JPH10210295A - Image forming device - Google Patents

Abstract

(57) [Summary] [Problem] To prevent a density jump at the time of connecting dots from a state where dots are not connected to a state where dots are connected by delicately controlling the interval between dots. SOLUTION: After the area ratio of the first pixel 1201 reaches a first predetermined value, the area ratio of the first pixel 1201 is kept constant, and the growth direction of the first pixel 1201 is The area ratio of the second pixels 1202 adjacent in different directions is increased.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an image forming apparatus, and more particularly to a halftone processing technique for preventing a density jump.

[0002]

2. Description of the Related Art Conventionally, printers based on various principles have been used as output devices such as personal computers and workstations. A laser beam printer (LBP) using an electrophotographic process has high output speed and high print quality. Therefore it is used in many places. Currently, LBPs mainly distributed in the market are monochrome, and are processed by performing binarization processing on data such as characters and line drawings. Also, the binary dither method is often used for image data requiring gradation. However, in the binary dither method, a large dither matrix is required to obtain sufficient gradation, and the resolution, which is a characteristic of LBP, is impaired.

Recently, the demand for color conversion of LBP has been increasing, and an image forming apparatus having more excellent gradation has been demanded.

As an example of an image forming apparatus having better gradation, as described in JP-A-5-176163, a block composed of a plurality of pixels on image data is set and a space in the block is set. In a region where dots having a specific priority continue to grow continuously in a region where dots having a specific priority continue to grow, the dots are grown in order from the dot corresponding to the lowest priority recording pixel position according to a predetermined priority order. There is proposed an apparatus for starting the growth of a dot having a lower priority than a dot having a lower priority.

In the prior art, when a block composed of a plurality of pixels on image data is set as shown in FIG. 2, as shown in FIG. 3, as the input data gradually increases, the highest priority pixel, for example, The value of the pixel A increases first. Next, when the value of the pixel A approaches the upper limit, the value of the pixel having the next priority, for example, the value of the pixel B increases. Here, it is described that the gradation expression is improved by overlapping the growth processes of the pixel A and the pixel B.

[0006]

Here, each pixel is represented by a rectangle, but in the case of LBP, for example, the potential gradient actually formed on the photosensitive member by exposure is not necessarily rectangular. FIG. 4 shows the shape of the potential gradient on the photoconductor with respect to the area ratio of the pixel.

[0007] As shown in FIG. 5, the contour of the dot formed when the above-described area ratio growth is performed using such a potential gradient is such that the pixel A501 becomes larger to a certain extent when the pixel B502 becomes larger to some extent. Influences the potential gradient to be formed and leads to one. At this time, an extra area 503 is generated,
The concentration will rise sharply.

This phenomenon also occurs when the pixel having the second priority is the pixel D. When the value of the pixel D is increased to some extent, the outline of the dot is represented by the potential gradient formed by the pixel A 601 and the pixel D 60 as shown in FIG.
Since the potential gradients formed by the two influence each other, they are connected to one another. At this time, the area of the dot formed on the medium also includes the extra area 603. Therefore, comparing the total areas of the pixels A and D before and after connection as shown in FIG. The rate of increase of the area at the moment of connection when compared to the rate becomes abnormally large.

That is, in the improvement area 801 shown in FIG. 8, the gradation is improved as described in the previous section, but the density is such that the pixels A and D are influenced by each other and the dots are connected. The jump area 802 indicates that a density jump occurs.

This abnormal increase in dot area cannot be avoided in a system in which the area ratio of each pixel is increased or decreased by an electrophotographic method, and it is difficult to solve the problem. When an image image is expressed using dots whose area ratio changes greatly in this way, a density jump may occur, or in a severe case, a false contour may be caused and image quality may be impaired. Further, FIG. 9 shows a contour of a dot formed when there is main scanning jitter in the scanning direction of the exposure beam or sub-scanning jitter in the sub-scanning direction in such a situation where the potential gradients interfere with each other.

As can be seen from FIG. 9, there is a problem that the area of the formed dots becomes unstable, and the density of the image becomes uneven, which causes deterioration of the image quality.

Accordingly, an object of the present invention is to solve the above-mentioned problems, and the distance between the pixels is short, and the potential gradients on the respective photoconductors influence each other, so that the pixel area formed on the medium is contrary to expectation. An object of the present invention is to provide an image forming apparatus which is excellent in gradation and does not become large.

[0013]

A gradation processing means for determining an area ratio of each pixel based on image information and generating an image signal;
In an image forming apparatus having an image forming unit that forms an image based on an image signal from a gradation processing unit, the gradation processing unit may be configured to perform processing after the area ratio of a first pixel reaches a first predetermined value. The area ratio of the first pixel is kept constant, and the area ratio of a second pixel adjacent in a direction different from the growth direction of the first pixel is increased.

For this reason, even when pixels which are close to each other and the potential gradient formed on the photoreceptor mutually affect each other and pixels larger than expected are formed on the recording medium, the second order is obtained. And the degree of influence of the potential gradient can be delicately controlled, so that a density jump can be suppressed, and a high-quality recorded image excellent in gradation without pseudo contours or density unevenness can be obtained.

The first predetermined value is adjacent to a dot formed at a first pixel position by the image forming means in accordance with an area ratio of the first pixel in a growth direction of the first pixel. The dot formed at the third pixel position by the image forming means is set to a value that is close to or connected to the third pixel position according to the area ratio of the third pixel.

The image forming means has dot writing means for forming dots based on the area ratio of each pixel, wherein a is a ratio of the size of one pixel to the minimum dot diameter by the dot writing means. Assuming that the ratio of the size of the pixel formed by the pixel having the maximum area ratio to the pixel size is n, the first predetermined value is 1 when the maximum area ratio is 1.
The value is set to be equal to or more than -1 / 2na and less than 1.

The image forming means comprises an image exposing means capable of pulse width modulation, which is the dot writing means, and an image carrier on which an image corresponding to image exposure from the image exposing means is formed. The first predetermined value is set to a value of 0.3 or more and less than 1 in terms of pulse width, assuming that the pulse width necessary for the image exposure means to expose the entire area of the first pixel is 1. It is characterized by the following. The first predetermined value is set to 0.75 to 0.9 in terms of pulse width. The center of the second pixel is arranged obliquely with respect to the center of the first pixel.

After the area ratio of the second pixel reaches a second predetermined value, the gradation processing means reduces the area ratio of the second pixel to a second predetermined value or less; The area ratio of the first pixel is increased beyond the first predetermined value.

