JP2009194660A - Image processor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image processor which generates high-quality images with halftone dots giving little feeling of noise, can obtain a stable quality even at the boundaries of input gradations, executes processing at a high speed, and is suitable for application to electrophotography. <P>SOLUTION: The image processor is provided with a first halftoning part 12-1 which generates halftone dot information for each of areas which are divided for each gradation value of input images by an area dividing part 11, an estimating part 14 which estimates whether halftone dots should be generated by the halftone dot generating means at the boundaries of the areas where the gradation value changes and a second halftoning part 12-2 which performs correction so that halftone dot information at the boundaries may be deleted or left according to the result of estimation by the estimating part 14. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image processing apparatus that performs optimal image processing on an input image. More specifically, the present invention relates to an image processing apparatus that optimally selects an image spatial frequency of an output image and an image forming apparatus having the image processing apparatus.

In an electrophotographic printer capable of binary output, an image is expressed by arranging fine dots called halftone dots on paper, and by changing the density of lines (screen lines) drawn by halftone dots. It is well known that grayscale and color images can be expressed as binary images.
In such an electrophotographic printer, the required image spatial frequency (number of screen lines) differs depending on the type of picture to be output, such as a person's face and a simple figure or one color.
In addition, when the performance of an image forming apparatus such as electrophotography affects the image quality, the image quality may be more stable when the number of lines is lower than when it is higher.
Specifically, a processing method is generally used in which the number of lines is reduced to improve the graininess and uniformity, and the number of lines is increased to improve the sharpness.

In recent years, it has been common practice to create a document for presentation using a personal computer, print it, and distribute it. To give such a document a visual effect, Pale uniform backgrounds and gentle gradations are often used over a wide range, and how to output such graphic images stably and with high image quality is an output device for offices. It becomes a big selling point of an electrophotographic printer.
In the conventional method, as a method of performing appropriate image processing, based on the input image and the predicted image after binarization, the halftone dot arrangement is corrected so as to reduce the visual difference, thereby increasing the Some have tried to get quality images. Patent Document 1 is an example of this, and halftone dots are arranged for the entire input image, and evaluation based on the predicted image of the binarized image is performed.
Japanese Patent Application Laid-Open No. 2004-228561 proposes that a halftone dot dither mask is provided for each input gradation, and thereby halftone dots are arranged according to the input gradation value. With this method, it is expected to obtain both stability and good image quality by optimizing the arrangement of halftone dots for each input gradation.
Patent Document 3 focuses on a gradation switching portion. In other words, the number of lines of halftone dots is limited to two types of lines, A and twice that of 2A, and the input gradation area is slightly expanded or reduced, so that both stability and good image quality are achieved. Aiming.
JP 2005-144873 A JP-A-11-157132 JP 2003-87566 A

However, in the method of Patent Document 1, since the position of the halftone dots deviates from the aligned state, it is given to a person who sees a blue noise texture as a whole, and a stable output can be obtained as a method corresponding to electrophotography. The image quality will be slightly inferior.
In the method of Patent Document 2, halftone dots are determined according to the gradation value of the input image, and so-called missing halftone dots that are not completely formed are generated at the gradation switching portion. Then, since an electrophotographic engine produces an unstable image, a stable image is not always obtained.
Further, even in the method of Patent Document 3, it is more preferable to gradually change the number of lines according to the input gradation value without limiting the number of lines to two in terms of good image quality. In order to achieve this, it is necessary to finally use a technique such as a sub-matrix, and the image quality cannot be sufficiently exhibited.
The object of the present invention is to solve the above-mentioned problems, and is good in image quality with little noise in the generated halftone dot, and moreover, stable quality can be obtained even at the input gradation switching point, Another object of the present invention is to provide an image processing apparatus suitable for application to electrophotography and an image forming apparatus using the same.

In order to solve the above problems, the present invention provides an image processing apparatus that generates a binary image from a multi-valued input image, and is divided for each gradation value of the input image. Halftone dot generating means for generating halftone dot information for each area, evaluation means for evaluating whether or not halftone dots are generated by the halftone dot generating means at the boundary portion of the area where the gradation value changes, The image processing apparatus includes: a correction unit that performs correction so as to erase or leave halftone dot information in the boundary portion based on an evaluation result by the evaluation unit.
According to a second aspect of the present invention, the evaluation means includes a filter processing means for performing frequency filter processing on the image information at the boundary portion, and a non-linear operation on the image information at the boundary portion subjected to frequency filter processing. The image processing apparatus according to claim 1, further comprising: a non-linear calculation unit that performs density conversion, wherein the evaluation is performed on the converted image information of the boundary portion.

