the pixel on the right is to be processed, if the number of dots is 2, dots are output to the lower two pixel positions (threshold values 77 and 98) as shown in FIG. 5B. become. In this way, by changing the dot output order (arrangement order) according to the corresponding position of the pixel to be processed on the dither threshold matrix, halftone dots can be easily formed.
[0034]
Thus, at edge level = 0 (region with a small edge degree), halftone dots are formed using the dither threshold value, so that an image having excellent graininess and stability can be formed. On the other hand, since a fixed threshold value is used at edge level = 1 (region with a large edge degree), a sharp image can be obtained in the character portion, and moire can be prevented from occurring in the halftone image.
[0035]
The edge level calculation unit 2 will be described in detail below. A processing flowchart of the edge level calculation unit is shown in FIG. First, the edge amount of each pixel of 600 dpi is calculated by the edge extraction filter shown in FIG. 7 (step 101). FIG. 7A extracts vertical lines, (b) horizontal lines, and (c) and (d) diagonal lines. Specifically, first, coefficients corresponding to the four edge amounts calculated in FIGS. 7A to 7D are respectively applied. For example, the coefficient of (a) and (b) is 2, and the coefficient of (c) and (d) is 1. The maximum value of the edge amount of each filter multiplied by the coefficient is set as the edge amount of the pixel.
[0036]
Next, it is determined whether or not the pixel of interest belongs to a white background area (step 102). When a predetermined number (for example, 6 pixels) or more of white pixels exist in a predetermined area (for example, 5 × 5 area) centered on the target pixel, it is determined that the target pixel belongs to the white background area. Here, the white pixel is preferably a pixel taking noise into consideration, for example, a pixel whose data (density) is 5 or less.
[0037]
Next, the edge amount threshold value is switched depending on the result of the white background determination (steps 103 and 104), and the calculated edge amount is quantized to a plurality of levels (step 105). When the target pixel belongs to the white background area, the quantization threshold is set to a lower value than when the target pixel does not belong to the white background area. The reason for performing such switching is to make it easy to determine even a character with a relatively low edge amount such as a low-density character as an edge (since there are many white pixels around the character area, a low threshold is easily selected. ). Finally, it is determined whether or not the target pixel is a black pixel (step 106). If the data of the pixel of interest is greater than or equal to a predetermined value, the pixel is black, and the edge level of the pixel is set to the maximum level (edge level = 1 in this embodiment) (step 107). This is to make it easy to determine an edge even in a high density halftone dot portion having a relatively small edge amount.
[0038]
As described above, in this embodiment, an edge level is calculated, and an image having both graininess and sharpness can be formed by performing quantization by switching the dither threshold according to the edge level. In particular, since the edge level is calculated from the image data immediately before the γ correction process, there is an advantage that the data subjected to the filter process or the scaling process is quantized using an optimum threshold value.
[0039]
In addition to the example shown in FIG. 7, the edge extraction filter may be, for example, a filter as shown in FIG. 8A and 8B extract vertical and horizontal edges, respectively, and the filter of FIG. 8C extracts halftone and diagonal edges. A relatively large edge amount can be extracted from a halftone image.
[0040]
(Example 2)
As a second embodiment of the present invention, an example in which processing parameters are switched according to an output mode will be described.
[0041]
FIG. 9 shows the configuration of the second embodiment of the present invention. The output mode is designated by the operator, and in this embodiment, there are two types of cases: “character mode” that emphasizes character images and “character / photo mode” that is suitable for images in which characters and photos are mixed. Show.
[0042]
First, as in the first embodiment, the edge level calculation unit 22 uses the edge level calculation unit 22 to quantize the edge amount into a plurality of levels from the input data that has been subjected to the filter processing / magnification processing by the filter / magnification processing unit 21. Calculate the level. However, in this embodiment, the edge level is set in four stages from 0 to 3 (edge level = 3 has the largest edge degree). The four levels of edge levels are subjected to predetermined area expansion processing in the edge level expansion section 23 in the same manner as in the first embodiment. The edge level after the expansion process is contracted in the edge level contraction unit 24 in the same manner as in the first embodiment, but the contraction area is switched depending on the output mode. Specifically, in the character mode, the contraction area is 1 × 1 (no contraction processing), and in the character / photo mode, the contraction area is the same as the expansion area. By switching the contraction area according to the output mode in this way, in the character mode, an area with a large edge level is increased to emphasize sharpness, and in the character / photo mode, an area with an excessive edge level is prevented from increasing more than necessary.