After the area ratio of the second pixel reaches a second predetermined value, the area ratio of the second pixel is set to 0.

The gradation processing means performs multi-value dither processing, and stores a plurality of dither matrices corresponding to respective area ratios of pixels, and stores the image density of each pixel based on the image information. The multi-value dither processing means for reading out the largest area rate among the area rates corresponding to the dither matrix having the input density equal to or higher than the threshold value of the input density stored in the matrix, and the multi-value dither processing means having the obtained area rate It is characterized by having a conversion means for converting into a value equal to or less than the number of gradations.

The number of dither matrices stored in the storage means is stored in excess of the number of tones that each pixel can take.

[0022]

FIG. 10 is a block diagram showing the configuration of an image forming apparatus according to the present invention. In FIG. 10, 1001
Reference numeral 1002 denotes an image forming apparatus which forms image data output from the image data output device 1002 on a recording medium such as paper and outputs the image data.

The image forming apparatus 1001 includes a buffer 1003 for storing the image data output from the image data output device 1002, a conversion processing unit 1004 for converting the image data extracted from the buffer 1003 into data suitable for image recording, It comprises a selector 1005 for selecting data to be recorded at each time, a gradation processing unit 1006 to which the present invention is applied, and an image forming unit 100 for recording on a recording medium.

Next, the operation of the entire image forming apparatus will be described in detail.

First, image data to be output is transferred from the image data output device 1002. Generally, the image data output device 1002 corresponds to a device such as a personal computer or a workstation, and outputs images, documents, and the like created by these devices. Therefore, data transferred from the image data output device 1002 is generally assumed to be displayed on a CRT in RGB.
Data.

Therefore, the image forming apparatus of the present invention is also effective when outputting a monochrome image. Here, an image forming apparatus which outputs a color image will be described as an example.

The image data output from the image data output device 1002 enters the image forming device 1001 and is first stored in the buffer 1003. Usually, there is a great difference between the speed of the image data output from the image data output device 1002 and the speed of the image data processed by the image forming means 100, and this difference is absorbed by the buffer 1003. Since the image data to be transferred is a color image, the data size may be extremely large, and a configuration in which the compressed image data is transferred to the buffer 1003 is also conceivable. In this case, the decompression process is performed in the buffer 1003 before the image data is sent to the subsequent process, and then the image data is sent to the next conversion processing unit 1004.

The image data sent to the conversion processing means 1004 is RGB data because it is image data on the premise of being displayed on the CRT of the image data output device 1002. In order to record RGB image data on an output medium such as paper, the three primary colors of printing from RGB, C (cyan)
It is necessary to convert the data into M (magenta) and Y (yellow) data. Generally, printing is performed not only with CMY of three primary colors but also with four colors of CMYK including K (black) in order to improve color reproducibility of an image. Therefore, the conversion processing unit 1004 performs this RGB → CMYK conversion.

Although the data is converted into four CMYK data by the conversion processing means 1004, when the image is actually formed by the image forming means 100 as described later in detail, 1
In order to form a single image by repeating the formation of a single-color image for a page for the number of colors used, CMYK4
Only one color must be selected from the colors. The selector 1005 is a unit for selecting valid single-color data from data for four colors.

As described above, CMYK monochromatic image data is generated from the RGB data. The obtained image data is obtained by converting the RGB data into CMYK data in accordance with the gradation characteristics of the image forming means 100. The number of tones has the same number of tones as the RGB data.

However, since the number of tones of the image forming unit 100 is generally very small compared to the number of tones of the CRT, the number of tones is obtained by expressing the number of tones by a plurality of pixels, and the input tones are rounded. Image forming unit 1 by approximation
It is necessary to convert to image data that can be represented by 00. This processing is performed by the gradation processing unit 1006.

Next, the operation of the image forming means 100 will be described in detail. FIG. 1 is a main sectional view of an image forming means 100 of the image forming apparatus of the present invention.

The charging roller 102 charges the photosensitive member 101 to a uniform potential (for example, -700 V). 600 dpi (dot) formed by the laser scanner 103
The laser beam having a resolution of (per inch) is guided onto the photoreceptor 101 by the return mirror 104 to form an electrostatic latent image. The laser beam of this apparatus has a beam diameter defined at a point where the intensity becomes 1 / e 2, which is 6 in the main scanning direction.
0 μm, and 70 μm in the sub-scanning direction. Here, the main scanning direction is a direction in which the laser beam is scanned by the laser scanner 103. The sub-scanning direction is a direction orthogonal to the main scanning direction, and is the rotation direction of the photoconductor 101 and the intermediate transfer body 106 and the traveling direction of the recording material 113.

One-component contact type developing device 1 as a developing means
05 can be moved in and out in the direction of the arrow in the figure. Among them,
First, the yellow (Y) developing unit 105Y is brought into contact with the other developing units, and the other developing units are separated from each other, and the negatively chargeable Y toner is reversely developed by the action of an electric field of a power supply (not shown) to be visualized on the photoreceptor 101. The visualized Y toner is ETF
E (ethylene-tetrafluoroethylene copolymer) dispersed carbon in the intermediate transfer member 106 adjusted to an appropriate resistance
A bias having a polarity opposite to that of the toner is applied to the primary transfer roller 107 by the primary transfer power source 108, and the primary transfer roller 107 is transferred by the action of the electric field. Further, the intermediate transfer member 106 is adjusted so that an appropriate tension is applied by the tension roller 118. Then, the transfer residual toner on the photoconductor 101 is
Photoreceptor cleaner 109, which cleans by contacting the blade, collects the photoreceptor potential.
Reset by 10.

The same operation is performed by synchronizing the position of the intermediate transfer member 106 and the light emission timing of the exposure means 103 with a magenta (M) developing device 105M, a cyan (C) developing device 105C,
By repeating the process for the black (K) developing device 105K, the toner of each color is superimposed on the intermediate transfer member 106 to form a full-color image. During this time, the secondary transfer roller 116 and the intermediate transfer member cleaner 119 are kept separated.

On the other hand, a recording material 113 such as paper or OHP is conveyed from a paper supply cassette 112 to a pair of registration rollers 114 by paper supply means 111, and then the intermediate transfer member 106.
The sheet is conveyed to a secondary transfer section formed by a secondary transfer roller 116 which can be brought into contact with and separated from the driving roller 115 in the direction of the arrow in FIG. In the secondary transfer section, a secondary transfer roller 116 contacts the intermediate transfer body 106 to form and press a nip in synchronization with the recording material 113, and also calculates a voltage obtained from a primary transfer power supply 108. The voltage determined in 121 is controlled by the secondary transfer power supply 117 at a constant voltage, and a full-color toner image is formed on the recording material 113 by the action of the electric field. Then, the recording material 1
13 is fixed by the fixing means 120 and discharged out of the apparatus.