The invention according to claim 3 is characterized in that the evaluation means includes continuity calculation means for calculating the continuity of the image information at the boundary, and performs the evaluation based on the continuity. The image processing apparatus according to 2, is characterized.
Further, the invention according to claim 4, wherein the evaluation unit applies a halftone dot shape regardless of a gradation value of the input image for a halftone dot related to a surrounding gradation value in the boundary portion, The continuity of image information when not applied is integrated, and for halftone dots not related to surrounding gradation values, the evaluation is performed according to the number of constituent pixels of the halftone dots. The image processing apparatus according to claim 1 is characterized.

According to a fifth aspect of the present invention, in the image processing apparatus according to any one of the first to fourth aspects, the halftone dots arranged by the halftone dot generating means are arranged in a substantially lattice shape at the center. It is characterized by.
According to a sixth aspect of the present invention, there is provided an image forming apparatus including the image processing apparatus according to any one of the first to fifth aspects.

  According to the present invention, it is possible to provide an image processing apparatus and an image forming apparatus in which halftone dots can be arranged with good graininess and image density can be appropriately expressed.

Hereinafter, embodiments of an image processing apparatus of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a functional block diagram showing the main configuration of the image processing apparatus of the present invention.
In FIG. 1, the main parts of the image processing apparatus of the present invention are an area dividing unit 11 that performs area division for each gradation value of an input image, and a halftoning processing unit that performs halftone processing for each divided area. 12. Pattern storage unit 13 which is a ROM (Read Only Memory) storing a halftone pattern used when halftone processing is performed by the halftone processing unit 12; whether to generate a halftone dot at the boundary of each divided area It is comprised from the evaluation part 14 for determining whether or not.
Hereinafter, processing in each element of the image processing apparatus of the present invention will be described with reference to FIG.
In FIG. 1, an input image input to the image processing apparatus is an image having a gradation of 8 bits after interpreting a command in a printer description language.
The input image is first divided (area division) into areas having different input levels (gradation values) by the area dividing unit 11.
Next, each divided area is sent to the halftone dot processing unit 12.
Here, the halftone dot processing unit 12 includes a first halftone dot conversion unit (halftone dot generation unit) 12-1 and a second halftone dot conversion unit (correction unit) 12-2. In the input image that has been sent, first, halftone dots are provisionally arranged using a pattern stored in the pattern storage unit 13 in the first halftone dot conversion unit 12-1.
The pattern storage unit 13 stores a halftone dot pattern corresponding to the gradation value of the input image, and an appropriate pattern is applied according to the gradation value of each area.

By the way, in the processing in the first halftone dot forming unit 12-1, the generation of halftone dots at the input gradation switching portion (the boundary portion between the divided areas) is not fixed.
The evaluation unit (evaluation means) 14 determines whether to generate a halftone dot at the input gradation switching unit.
As a result of the evaluation by the evaluation unit 14, if it is determined that a halftone dot should be generated by the switching unit, the second halftone forming unit 12-2 sets the image with the halftone dot generated by the switching unit. Conversely, if it is determined that a halftone dot is not generated, the information on the halftone dot of the switching portion that has been temporarily arranged is deleted, and an image that does not generate a halftone dot at the switching portion is obtained.
As a result of the above processing, the second halftone dot forming unit 12-2 stores the image data in a state where the ON / OFF information (whether to generate halftone dots) of the pixels is developed.
As a result, pixel ON / OFF 1-bit information is obtained, and laser exposure is performed in accordance with this information to form a halftone image.

FIG. 2 is a diagram for explaining a halftone dot pattern corresponding to the gradation value stored in the pattern storage unit 13.
As shown in FIG. 2, the pattern to be used is determined in advance according to the input gradation value N of the input image. In this embodiment, since the gradation of the input image is 8 bits, the gradation value N is a value from 0 to 255.
In FIG. 2, the pattern sizes for all gradations appear to be the same, but in practice, the sizes may differ slightly for each gradation, so that these patterns can be repeatedly applied in the two-dimensional direction. It has become.