[0043]
The contracted edge level is sent to the γ correction processing unit 25 and the dither threshold amplitude control unit 26. The γ correction processing unit 25 switches the γ table according to the edge level. In this embodiment, the edge level has four stages. However, the number of γ is not necessarily the same as the edge level, such as switching between two γ tables when the edge level is 0 and 1 and when the edge level is 2 and 3. There is no need to switch tables. When switching between two γ tables, for example, the same as in the first embodiment is performed.
[0044]
The dither threshold amplitude control unit 26 determines the dither threshold matrix amplitude (step width). The step width A of the dither threshold matrix is A = (3-edge level). That is, the step width A is 3 when the edge level = 0 (minimum edge degree), and the step width A decreases as the edge level increases, and the step width A becomes 0 when the edge level = 3 (maximum edge degree). .
[0045]
Next, the dither threshold value calculation unit 27 calculates a dither threshold value matrix. First, a dither coefficient matrix corresponding to the output mode designated by the operator is selected. FIG. 10 shows a dither coefficient matrix corresponding to the character mode, and FIG. 11 shows a dither coefficient matrix corresponding to the character / photo mode. Multiplying the dither coefficient matrix by the step width A determined by the dither threshold amplitude control unit 26 and adding 128 is a dither threshold matrix at 1200 dpi.
[0046]
FIG. 12 is a dither threshold matrix at 1200 dpi calculated when the step width is 3 in the dither coefficient matrix in the character mode (FIG. 10). From this dither threshold matrix at 1200 dpi, four 3 × 5 dither threshold matrices at 600 dpi are created in the same manner as in the first embodiment (FIGS. 13A to 13D), and five values are obtained using this dither threshold matrix. Is converted into binary image data of 1200 dpi and output.
[0047]
The dither coefficient matrix shown in FIGS. 10 and 11 will be described. In FIG. 10, the four smallest coefficients (−17) are arranged in the same 600 dpi pixel, whereas in FIG. 11, the lower four coefficients (−18 to −16) are arranged in different 600 dpi pixels. Yes. In the arrangement as shown in FIG. 10, the four threshold values are the same in the pixel in which four −17s are arranged, and therefore the pixel is output only in units of 4 dots. For this reason, the graininess at the low density portion of the image is deteriorated, but the uncomfortable feeling at the portion where the region having a large edge level and the region having a small edge level are mixed is reduced. On the other hand, with the arrangement as shown in FIG. 11, for example, when a dot is turned on in a pixel where −16 is arranged, negative errors are diffused around the area, so −18, −17, and −15 are arranged. Dot off tends to occur in some pixels. Accordingly, since the low density portion of the image is output from 1 dot, the graininess is improved. In the character mode, to prevent show-through, γ with the low density side dropped to 0 (white) is often used, and in that case, the mixed part feels more strange than the appearance of dots in the low density part. Emphasis is placed on the dither coefficient matrix of FIG. 10, and in the character / photo mode, the dither coefficient matrix of FIG.
[0048]
As described above, in this embodiment, an image optimal for the output mode is formed by switching the processing parameters in accordance with the output mode specified by the operator while achieving both the graininess and sharpness of the image. It is possible.
[0049]
In the present embodiment, an example is shown in which the arrangement of the dither threshold matrix coefficients differs depending on the output mode. However, the dither threshold matrix coefficients are switched depending on the output mode (for example, in the character mode, a matrix having a small amplitude is selected and the character / It is also possible to select a matrix with a large amplitude in the photo mode) and switch the size of the dither threshold matrix (for example, select a small matrix in the character mode and select a large matrix in the character / photo mode). is there.
[0050]
As described above, the present invention may of course be implemented by dedicated hardware, but can also be implemented by software (program) using a general-purpose computer system. When implemented by software, a program that realizes the image processing functions (edge level calculation, γ correction, multi-value error diffusion processing, etc.) and processing procedures of the present invention is recorded on a recording medium and the like. The program is read into the computer system and executed by the CPU, whereby the image processing procedure of the present invention is performed. The input image data is original image data read from a scanner or the like, image data prepared in advance on a hard disk or the like, or image data acquired via a network. Furthermore, high-resolution binary image data is output to an external computer or the like via a laser printer or a network.
[0051]
【The invention's effect】
As described above, according to the present invention, the following effects can be obtained.
(1) By performing multilevel error diffusion processing using a dither threshold value corresponding to the edge degree of an image, low resolution input multilevel image data can be converted to high resolution binary image data at high speed.
(2) By concentrating dots in a dot pattern, an image having excellent graininess and stability can be formed.
(3) Quantization can be performed using a dither threshold matrix optimum for the designated output mode.
(4) By controlling the amplitude of the dither threshold matrix according to the edge degree of the image, it is possible to form an image having both graininess and sharpness.