In the electrophotographic process section, a patch sensor 122 as a density measuring means is provided at a position downstream of the primary transfer roller and opposed to the surface of the intermediate transfer member 106. The patch sensor 122 is a sensor combining an LED and a photo sensor, and the read value is A / D converted and sent to the control system. The patch formed on the surface of the intermediate transfer member 106 is read by the patch sensor 122 when the intermediate transfer member 106 is sent.

Next, a first embodiment of the present invention will be described. The first mode is a configuration in which the area ratio is determined for each pixel of image data, and a pixel corresponding to the second pixel is arranged.

Normally, image data often has 256 gradations for each color of RGB. Therefore, data obtained by RGB → CMYK conversion is usually converted as 256 gradations for each color. However, the number of tones per pixel at the resolution of the image forming means is 2
In many cases, about 56 gradations cannot be obtained.

Therefore, as shown in FIG. 11, in a system in which the area within a pixel is increased or decreased according to the value of each pixel, an image forming apparatus capable of expressing the gradation of 0 + 1 to m + 1 steps from the left end. An example is described.

FIG. 12 shows how the pixels grow with respect to the input image data. In the growth process of FIG. 12, the first pixel 1
When the area ratio of the first pixel 1201 reaches the first predetermined value of 1-1 / 2na, the size of the first pixel 1201 becomes constant,
Second pixel 120 located immediately below first pixel 1201
2 shows how it begins to grow instead.

The first predetermined value 1-1 / 2na is a ratio when the maximum value of the area ratio is set to 1. A represents the ratio of the size of one pixel to the diameter of the dot formed by the dot writing means of the image forming apparatus, and n represents the ratio of the size of the pixel having the maximum area ratio to the size of one pixel. Represents.

For example, when the maximum value of the area ratio of one pixel is 5 m, n = 5, and each pixel can have a maximum size of 5 pixels. FIG. 13 shows the relationship between the sizes of a and n.

In general, the dots formed by the dot writing means have a distribution in which the density is higher at the center of the dot than at the periphery, as shown in FIG. That is, the outline of the dot is not clear at the peripheral portion of the dot. Therefore, when two dots are adjacent to each other, the density distribution between the dots changes depending on the interval between the dots.

FIG. 15 shows the density distribution when the first pixel and the adjacent third pixel adjacent in the growth direction of the first pixel both have the first predetermined value of 1-1 / 2na. . Since the first predetermined value is a ratio with respect to the maximum area ratio, the size of the region 1501 where no dot is written is 1 / 2na. Then, 1 / 2na corresponds to a ratio of 1/2 of the diameter of the dot to the maximum area ratio.
03 overlaps by half the dot diameter.

Therefore, when the overlapping portion becomes larger, the density is sufficiently increased even in the area 1501 in which writing is not performed, that is, the dot formed by the first pixel and the dot formed by the third pixel are connected to each other.
One dot.

Thus, the growth of the first pixel is stopped at the first predetermined value 1-1 / 2na, and the gradation is secured by growing the second pixel.

Since the first predetermined value is a ratio to the maximum area ratio, the maximum value is 1;
Since 2 and the dot 1503 completely overlap each other, it is not appropriate as the first predetermined value, and the first predetermined value must be set to less than 1.

Further, with respect to the first predetermined value, in the case of an image forming means constituted by using an image exposing means capable of pulse width modulation as the dot writing means, it is necessary to expose the entire area of the first pixel. Assuming that the required pulse width is 1, it is desirable to set the pulse width in a range of 0.3 or more and less than 1.

For example, the configuration of the image forming apparatus described with reference to FIG. 1 has a laser scanner 103 using a laser beam as image exposure means capable of pulse width modulation. When writing dots with a laser beam, the dot diameter is equal to the spot diameter of the laser beam, and the shape is a circle or an ellipse.

On the other hand, the shape of the pixel is a rectangular shape whose one side is a unit length obtained from a value such as "dpi" which is a measure of the resolution of the image forming apparatus. Therefore, for example, as in the image forming means having a resolution of 600 dpi described with reference to FIG. 1, the relationship as shown in FIG. 16 should be established at least so that the beam diameter becomes larger than the size of the pixel in order to write the whole pixel. Is common.

Therefore, in this case, since the ratio of the size of one pixel to the spot diameter represented by a is 0.7 from a = 1 / √2, the area ratio of one pixel unit where n = 1 is obtained. When expressing an image, the pulse width required to expose the entire area of the first pixel is 1, that is, the pulse width necessary to expose one pixel is 1, and the first predetermined pulse width is converted to a first predetermined value. The value is set to 1-1 / 2na = 0.3.

Further, as shown in FIG. 17, an image forming apparatus designed so that the spot diameter 1701 in the pixel growth direction is smaller than the pixel size 1702, that is, a is larger, as shown in FIG. Is preferably set to a range of 0.3 or more and less than 1 in terms of pulse width.

Next, a second embodiment will be described.
This embodiment is an example in which the image forming unit having a resolution of 600 dpi shown in FIG. 1 is used, and when each pixel has a maximum area ratio, the size becomes two or more pixels in order to secure more gradation.

In the above embodiment, the value of 0 to m is taken per pixel, and the density of m + 1 gradation is expressed. But m
In general, since about 255 cannot be obtained, by expressing the gradation by increasing the size of the pixel, it is possible to express more gradations and improve the image quality.

In other words, as shown in FIG. 18, if the pixel is configured to grow beyond one pixel to the size of n pixels, one pixel can express m + 1 gradations of 0 to m. Therefore, each pixel takes a value of 0 to nm, and nm +
One-step gradation can be expressed.

However, increasing the value of n in order to increase the number of gradations increases the size of each pixel, and at the same time lowers the resolution. Therefore, there is a good range of n that balances the number of gradations and the resolution.

FIG. 19 shows the resolution, the number of gradations, and the first predetermined value when n is 1 to 10. For the first predetermined value, as in the previous embodiment, when the pulse width required to expose the entire area of the first pixel, that is, the pulse width required to expose the entire n pixels is 1, This is a value represented by 1-1 / 2na in pulse width conversion. Where a is 42
μm / 60 μm = 0.7, and although not illustrated in FIG. 1, the number of gradations m + 1 that can be expressed in one pixel is 33, that is, m = 32.