  FIG. 3 is a diagram for explaining halftone dots formed in the present invention. In FIG. 3, (a) shows halftone dots that are actually formed, and (b) shows a plurality of dots that form halftone dots. That is, since the present invention is applied to an electrophotographic engine and the resolution can be as high as 2400 dpi, the halftone dots are not formed by exposure dots by one pixel, but as shown in (b) The plurality of pixels are turned on. When toner adheres thereto, a roughly round halftone dot is formed as shown in FIG.

Next, the halftone dot pattern in the present invention will be described in more detail.
FIG. 4 is a diagram for explaining a halftone dot pattern applied to an area where the gradation value N is 1 to 32 and the density is low.
4A is a diagram showing an example of a halftone dot pattern applied to a highlight portion of a certain color, and FIG. 4B is a diagram showing an example of a pattern at a slightly darker portion. As shown in the figure, the centers of the halftone dots are arranged on the grid for each gradation. (C) is an arrangement of a halftone dot pattern used for another color, and ON pixels are arranged so that halftone dots are generated on a parallelogram lattice or a triangular lattice.
By forming halftone dots in this manner, the graininess of the appearance is improved and a rough impression is not given.
In the present invention, in the range of gradation values N = 1 to 32, gradation expression (expression based on the difference in the number of lines) is performed by such halftone dot arrangement intervals (FM screening).
On the other hand, in the range of N = 33 to 254, the halftone dot disposition interval is constant (the number of lines is the same) as formed by normal halftone dot dither, and the size of the halftone dot itself is different. Tone expression (AM screening).
Note that the tone value 255 is a so-called “solid” image with all pixels turned on. A gradation value of 0 is a state where no toner adheres.

Next, an example in which the evaluation unit 14 of the present invention determines whether or not there is a halftone dot at the boundary will be described.
[First Embodiment]
FIG. 5 is a diagram showing a configuration of the evaluation unit 14 in FIG.
In FIG. 5, the evaluation unit 14 includes first and second bitmap information generation units 21 and 27, first and second smoothing filter calculation units (filter processing means) 22 and 28, and first and second nonlinearities. Part calculation (nonlinear calculation means) 23 and 29, first and second continuity calculation parts (continuity calculation means) 24 and 30, first and second integration parts 25 and 31, and determination part 26. .
The first and second bitmap information generation units 21 and 27 include the image information of the gradation switching unit (boundary) from the area dividing unit 11 and the switching unit from the first halftone conversion unit 12-1. Halftone dot information is input.
From these pieces of information, the first bitmap information generating unit 21 generates bitmap image information in which a target halftone dot is set at the area boundary, and the second bitmap information generating unit 27 is a bit in which the target halftone dot is not set. Map image information is generated.
Next, the first and second smoothing (frequency) filter calculation units (filter processing means) 22 and 28 perform smoothing filter processing for reducing noise on the generated bitmap image information.
The first and second nonlinear portion calculations 23 and 29 perform nonlinear density conversion on the bitmap image information subjected to the filter processing, thereby improving contrast.

FIG. 6 is a diagram showing the relationship between the input density value and the output density value after density conversion.
As shown in FIG. 6, the nonlinear arithmetic units 23 and 29 convert the relationship between the input density and the output density so that the histogram concentrates on the intermediate density value (toner density increases rapidly). A sigmoid function is used for this conversion. A Gaussian function may be used.

7 and 8 are diagrams for explaining processing in the first and second continuity calculation units 24 and 30 in FIG.
In FIG. 7, (A) is an area of gradation value N1, (B) is an area of gradation value N2, and (A) is an area having a higher density than (B).
Here, as an example, a continuity calculation process for bitmap image information in which a target halftone dot is set by the first continuity calculation unit is described.
In FIG. 7, P is the center of the target halftone dot, and the continuity of gradation is measured at the boundary between both areas sandwiching P from above in the figure. In the figure, the upper part is the processed range, and the lower part is the unprocessed range.
In this embodiment, the direction of AA ′ and BB ′ on two straight lines intersecting with P at about 120 degrees is examined. Each interval is set to about 0.5 mm.