(5) By switching the quantization threshold of the edge amount according to the white background determination result, the character region can be easily taken as the maximum edge level, and an image with high sharpness can be formed.
(6) A high density halftone dot can be easily set as the maximum edge level, and moire can be prevented.
(7) By performing the expansion / contraction of the edge level, it is possible to prevent an area with a large edge level other than the halftone image from spreading more than necessary. Further, by switching the expansion / contraction region in accordance with the designated output mode, the optimum expansion / contraction can be performed for the designated output mode.
[Brief description of the drawings]
FIG. 1 shows a configuration of Embodiment 1 of the present invention.
FIG. 2 shows an example of a γ table.
FIG. 3 shows a 600 dpi dither threshold matrix.
FIG. 4 shows a dither threshold matrix expanded to 1200 dpi.
FIG. 5 shows an example of a dot output position.
FIG. 6 shows a flowchart of edge level calculation processing.
FIGS. 7A to 7D show examples of edge extraction filters.
FIGS. 8A to 8C show other examples of edge extraction filters.
FIG. 9 shows a configuration of Embodiment 2 of the present invention.
FIG. 10 shows a dither coefficient matrix (character mode).
FIG. 11 shows a dither coefficient matrix (character / photo mode).
FIG. 12 shows a dither threshold matrix (character mode: step 3) expanded to 1200 dpi.
FIG. 13 shows a dither threshold matrix (character mode: step 3).
FIG. 14 shows an example of an image forming apparatus including a digital copying machine to which the present invention is applied.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Filter and scaling process part 2 Edge level calculation part 3 Edge level expansion part 4 Edge level contraction part 5 Gamma correction process part 6 Dither threshold value selection part 7 Quantization part 8 Dot output position control part 9 Subtractor 10 Error calculation part 11 Adder

Claims (14)

  An image forming apparatus that converts low-resolution input multilevel image data into high-resolution binary output image data, and an edge level calculation unit that calculates an edge level from input multilevel image data immediately before γ correction. Input multi-value by multi-value error diffusion using γ correction means that performs gradation correction by a printer γ selected according to the edge level and a first dither threshold matrix selected according to the calculated edge level Quantization means for quantizing the image data, and dot position control for converting the quantization result into the number of dots-on pixels of high-resolution pixels and controlling the pixel positions where the dots are turned on based on the second dither threshold matrix And an image forming apparatus. In the second dither threshold matrix, when the edge level is the lowest , coefficients are arranged so as to form a dot-concentrated halftone dot, and the dot position control means includes a second dither threshold matrix. 2. The image forming apparatus according to claim 1, wherein dots are output in order from a pixel having a low coefficient.   An image forming apparatus that converts low-resolution input multilevel image data into high-resolution binary output image data, and an edge level calculation unit that calculates an edge level from input multilevel image data immediately before γ correction. Multi-level error diffusion using γ correction means for performing gradation correction with a printer γ selected according to the edge level and a first dither threshold matrix selected according to the calculated edge level and output mode. Quantization means for quantizing the input multivalued image data, and the quantization result is converted into the number of dots-on pixels of high-resolution pixels, and the pixel positions where dots are turned on are controlled based on the second dither threshold matrix An image forming apparatus comprising a dot position control unit.   4. The image forming apparatus according to claim 3, wherein the dither threshold value matrix having a different coefficient or coefficient arrangement of the first dither threshold value matrix is switched according to the output mode.   4. The image forming apparatus according to claim 3, wherein a dither threshold value matrix having a different size is switched according to the output mode.   4. The image forming apparatus according to claim 1, wherein the amplitude of the dither threshold value of the first dither threshold value matrix increases as the edge level decreases.   4. The image forming apparatus according to claim 1, wherein the edge level is a plurality of levels obtained by quantizing edge amounts calculated from input multi-value image data immediately before γ correction.   8. The image forming apparatus according to claim 7, wherein a threshold value for quantizing the edge amount is switched according to a white background determination result for determining whether or not the target pixel is a white background region.   9. The determination of the white background is performed when the number of pixels whose input pixel data is less than or equal to a predetermined value is greater than or equal to a predetermined number within a predetermined area centered on the target pixel. The image forming apparatus described.   8. The image forming apparatus according to claim 1, wherein the edge level is maximized when the target pixel data is a predetermined value or more.   4. The image forming apparatus according to claim 1, wherein the predetermined area is contracted after the edge level is expanded by a predetermined area.   12. The image forming apparatus according to claim 11, wherein the expansion and contraction regions are switched according to an output mode.   13. The image forming apparatus according to claim 12, wherein the shrinking area is smaller than the expanding area.   13. The image forming apparatus according to claim 12, wherein the image forming apparatus does not contract in an output mode for character images.

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