As shown in FIG. 19, if n is set to 8 or more, 257 or more gradations can be expressed, so that the input image data can be sufficiently reproduced even with 256 gradations. However, in that case, the resolution becomes 75 dpi or less, which is not preferable. Usually, when expressing an image having a gradation such as a photograph, it is desired to secure at least 100 dpi or more.

Therefore, in order to maintain good resolution and gradation, the first predetermined value is 0.75 to 0.75 in terms of pulse width.
Preferably, it is set to a value of 0.9.

Next, a third embodiment for realizing the present invention will be described in detail. The third mode is a mode in which a plurality of pixels are treated as one block and a density pattern method, which is general when gradation is expressed by the area ratio of the block, is used.

In this embodiment, as shown in FIG. 20, in a system in which the area within a pixel increases or decreases according to the value of each pixel, the pixel growth starts at the center and each pixel has 97 levels of 0 to 96 gradations. An example of the image forming apparatus will be described in which the area of one pixel is obtained by 32 and the area of three pixels is obtained by the maximum 96.

In this image forming apparatus, in order to ensure sufficient gradation, 9 × 3 pixels are represented as one block, and gradation is expressed by the area ratio in the block. Since each pixel can have an area ratio of 33 steps (0 to 32), the entire block has 289 steps (32 × 9).
+1).

Therefore, as shown in FIG. 21, processing is performed using a density pattern method in which each of the input 256 levels of values corresponds to one of the 289 levels of block patterns. Therefore, the relationship between the input density and the output density can obtain an arbitrary gradation curve by changing the correspondence shown in FIG.

FIG. 22 shows how the area ratio of the blocks is increased.

First, a block 2201 indicating a density 0 starts from a state where all the pixels in the block are 0. As a step 1, a pixel 2202 located at the center (X position 2, Y position 2) of the block 2201 grows. Maximum value of 96
Grows into dots 2205 for three horizontal pixels.

Next, as step 2, immediately above the pixel 2202 that has already grown to the maximum value (X position 2, Y position 1)
Are grown to form dots 2206.

When the dot 2206 has grown to the size of three horizontal pixels, finally, immediately below the dot 2202 (X position 2,
Pixel 2204 located at Y position 3) grows to form dot 2
207. When the dot 2207 grows to the size of three horizontal pixels, all the pixels in the block 2201 are buried and have the maximum density (step 3). The input value at this time is 255.

When the above-described pixel growth is observed macroscopically, when the density is low as shown in FIG.
Exist at positions separated from each other, and each grows laterally as the concentration increases. When each point grows to some extent in the horizontal direction, adjacent points are connected at a certain place to form one horizontal line 2302. This is step 1
Is equivalent to

When the density further increases, bump-like points 2303 are generated adjacent to the upper part of the horizontal line 2302 and apart from each other, and each of them grows in the horizontal direction. When the point 2303 grows to some extent in the horizontal direction, the adjacent points 2303 are connected to each other at a certain point as in Step 1 to form a horizontal line. At this time, two continuous horizontal lines are formed in the Y direction.
Is the same situation that exists. This is step 2
Is equivalent to

In the last step 3, points 2305 are generated so as to fill the white lines 2304 left in step 2, and each of them grows in the horizontal direction. Finally, all the white lines are buried to form a solid image 2306.

FIG. 24 shows a gradation curve 2401 representing the output density with respect to the input value. This gradation curve 2401 is a result of associating the above-described block pattern with each input value.
The block pattern is made to correspond to the input value so that the density is proportional to the luminance.

When actually processing an image, the gradation of the highlight portion (low-density portion) greatly affects the image quality, and it is necessary to sufficiently secure the gradation of the low-density area 2402 in FIG. is there. In addition, human flesh color, which is particularly noticeable to human eyes, cannot be expressed satisfactorily unless the gradation of the magenta component is sufficiently ensured. Since the input value of the magenta component of the flesh color substantially indicates an intermediate portion, it is necessary to ensure sufficient gradation even in the intermediate density region 2403 in FIG.

When the above-described gradation expression method is used, if the steps described in FIG. 22 are made to correspond to the gradation curve 2401 in FIG. 24, the switching from step 1 to step 2 is included in the intermediate density area 2403. .

Switching from step 1 to step 2 means a state at the moment when adjacent points are connected to form a horizontal line. As can be seen from the description of the previous embodiment, the phenomenon that the points are connected to each other occurs during step 1 before switching to step 2 rather than at the moment when switching from step 1 to step 2. There are many things.

FIG. 25 shows an enlarged view of a contact point at two points and a block pattern of both states in the state A immediately before the adjacent points are connected to each other in step 1 and the state B at the moment of connection. From FIG. 25, it can be seen that when the value of the pixel at the center of the block becomes 81, a horizontal line is formed by connecting to the adjacent point.

In terms of data, it is ideal that a horizontal line is formed when the value of the pixel at the center of the block becomes 96, which means that the growth of points has ended earlier than expected. This means that the density of the block pattern in which the value of the pixel in the center of the block is 80 and the value in the center of the block are
This indicates that a density jump exists between the densities indicated by one block pattern.

This phenomenon occurs because the potential gradients for forming the respective points on the photoreceptor are too close to each other and are connected to each other. FIG.
FIG. 26 shows a potential gradient corresponding to each of A and B in FIG.

Therefore, as shown in FIG. 27, between the block pattern in which the value of the pixel in the center of the block is 80 and the block pattern in which the value of the pixel in the center of the block is 81, A ′,
Block patterns such as A '', A ''', and A''''are inserted.

The inserted block pattern corresponds to the upper right (X) of the block pattern in which the value of the pixel at the center of the block corresponding to the first pixel whose points are not connected is 80.
This is a pattern in which a very small value corresponding to the second pixel is arranged at position 3, Y position 1) and is increased.

A ′, A ″, A ′ ″,
A method of expressing density gradation by associating a block pattern of 293 steps (289 + 4) including four block patterns of A ″ ″ with 256 input values,
After the first pixel reaches 80 corresponding to the first predetermined value, it takes a constant value and the second pixel increases in value.

Here, the first predetermined value of 80 is equivalent to 0.83 when the pulse width at 96 is 1 and 0.75 to 0.75 since the maximum value that the pixel can take is 96.
It is set in the range of 0.9. That is, the value is set to a value at which the dot formed by the first pixel and the dot formed by the third pixel adjacent in the growth direction of the first pixel are close to or connected to each other.