FIG. 8 is a diagram showing a change in gradation value on the line AA ′ in FIG. 7 as an example.
In FIG. 8, ideally, the gradation continuity across P between AA ′ represented by the difference between the area of the region (C) and the area of (D) shown in FIG. ) And (D) cancel each other out.
However, actually, as shown in (b), the continuity calculation value represented by (C)-(D) becomes a positive value, or as shown in (c), it becomes a negative value. Or
This area information is added (or averaged) on the AA ′ and BB ′ sides to obtain a calculated value of continuity.
In addition, the second continuity calculation unit 30 performs the same measurement and obtains a calculated value of continuity when the target halftone dot is not set.
The first and second integration units 25 and 31 compare the continuity calculation values in the first and second continuity calculation units 24 and 30, respectively. The determination unit 31 in FIG. According to the halftone dot formation information on the side close to 0, the determination unit 31 in FIG. 5 determines whether or not halftone dots are generated.
That is, when the integrated value in the first integrating unit 25 is closer to 0, a halftone dot is formed according to the halftone dot information on the side where the target halftone dot P is set.
The result of the determination is transmitted to the second halftone dot conversion unit, and the state of the halftone dot is determined.
Further, in response to the decision to generate / not generate halftone dots, the integration result is rewritten with the value of the adopted side for the value of the integrating unit on the side that has not been adopted in the integrating unit 25.
As described above, the frequency filter is superimposed on the region corresponding to the portion (area boundary) where the gradation value of the generated binary input image changes, and the image obtained as a result of nonlinear transformation is evaluated. By performing the above, halftone dots suitable for the development characteristics peculiar to electrophotography can be formed.
Furthermore, it is possible to make it difficult to visually see an undesirable density change in the input gradation switching portion.

[Second Embodiment]
By the way, an actual input image may have a complicated shape in the gradation switching portion. In such a case, it is complicated to determine the presence or absence of halftone dots in the boundary portion for all areas. It is. Therefore, for example, as shown in FIG. 9, the gradation levels in the eight directions around the target halftone dot P are sampled by the same method as in FIG. Then, the presence / absence of a halftone dot at the boundary is determined for the area having the largest area.

[Third Embodiment]
Next, a third embodiment for determining the presence or absence of a halftone dot at the boundary will be described.
FIG. 10 is a diagram for describing evaluation in the first and second embodiments when the area information indicating the gradation continuity across the target halftone dot is the same.
In this case, if it has already been decided on either side of the upper boundary, it may be decided accordingly, and if it is still not decided, it may be decided by a random number.
In FIG. 10, P1 to P3 are halftone dots formed by the left area, and Q1 and Q2 are halftone dots formed by the right area.
In the case of the first embodiment, when P1 is ON,
Q1 OFF, P2 ON, Q2 ON, P3 OFF
Thus, the density can be appropriately expressed in the gradation switching portion.
In the case of the second embodiment, when P1 is ON,
Q1 ON, P2 OFF, Q2 ON, P3 OFF
Thus, the density can be expressed almost appropriately at the gradation switching portion.

[Fourth Embodiment]
Next, still another embodiment will be described.
FIG. 11 is a flowchart for explaining the flow of processing in the fourth embodiment of the present invention.
First, it is determined whether the processing has been completed for all gradations (S100). If the process is finished (Yes in S100), the process is finished.
If the processing has not been completed (No in S100), the pixel in the vicinity of the gradation switching portion where the halftone processing has not been completed is examined. If there is no unprocessed pixel (No in S101), the process returns to S100 to start processing for the next gradation. In this embodiment, since the grayscales are examined in order, the grayscale to be investigated is reduced by one.

If there is a pixel that has not been processed (Yes in S101), pixel information that forms a halftone dot is developed on the memory and a halftone dot is assigned (S102). As a result of the assignment, it is checked whether the halftone cell is missing, that is, whether the halftone dot is related to the surrounding gradation value. If not missing (No in S103), the integrated value when there is a dot is calculated (S109), and the binarized information is determined (S106).
If it is determined that a halftone cell is missing, it is determined whether or not the number of pixels constituting the halftone dot is greater than or equal to a predetermined number (S104). If the number is greater than or equal to the predetermined number (Yes in S104), the integrated value when there is a dot is calculated (S109), and the binarized information is determined (S106).
If the number of pixels constituting the halftone dot is less than or equal to the predetermined number (No in S104), evaluation and integration are performed under halftone dot generation and halftone dot disappearance conditions (S107), and the one with the integrated value close to 0 is selected (S108). Binarization information is determined (S106).
In this way, halftone dot application / non-application by the halftone processing unit to the part where the input image value changes is to input the halftone dot shape only for halftone dots related to at least the surrounding gradation values. The output image is evaluated in each case when applied regardless of the gradation value and when it is not applied at all. For the gradation value not considered, the number of pixels constituting the halftone dot is used. Since it is determined whether or not halftone dots can be generated, the complexity of the processing can be prevented and the processing can be performed at high speed.

