JP3733983B2 - Image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus, and more particularly to a halftone processing technique for preventing a density jump.
[0002]
[Prior art]
Conventionally, printers of various principles have been used as output devices such as personal computers and workstations. Laser beam printers (LBP) using an electrophotographic process are used in many places because of their high output speed and print quality. It is used in.
Currently, the LBP mainly distributed in the market is monochrome, and binarization processing is performed on data such as characters and line drawings. The binary dither method is often used for image data that requires gradation. However, the binary dither method requires a large dither matrix in order to obtain sufficient gradation, and the high resolution, which is a feature of LBP, is lost.
[0003]
Recently, there is an increasing demand for colorization of LBP, and there is a demand for an image forming apparatus with better gradation.
[0004]
As an example of an image forming apparatus having more excellent gradation, as described in Japanese Patent Laid-Open No. 5-176163, a block composed of a plurality of pixels on image data is set and spatially preliminarily stored in the block. In a region where dots are grown in order from the dot corresponding to the minimum recording pixel position having a higher priority according to the predetermined priority order, and the dots having a specific priority continue to grow, the specific priority An apparatus has been proposed that initiates the growth of dots with a lower priority than the previous dot.
[0005]
In the prior art, if a block consisting of a plurality of pixels on the image data is set as shown in FIG. 2, as the input data gradually increases as shown in FIG. The value increases first. Next, when the value of the pixel A is close to the upper limit, the value of the pixel having the next priority, for example, the pixel B is increased. Here, the gradation expression is improved by overlapping the growth processes of the pixel A and the pixel B.
[0006]
[Problems to be solved by the invention]
Here, each pixel is represented by a rectangle, but in the case of LBP, for example, the potential gradient actually formed on the photoreceptor by exposure is not necessarily a rectangle. FIG. 4 shows the shape of the potential gradient on the photoreceptor with respect to the area ratio of the pixels.
[0007]
The outline of the dots formed when the area ratio is grown using such a potential gradient is the potential formed by the pixel A501 when the pixel B502 becomes somewhat large as shown in FIG. It influences the gradient and leads to one. At this time, an extra area 503 is generated, and the concentration rapidly increases.
[0008]
This phenomenon also occurs when the pixel having the second priority is the pixel D. The outline of the dot when the value of the pixel D is increased to some extent is connected to one because the potential gradient formed by the pixel A601 and the potential gradient formed by the pixel D602 are affected as shown in FIG. At this time, the area of the dots formed on the medium also includes an extra area 603. Therefore, when the total areas of the pixels A and D before and after connecting are compared as shown in FIG. Compared to the rate, the rate of increase of the connected area is abnormally large.
[0009]
That is, in the improvement area 801 shown in FIG. 8, the gradation is improved as described in the previous section, but the density jump area 802 in which the pixels A and D are influenced by each other and the dots are connected to each other. Indicates that a density jump occurs.
[0010]
This abnormal increase in dot area is unavoidable and difficult to solve in a system that increases or decreases the area ratio of each pixel by electrophotography. If an image is expressed using dots whose area ratio changes greatly in this way, density jumps may occur, or in the worst case, it may become a pseudo contour and impair image quality.
Further, FIG. 9 shows the outline of a dot formed when there is main scanning jitter in the scanning direction of the exposure beam and sub-scanning jitter in the sub-scanning direction in a situation where the potential gradients interfere with each other.
[0011]
As can be seen from FIG. 9, there is a problem in that the area of the dots formed becomes unstable, resulting in uneven density of the image and deteriorating the image quality.
[0012]
Accordingly, an object of the present invention is to solve the above-described problems, and the distance between the pixels is close, and the potential gradient on each photoconductor is affected, so that the pixel area formed on the medium becomes larger than expected. An object of the present invention is to provide an image forming apparatus having excellent gradation characteristics.
[0013]
[Means for Solving the Problems]
An image forming apparatus comprising: gradation processing means for determining an area ratio of each pixel based on image information and generating an image signal; and image forming means for forming an image based on an image signal from the gradation processing means. The gradation processing means is Means for determining the area ratio for each pixel; After the area ratio of the first pixel reaches the first predetermined value, the area ratio of the first pixel is made constant and adjacent to the second pixel in a direction different from the growth direction of the first pixel. The area ratio of the pixels is increased.
[0014]
For this reason, even if pixels at close distances from each other affect the potential gradient formed on the photosensitive member, and even if pixels larger than expected are formed on the recording medium, the second pixel is Since the degree of influence of the potential gradient formed can be finely controlled, density jump can be suppressed, and a high-quality recorded image excellent in gradation can be obtained without pseudo contour and density unevenness.