For this reason, as shown in FIG. 28, the potential gradient formed by the first pixel 2801 and the potential gradient formed by the second pixel 2802 affect each other, and
By slightly changing the interval between dots by the 01 and third pixels 2803, it is possible to express an intermediate density of the densities of the patterns represented by A and B in FIG. 25, and it is possible to prevent a density jump. .

If the position of the second pixel is different from the growth direction of the first pixel, the potential gradient of the first pixel is controlled even if it is directly below or directly above the first pixel. However, dots are likely to be connected in the vertical direction, and texture is likely to occur. Therefore, in order to prevent unnecessary texture and obtain more control effects, as in the present embodiment, the first pixel and the second pixel are arranged so that the respective pixel centers have an oblique positional relationship. desirable. In the present embodiment, the position of the second pixel is at the upper right, but may be at the lower right, upper left, or lower left.

Then, after the second pixel has reached the second predetermined value and the dots formed by the first and second pixels have been smoothly connected, the first pixel becomes the first predetermined value. By increasing again beyond the range, a higher density gradation can be expressed. At this time, it is desirable that the value of the second pixel that has completed the purpose of controlling the potential gradient formed by the adjacent first and third pixels is reduced to a value equal to or less than the second predetermined value.

Further, in order to prevent the unnecessary second pixel from adversely affecting the image, the second pixel is reduced to 0 after reaching 6 in the present embodiment. It is desirable that the area ratio of the second pixel is set to 0 after reaching the second predetermined value.

Although the above description has been made in a 3 × 3 block, the present invention is not limited to this size.
The same applies to the case where the block size is other than 3 × 3.

Next, a fourth embodiment of the present invention will be described. The fourth mode is a mode using a multi-value dither method.

FIG. 29 shows a processing flow of the fourth embodiment.

In FIG. 29, the number of gradations of the input value 2901 is 256 which is the number of gradations of general image data, and is converted by the multi-value dither processing means 2902 into a temporary conversion value 2903 which takes multi-values. Similarly, the number of gradations of the temporary conversion value 2903 is larger than the number of gradations of the output value 2905 which takes multi-values.
Is converted to

Here, the number of gradations of the output value 2905 is 97, and the number of gradations of the temporary conversion value 2903 is 227. Then, the conversion processing means 2904 outputs a temporary conversion value 2 exceeding 96.
In order to convert the value of 903 into a value within 96, the remainder is obtained by dividing the temporary conversion value 2903 by 96 to obtain an output value 2905.

FIGS. 30 to 38 show dither matrices used by the multi-value dither processing means 2902 when the third embodiment is performed in this embodiment.

The multi-value dither processing means 2902 has 226 dither matrices for converting a 256-value input into 227 values, and each matrix has a size of 3 × 3 pixels. Each of the 226 dither matrices corresponds to a value after multi-value dither processing,
The numbers in each matrix correspond to the input values.

That is, when the input value is equal to or larger than the numerical value in the matrix, the pixel at that position outputs the value of the matrix as the output value.

When the input values of a plurality of matrices are equal to or larger than the numerical values in the matrices at the same time, the value of the larger matrix becomes effective.

That is, focusing on the lower right pixel in the matrices of FIGS. 30 to 38, first, when the input is 139,
Since the value is equal to or larger than the lower right value of the matrix of 1, the output of this pixel is 1. Next, when the input is 140, the output becomes 2 because the value is equal to or more than the value of the matrix of 2.

When the input becomes 150, the value of the matrix of 97 becomes 141 or more, so that 97 is output.

Then, when the input becomes 254, the value 253 of the matrix of 129 and the value 25 of the matrix of 226 are obtained.
In this case, the result of the matrix corresponding to the larger value is valid, and the output is 226.

Since the values of all other matrices are 255, they are valid only when the input value is 255. However, when the input is 255, the value of the matrix of 226 is also valid, so that the output is the largest 22
It becomes 6.

Therefore, it can be seen that the lower right pixel in the matrix takes one of four outputs of 1, 2, 97 and 226 for input values 0 to 255.

In FIG. 30, numerical values with [] are arranged beside the values exceeding 96 in the temporary conversion value of each dither matrix. This indicates an output value finally output through the conversion processing means 2904, that is, a value of 0 to 96.

Therefore, the value converted by the multi-value dither processing means 2902 is reset to 0 three times while increasing from 0 to 226. Therefore, the temporary conversion value is 97, 194
In the case of, the final output is 0. Then, it can be seen that the final output value when converted to the maximum value 226 is 32.

Next, it is confirmed that the processing using the dither matrix shown in FIGS. 30 to 38 is the same as the contents of the third embodiment.

The pixel value at the center of the block corresponding to the first pixel described in the third embodiment is changed from 80 gradations to 81
The process of obtaining the gray scale is seen on the dither matrix in FIGS.

First, the gradation at which the value of the first pixel is 80 is obtained when the input value is 91 or more from FIG. 32, and the gradation at which the value of the first pixel is 81 is obtained when the input value is 100 or more. It is.

Further, from FIG. 30, when the input value is 94, the upper right pixel value corresponding to the second pixel is 1, and thereafter, 2, 95 at 95, 3, 97 at 4, 97 at 4, 98 at 5, 99 at 6, and so on. Become.

Then, the value 97 of the second pixel when it is 100 or more passes through the conversion processing means 2904, and becomes 0 as the final output value. At this time, the first pixel is 81, and the same processing as in the previous embodiment can be realized.

As described above, the multi-value dither processing means and the conversion processing means for converting the output of the multi-value dither means to a value smaller than the value are provided, and the number of dither matrices used in the multi-value dither processing is determined by the output value. By setting the value to exceed the number of gradations, the same processing as the gradation processing realized by the density pattern method can be realized, and processing with higher resolution can be realized.

Next, a fifth embodiment of the present invention will be described. The fifth mode is a mode that can be applied even when it is difficult to express a minute area ratio due to the characteristics of the engine, or when the formation of a potential gradient is unstable due to environmental fluctuations.

First, the characteristics of the image forming means used in the following description will be described.

Here, the image forming means has seven values (0, 1, 2, 3, 4,...) Per pixel in order to stably represent dots.
5 and 6), and the formed dots are shown in FIG.
As shown in FIG. 9, as the value of each pixel increases, the area of the dot formed by that pixel increases.
(Maximum 1 pixel) However, the growth process of the rectangular dots shown in FIG. 39 is logical, and the latent image actually formed on the photoconductor and the dot shape formed by the latent image are Due to the influence of the spot diameter of the beam scanned by the exposure device and the sensitivity of the photoreceptor, the dot shape becomes round as shown in FIG.