FIG. 12 is a diagram showing an outline of an image forming apparatus to which the image processing apparatus of the present invention is applied.
By including the image processing apparatus of the present invention, it is possible to provide an image forming apparatus capable of forming a stable halftone dot suitable for electrophotography.
An apparatus 100 such as a workstation or a personal computer is connected to the image forming apparatus, and printing is performed based on information sent from the apparatus.
Referring to FIG. 12, in this image forming apparatus, a sheet on which an image is to be formed is set on the main body tray 101 or the manual feed tray 102, and the conveyance of the sheet is started from the tray 101 or 102 by the paper feed roller 103. . Prior to the conveyance of the sheet by the sheet feeding roller 103, the photosensitive member (photosensitive drum) 104 rotates, the surface of the photosensitive member 104 is cleaned by the cleaning blade 105, and then uniformly charged by the charging roller 106. The Here, a laser beam modulated in accordance with an image signal is exposed from the laser optical system unit 107 to form a latent image, which is developed by the developing roller 108 to which toner adheres. Paper is fed from the roller 103. The sheet fed from the sheet feeding roller 103 is conveyed while being sandwiched between the photosensitive drum 104 and the transfer roller 109, and at the same time, a toner image is transferred to the sheet. The toner on the photoreceptor 106 that has been transferred is scraped off again by the cleaning blade 105. A toner density sensor 110 is provided in front of the cleaning blade 105, and the density of the toner image formed on the photoreceptor 104 can be measured by the toner density sensor 110. The sheet on which the toner image is placed is conveyed to the fixing unit 111 along the conveyance path, and the toner image is fixed on the sheet in the fixing unit 1. The printed sheets are finally discharged in the order of pages with the recording surface facing down through the discharge roller 113.
The toner and the developer supplied to the developing roller 108 are supplied after the toner and the developer are stirred by the stirring screw 117 before use. The toner is conveyed from the toner cartridge (not shown) to the stirring unit by an air pump (not shown). The conveyance is performed by replenishing the toner for a certain time after detecting that the toner has become thin. A series of these sequence controls is performed by the CPU of the control board 116.

In the present embodiment, exposure is performed at a resolution of 2400 dpi as the resolution of the exposure position by the optical scanning of the laser optical system unit 107 and the rotation of the photosensitive member 106 as the resolution of the exposure position. The laser optical system unit 107 is connected to an image processing control unit 113 and an LD driving circuit 114 which are image processing apparatuses of the present invention. In the image processing control unit 113, image commands from a personal computer or a workstation 100 are connected. Are processed and controlled, or an evaluation chart (test pattern) signal held inside is generated. The developing roller 108 is applied with a high voltage bias by a bias circuit 115. By controlling the bias in the bias circuit 115, the overall density of the image can be controlled. Yes.
The image command sent from the personal computer or the workstation 100 is a so-called printer description language, and actually the image signal information is compressed. This is once restored to the original 8-bit or 6-bit image signal.
An image is generally formed by driving the LD driving circuit 114 with a signal obtained by binarizing the returned image signal by the dither method or by a signal binarized by the density pattern method. Yes. When the LD drive circuit accepts only 1-bit input, such binarization is performed, but if 2-bit or 3-bit can be handled, a dither method with a small value is performed.

FIG. 13 is a diagram illustrating a specific hardware configuration of the image processing control unit 113.
The image processing control unit 113 includes an input interface (hereinafter referred to as input I / F) 120, a CPU 121, a ROM 122, a hard disk 123, a RAM 124, a memory slot 125, and an output interface (hereinafter referred to as output I / F) 126. They are connected to each other via a bus 127. The input I / F 120 serves as an interface between the host computer 100 and the image processing control unit 113. Image data from the host computer 100 transmitted by a predetermined transmission method is input to the input I / F 120 and temporarily stored in the RAM 124. The RAM 124 serves as a working memory for each process executed under the control of the CPU 121, and serves as a buffer memory for storing image data and flag data described later.
The CPU 121 is connected to each of the input I / F 120, the hard disk 123, the ROM 122, the RAM 124, and the output I / F 126 via the bus, reads the image processing program stored in the hard disk 123 or the ROM 122, and the CPU 121 reads the image processing program. By executing the above, the various processes shown in FIGS.