FIG. 41 shows how dots are formed when pixel data exists at positions close to each other. FIG.
I is the case where pixel data is arranged side by side, II is the case where there is pixel data at an oblique position, and III is the case where there is pixel data arranged vertically.

In the state of I, when pixels having pixel data of 6 and 3 are arranged side by side, dots formed by each are connected to form one dot as shown in a. However, when 6 pixels and 2 pixels are arranged, each dot forms an independent dot as in b.

In the state II, when the pixel data of the pixel 4 and the pixel 3 are arranged side by side, the dots formed by each are connected to form one dot as shown in a, and the three pixels and the three pixels are formed. When dots are arranged, independent dots are formed.

In the state III, the pixel data is 6 pixels and 2 pixels.
When one pixel is arranged, one dot is formed as shown in a, but when six pixels and one pixel are arranged, independent dots are formed.

As described above, dots are connected or not connected depending on the value of the pixel data, so that a density jump may occur.

Therefore, an image is formed by applying the dither matrix according to the present invention shown in FIG.

The dither matrix shown in FIG. 42 is composed of six matrices each having a size of 8 × 8 pixels, and corresponds to pixel data 1, 2, 3, 4, 5, and 6, respectively.

That is, looking at the pixel 4201 in FIG. 42, when the input value to the pixel 4201 is 5, the output of the pixel 4201 becomes 1 because it is larger than 4 in the matrix of 1; Since the value is larger than 9 in the matrix, the output value is 2.

Further, when the input value is 15, the output value exceeds 3 in the matrix of 3 and the output value is 3, and when the input value is 40, it exceeds 31 in the matrix of 4 and the output value is 4 and the input value is 70. In this case, the output value exceeds 5 in the matrix of 5, so the output value is 5, and when the input value is 255, it exceeds 72 in the matrix of 6, so that the output value is 6, that is, a solid pixel.

Looking at the structure of each matrix in FIG. 42, the matrix 1 is a block group formed by repeating the dither matrix composed of blocks of 2.times.2 pixels shown in FIG. , 3 and 4 are shown in FIGS. 44 and 4 respectively.
5, a block group composed of nine types of 2 × 2 pixel blocks shown in FIG.

The matrix 5 and the matrix 6 are a block group formed by repeating one type of block shown in FIGS. 47 and 48, respectively.

As shown in FIG. 49, these matrices are configured such that each pixel in a 2 × 2 pixel block has a priority and grows in accordance with the priority. Also, as shown in FIG. 50, in a block group of 4 × 4 blocks, each block is given a priority, and is configured to grow according to the priority.

The following will describe in detail the dot growth process when the matrix shown in FIG. 42 is applied. Here, a case where processing is performed on image data composed of pixels having uniform values is considered. Hereinafter, a block of 2 × 2 pixels is referred to as a “block”, and a block group of 4 × 4 blocks is referred to as a “block group”.

First, when the pixel value is 0 to less than 4, the output values of all the pixels are 0. When the pixel value is 4 or more, the pixels at the X position 1 and the Y position 1 having the highest priority in the block. Becomes 1, that is, (X,
Y) = (1, 1), (3, 1), (5, 1), (7,
1), (1, 3), (3, 3), (5, 3), (7,
3), (1, 5), (3, 5), (5, 5), (7,
5), (1, 7), (3, 7), (5, 7), (7,
The output value of the pixel of 7) becomes 1.

Next, when the input value becomes 9 or more, the output value of the above-mentioned pixel becomes 2.

When the input value becomes 13 or more, the output value becomes 3 and the output of the pixel at the X position 2 and the Y position 2 having the second priority in the block becomes 1.

Thereafter, the pixel of priority 1 remains at output value 3 and the pixel of priority 2 becomes 2 when the input value is 18 or more, and the input value 2
It becomes 3 with 2 or more.

Up to this point, each pixel forms a single dot.

Next, when the input value becomes 27 or more, the output value of the pixel of priority 1 becomes 4, but as described above, the pixel of priority 1 and the pixel of priority 2 become 4, 3 In the combination of the dots, the dots formed by the respective pixels are connected.

Therefore, when the dither matrix shown in FIG. 42 is used, only the block at the X position 1 and the Y position 1 of the priority 1 according to the priority of the blocks in the block group has the output of the pixel of the priority 1 at 4, and other than that. Block remains in its previous state, ie, both the priority 1 pixel and the priority 2 pixel.

Thereafter, the pixels of priority 1 of each block grow to 4 in order according to the priority within the block group.

FIG. 51 shows a table summarizing the dot growth when the input value is similarly viewed up to 255.

In the table, the column of gradation shows the order of each gradation which can be expressed by the dither matrix of FIG. The input value indicates an input value for outputting each gradation, and when the input value is equal to or more than this value, the corresponding gradation is output. The arrangement column shows how the blocks constituted by the pixel values shown in the following columns are arranged in the block group, and "A" indicates that all the blocks in the block group are in the following columns. Indicates that the block is filled with the pixel value shown, and in the case of a numerical value, the corresponding priority block in the block group is filled with the pixel value shown in the following column, and the other blocks are the previous blocks. This shows that there is no change in the pixel value filled with the gradation.

It can be seen from FIG. 51 that the dither matrix shown in FIG. 42 can represent 57 tones from 0 to 56.

In FIG. 51, the gray scales surrounded by the thick frames indicate the combinations of the pixel values that connect the dots formed by the respective pixels described above. It can be seen that the pixel values are filled in partly by blocks.

That is, even if a density jump occurs when dots are connected to each other, it is an extreme phenomenon, and when viewed macroscopically, it does not become a density jump, so that an image with excellent gradation can be formed.

In the above description, the image forming apparatus shown in FIG. 10 has been described. However, the configuration of the apparatus varies depending on the purpose of the apparatus, and is not limited to the configuration shown in FIG.

For example, in FIG. 52, the RGB data is converted into CMYK data by the conversion processing means 5203 inside the image data output device 5201, and the gradation processing means 520 is output.
4, the image data is output to the image forming apparatus 5202 after being compressed after being subjected to the gradation processing. In the image forming apparatus 5202, the image is expanded by the buffer 5205, and the data is sent to the image forming means 5206 and printed. is there.

In FIG. 53, the image data output device 53
01, only the RGB → CMYK conversion is performed by the conversion processing unit 5303 in the image forming apparatus 5302,
4 is subjected to gradation processing by the gradation processing unit 5305 and output by the image forming unit 5306.

The present invention can be realized in these different configurations. That is, in FIG. 52, it can be realized as a conversion process by software in the image data output device 5201, and in FIG. 53, it can be realized as hardware similarly to the configuration of FIG.

In this embodiment, the printer using the electrophotographic method has been described as an example. However, the image formation which is different from the area ratio of the pixel to be output in the shape of the formed dot, especially affected by the adjacent pixel. It is needless to say that the present invention can be applied to any device, for example, an ink jet printer.

[0143]

As described above, according to the image forming apparatus of the present invention, after the area ratio of the first pixel reaches the first predetermined value, the area ratio of the first pixel is kept constant. In addition, since the area ratio of the second pixel adjacent in the direction different from the growth direction of the first pixel is increased, a density jump caused by the continuous growth of the first pixel is suppressed, and the image quality is improved. There is.

Further, the first predetermined value is set to a value at which the dot formed at the first pixel position and the dot formed at the third pixel position adjacent in the growth direction of the first pixel are close to or connected to each other. By setting, the interval between dots can be delicately controlled using the second pixel from a state where the dots formed by the first pixel and the third pixel are not connected to a state where the dots are connected, so that when the dots are connected, Has the effect of preventing concentration jumps.

In particular, when the image forming means has dot writing means for forming dots based on the area ratio of each pixel, the ratio of the size of one pixel to the minimum dot diameter by the dot writing means is defined as a When the ratio of the size of the pixel formed by the pixel having the maximum area ratio to the size of one pixel is n, and the maximum area ratio is 1, the first predetermined value is 1-1 / 2na. By setting the value to less than 1, the second pixel can be grown so that the dots formed by the first pixel and the third pixel are not connected to each other, and the interval between the dots can be controlled. This has the effect of preventing concentration jumps.

Further, even when the image forming apparatus is composed of image exposure means capable of pulse width modulation and an image carrier on which an image corresponding to the image exposure from the image exposure means is formed, the first predetermined value is maintained. By setting the pulse width required for the image exposure means to expose the entire area of the first pixel to 1 and setting it to a value of 0.3 or more and less than 1 in terms of pulse width, the third pixel adjacent in the growth direction Since there is no connection with a dot formed by a pixel, a density jump does not occur.

By setting the first predetermined value to 0.75 to 0.9 in terms of pulse width, it is possible to secure the resolution and gradation without causing a density jump.

By arranging the center of the second pixel obliquely with respect to the center of the first pixel, the control efficiency of the first pixel by the second pixel is increased, the density jump is easily suppressed, and unnecessary texture is reduced. Has the effect of making it less likely to occur.

After the area ratio of the second pixel has reached the second predetermined value, the area ratio of the second pixel is reduced to the second predetermined value or less, and the area ratio of the first pixel is reduced. Since the rate is increased beyond the first predetermined value, a higher density can be expressed while preventing a density jump.

Further, after the area ratio of the second pixel reaches the second predetermined value, the area ratio of the second pixel is set to 0.
Image degradation due to unnecessary dots can be eliminated.

The gradation processing means performs multi-value dither processing, and stores a plurality of dither matrices corresponding to the respective area ratios of the pixels, and stores the image density of each pixel based on the image information. The multi-value dither processing means for reading out the largest area rate among the area rates corresponding to the dither matrix having the input density equal to or higher than the threshold value of the input density stored in the matrix, and the multi-value dither processing means having the obtained area rate Since there is a conversion means for converting the value to a value equal to or less than the number of gradations, it is possible to perform processing with higher resolution while preventing a density jump.

Further, since the number of dither matrices stored in the storage means exceeds the number of gradations that can be taken by each pixel, the dots once generated in the low-density part are eliminated in the high-density part. Thus, the deterioration of the image due to unnecessary dots can be eliminated.

As described above, since the distance between the pixels is short and the potential gradients on the photoconductors affect each other, the dot area formed on the medium does not increase unexpectedly. This has the effect that an image forming apparatus having excellent tonality can be configured.

[Brief description of the drawings]

FIG. 1 is a sectional view of an image forming unit of an image forming apparatus according to the present invention.

FIG. 2 is a diagram showing a priority order of pixels growing in a block of a plurality of pixels.

FIG. 3 is a diagram showing a pixel growth process.

FIG. 4 is an explanatory diagram of a shape of a potential gradient.

FIG. 5 is a diagram showing a connection of dots.

FIG. 6 is a diagram showing the connection of dots.

FIG. 7 is a diagram showing an abnormal increase in dot area.

FIG. 8 is a diagram showing an area where a density jump occurs.

FIG. 9 is a view showing density unevenness due to jitter.

FIG. 10 is a block diagram illustrating a configuration of an image forming apparatus.

FIG. 11 is a diagram showing a growth process of one pixel.

FIG. 12 is a diagram illustrating pixel growth according to the first embodiment.

FIG. 13 is a diagram illustrating a relationship between a pixel size, a dot size, and a dot size at the maximum area ratio.

FIG. 14 is a diagram showing a density distribution of dots.

FIG. 15 is a diagram showing a density distribution at a first predetermined value.

FIG. 16 is a diagram showing a relationship between a spot diameter and a pixel size.

FIG. 17 is a diagram showing a prefectural police of a spot diameter and a pixel size.

FIG. 18 is an explanatory diagram of pixel growth.

FIG. 19 is a relationship diagram between a resolution and a first predetermined value.

FIG. 20 is an explanatory diagram of pixel growth.

FIG. 21 is a diagram showing the relationship between input values and block patterns.

FIG. 22 is a diagram showing an increase in a block area ratio.

FIG. 23 is a diagram showing a dot shape to be formed.

FIG. 24 is a diagram showing a gradation curve with respect to an input value.

FIG. 25 is a diagram showing before and after connecting adjacent points.

FIG. 26 is a diagram showing potential gradients before and after adjacent points are connected.

FIG. 27 is a diagram showing a block pattern in which second pixels are arranged.

FIG. 28 is a diagram showing that a dot interval changes according to a second pixel.

FIG. 29 is a diagram showing a flow of processing according to the fourth embodiment.

FIG. 30 is a diagram showing a dither matrix used in multi-value dither processing means.

FIG. 31 is a diagram showing a dither matrix used in the multi-value dither processing means.

FIG. 32 is a diagram showing a dither matrix used in the multi-value dither processing means.