The image processing program may be stored in advance in the hard disk 123 or the ROM 122. For example, the image processing program may be supplied from the outside by a computer-readable recording medium such as a memory card 128 and output through the memory slot 125. It is good also as what is stored by memorize | storing in the hard disk 123 with which the apparatus was equipped. Of course, it may be stored by accessing a server or the like that supplies a program via a network means such as the Internet and downloading data.
Data processed by the image processing control unit 113 is output to the LD drive circuit 114 via the output I / F 126.

1 is a functional block diagram showing a main configuration of an image processing apparatus according to the present invention. The figure explaining the halftone dot pattern according to the gradation value stored in the pattern storage part. The figure explaining the halftone dot formed. The figure explaining the halftone dot pattern applied to an area with low density. The figure which shows the structure of an evaluation part. The figure which shows the relationship between the input density value after performing density conversion, and an output density value. The figure explaining the process in a continuity calculation part. The figure explaining the process in a continuity calculation part. The figure explaining the process in a continuity calculation part. The figure explaining evaluation when the area information which shows the gradation continuity which pinched | interposed the halftone dot is the same. The flowchart explaining the flow of the process in the 4th Embodiment of this invention. 1 is a diagram showing an outline of an image forming apparatus to which an image processing apparatus of the present invention is applied. FIG. 13 is a diagram illustrating a specific hardware configuration of the image processing control unit in FIG. 12.

Explanation of symbols

  11 area division unit, 12 halftone dot processing unit, 13 pattern storage unit, 14 evaluation unit, 26 determination unit, 21, 27 bitmap information generation unit, 23, 29 nonlinear operation unit, 24, 30 continuity calculation unit, 25 , 31 determination unit, 26 determination unit 100 host computer, 100 workstation, 101 main body tray, 102 tray, 103 paper feed roller, 104 photoconductor, 105 cleaning blade, 106 photoconductor, 106 charging roller, 107 laser optical system unit, 108 Developing roller, 109 Transfer roller, 110 Toner density sensor, 111 Fixing unit, 113 Image processing control unit, 113 Discharge roller, 114 LD drive circuit, 115 Bias circuit, 116 Control board, 117 Stir screw, 121 CPU, 122 ROM , 23 hard disk, 124 RAM, 125 memory slot, 127 a bus, 128 a memory card

Claims (6)

In an image processing apparatus that generates a binary image from a multi-valued input image,
Halftone dot generating means for temporarily generating halftone dot information for each area divided for each gradation value of the input image;
Evaluation means for evaluating whether or not to generate a halftone dot by the halftone dot generation means at the boundary portion of the area where the gradation value changes;
Correction means for performing correction so as to erase or leave halftone dot information at the boundary based on the evaluation result by the evaluation means;
An image processing apparatus comprising: The evaluation means includes
Filter processing means for performing frequency filter processing on image information in the boundary portion;
A non-linear operation means for performing density conversion by non-linear operation on the image information in the boundary portion subjected to frequency filter processing,
The image processing apparatus according to claim 1, wherein the evaluation is performed on the converted image information of the boundary portion. The evaluation means includes
The image processing apparatus according to claim 1, further comprising a continuity calculating unit that calculates the continuity of the image information at the boundary, and performing the evaluation based on the continuity. The evaluation means, for halftone dots related to surrounding gradation values in the boundary portion, continuity of image information when a halftone dot shape is applied regardless of the gradation value of the input image Is accumulated,
4. The image processing apparatus according to claim 1, wherein the evaluation is performed on halftone dots that are not related to surrounding gradation values according to the number of constituent pixels of the halftone dots. 5.   The image processing apparatus according to claim 1, wherein the halftone dots arranged by the halftone dot generating unit are arranged in a substantially lattice shape at the center.   An image forming apparatus comprising the image processing apparatus according to claim 1.

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