FIG. 33 is a diagram showing a dither matrix used in the multi-value dither processing means.

FIG. 34 is a diagram showing a dither matrix used in multi-value dither processing means.

FIG. 35 is a diagram showing a dither matrix used in multi-value dither processing means.

FIG. 36 is a diagram showing a dither matrix used in the multi-value dither processing means.

FIG. 37 is a diagram showing a dither matrix used in multi-value dither processing means.

FIG. 38 is a diagram showing a dither matrix used in the multi-value dither processing means.

FIG. 39 is an explanatory diagram of pixel growth.

FIG. 40 is an explanatory diagram of dot growth.

FIG. 41 is a view for explaining the connection of dots;

FIG. 42 is a diagram showing a dither matrix used in the fifth embodiment.

FIG. 43 is a diagram showing one block constituting a dither matrix.

FIG. 44 is a diagram showing nine blocks forming a dither matrix.

FIG. 45 is a diagram showing nine blocks forming a dither matrix.

FIG. 46 is a diagram showing nine blocks forming a dither matrix.

FIG. 47 is a diagram showing one block constituting a dither matrix.

FIG. 48 is a diagram showing one block constituting a dither matrix.

FIG. 49 is a diagram showing the priority of growth in a block.

FIG. 50 is a diagram showing growth priorities in block counties.

FIG. 51 is a view showing all gradations that can be expressed in a fifth mode.

FIG. 52 is a diagram illustrating an image forming apparatus according to another embodiment.

FIG. 53 is a diagram illustrating an image forming apparatus according to another embodiment.

[Explanation of symbols]

 REFERENCE SIGNS LIST 100 image forming means 101 photosensitive member 102 charging roller 103 laser scanner 104 folding mirror 105Y yellow developing device 105M magenta developing device 105C cyan developing device 105K black developing device 106 intermediate transfer member 107 transfer roller 108: primary transfer power supply 109: photoconductor cleaner 110: static elimination lamp 111: paper supply means 112: paper supply cassette 113: recording material 114: registration roller pair 115: drive roller 116: secondary transfer roller 117: secondary Transfer power supply 118 ... Tension roller 119 ... Intermediate transfer member cleaner 120 ... Fixing means 121 ... Calculation means 122 ... Patch sensor 501 ... Pixel A 502 ... Pixel B 503 ... Extra area 601 ... Pixel A 602 ... Pixel B 603 ... Extra Area 801 ... Improved area 802 Density jump area 1001 image forming apparatus 1002 image data output apparatus 1003 buffer 1004 conversion processing means 1005 selector 1006 gradation processing means 1201 first pixel 1202 second pixel 1501 dots are not written Area 1502, 1503 dot 1701 spot diameter in the growth direction of pixel 1702 pixel size 2201 block 2202 2203 2204 pixel 2205 2206 2207 dot 2301 point 2302 horizontal line 2303 bump-like point 2304 White line 2305 point 2306 solid image 2401 gradation curve 2402 low density area 2403 intermediate density area 2801 first pixel 2802 second pixel 2803 third pixel 2901 input value 2902 many Value dither processing Means 2903 Temporary conversion value 2904 Conversion processing means 2905 Output value 4201 Pixel 5201 Image data output device 5202 Image forming device 5203 Conversion processing unit 5204 Gradation processing unit 5205 Buffer 5206 Image forming unit 5301 Image data output device 5302 image forming device 5303 conversion processing means 5304 buffer 5305 gradation processing means 5306 image forming means

Claims (10)

[Claims] 1. An image comprising: a gradation processing means for determining an area ratio of each pixel based on image information to generate an image signal; and an image forming means for forming an image based on an image signal from the gradation processing means. In the forming apparatus, after the area ratio of the first pixel reaches a first predetermined value, the gradation processing unit keeps the area ratio of the first pixel constant, and An image forming apparatus, wherein an area ratio of a second pixel adjacent in a direction different from a growth direction is increased. 2. The method according to claim 1, wherein the first predetermined value includes a dot formed at a first pixel position by the image forming unit according to an area ratio of the first pixel, and a dot formed in a growth direction of the first pixel. 2. The image forming apparatus according to claim 1, wherein a dot formed at said third pixel position by said image forming means is set to a value close to or connected to said third pixel according to an area ratio of an adjacent third pixel. apparatus. 3. The image forming means has a dot writing means for forming dots based on an area ratio of each pixel, and the image forming means has one dot with respect to the minimum dot diameter by the dot writing means.
When the ratio of the size of the pixels is a and the ratio of the size of the pixels formed by the pixels having the maximum area ratio to the size of one pixel is n, the first area ratio is defined assuming that the maximum area ratio is 1. 2. The image forming apparatus according to claim 1, wherein the predetermined value is set to be equal to or more than 1-1 / 2na and less than 1. 4. The image forming means comprises: an image exposing means capable of pulse width modulation as the dot writing means; and an image carrier on which an image corresponding to image exposure from the image exposing means is formed. The first predetermined value is set to a value of 0.3 or more and less than 1 in terms of pulse width, assuming that the pulse width required for the image exposure means to expose the entire area of the first pixel is 1. The image forming apparatus according to claim 3, wherein: 5. The image forming apparatus according to claim 4, wherein the first predetermined value is set to 0.75 to 0.9 in terms of a pulse width. 6. The image forming apparatus according to claim 1, wherein the center of the second pixel is arranged obliquely with respect to the center of the first pixel. 7. The gradation processing unit reduces the area ratio of the second pixel to a second predetermined value or less after the area ratio of the second pixel reaches a second predetermined value. The image forming apparatus according to claim 1, wherein an area ratio of the first pixel is increased beyond the first predetermined value. 8. The image forming apparatus according to claim 7, wherein the area ratio of the second pixel is set to 0 after the area ratio of the second pixel reaches a second predetermined value. . 9. The gradation processing means for performing multi-value dither processing, wherein a storage means for storing a plurality of dither matrices corresponding to respective area ratios of pixels, and an image density of each pixel based on image information, Multi-value dither processing means for reading out the largest area ratio among the area ratios corresponding to the dither matrix which is equal to or more than the threshold value of the input density stored in the dither matrix; and 6. A conversion means for converting a value into a value equal to or less than the number of gradations possessed by the device.
An image forming apparatus according to any one of claims 6 to 7 and 8. 10. The image forming apparatus according to claim 9, wherein the number of dither matrices stored in said storage means exceeds the number of gradations that each pixel can take.