[0015]
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 third adjacent to the growth direction of the first pixel. According to a pixel area ratio, the dot formed at the third pixel position by the image forming unit is set to a value that is close or connected.
[0016]
The image forming means has dot writing means for forming dots based on the area ratio of each pixel, and a ratio of the size of one pixel to the diameter of the minimum dot 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 n is n, the first predetermined value is set to 1−1 / 2 na or more and less than 1 when the maximum area ratio is 1. It is characterized by that.
[0017]
The image forming means includes an image exposure 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 exposure means is formed, and the first The predetermined value is set to a value not less than 0.3 and less than 1 in terms of pulse width, where 1 is a pulse width necessary for the image exposure means to expose the entire area of the first pixel. And The first predetermined value is set to 0.75 to 0.9 in terms of pulse width. The second pixel center is arranged obliquely with respect to the first pixel center.
[0018]
The gradation processing means 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, and The area ratio of one pixel is increased beyond the first predetermined value.
[0019]
The area ratio of the second pixel is set to 0 after the area ratio of the second pixel reaches a second predetermined value.
[0020]
The gradation processing means performs multi-value dither processing and stores a plurality of dither matrices corresponding to each area ratio of the pixels, and stores the image density of each pixel based on image information in the dither matrix. Multi-value dither processing means for reading out the largest area ratio among the area ratios corresponding to the dither matrix that is equal to or greater than the threshold value of the input density, and the number of gradations of the obtained multi-value dither processing means. It has the conversion means which converts into the following values, It is characterized by the above-mentioned.
[0021]
The number of dither matrices stored in the storage means is stored in excess of the number of gradations that each pixel can take.
[0022]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 10 is a block diagram showing the configuration of the image forming apparatus of the present invention. In FIG. 10, reference numeral 1001 denotes an image forming apparatus, which is an apparatus that forms an image on a recording medium such as paper and outputs the image data output from the image data output apparatus 1002.
[0023]
The image forming apparatus 1001 includes a buffer 1003 for accumulating image data output from the image data output apparatus 1002, a conversion processing unit 1004 for converting image data extracted from the buffer 1003 into data suitable for image recording, and depending on the occasion. It comprises a selector 1005 for selecting data to be recorded, a gradation processing means 1006 to which the present invention is applied, and an image forming means 100 for recording on a recording medium.
[0024]
Next, the operation of the entire image forming apparatus will be described in detail.
[0025]
First, image data to be output is transferred from the image data output device 1002. In general, the image data output device 1002 corresponds to a device such as a personal computer or a workstation, and outputs an image, a document, or the like created by these devices. Therefore, the data transferred from the image data output device 1002 is generally RGB data on the assumption that it is displayed on the CRT.
[0026]
Therefore, the image forming apparatus of the present invention is also effective when outputting a monochrome image. Here, an image forming apparatus that outputs a color image will be described as an example.
[0027]
Image data output from the image data output apparatus 1002 enters the image forming apparatus 1001 and is first stored in the buffer 1003. Normally, there is a large difference between the speed of image data output from the image data output device 1002 and the speed of image data processed by the image forming unit 100, and this difference is absorbed by the buffer 1003. In addition, since the transferred image data 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 possible. In this case, the image data is sent to the next conversion processing means 1004 after decompression processing is performed in the buffer 1003 before sending the image data to the subsequent process.
[0028]
The image data sent to the conversion processing means 1004 is RGB data because it is assumed to be 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, it is necessary to convert RGB to C (cyan) M (magenta) Y (yellow) data which are the three primary colors of printing. In general, printing is performed not only with the three primary colors CMY but also with four colors CMYK including K (black) in order to improve the color reproducibility of the image. Therefore, the conversion processing means 1004 performs this RGB → CMYK conversion.
[0029]
Although it has been converted into four CMYK data by the conversion processing unit 1004, as will be described in detail later, when the image forming unit 100 actually forms an image, the formation of a single color image for one page is repeated for the number of colors used. In order to form one image, it is necessary to select only one color from the four CMYK colors depending on the occasion. Therefore, a selector 1005 is a means for selecting valid single color data from the data for four colors.
[0030]
As described above, CMYK single-color image data is generated from the RGB data. The obtained image data is converted from RGB data to CMYK data in accordance with the gradation characteristics of the image forming means 100, and the number of gradations thereof. Has the same number of gradations as RGB data.
[0031]
However, since the number of gradations of the image forming unit 100 is generally very small compared to the number of gradations possessed by the CRT, the number of gradations is increased by expressing the gradations with a plurality of pixels, the input gradation is rounded and approximated, etc. Thus, it is necessary to convert the image data into image data that can be expressed by the image forming unit 100. The gradation processing means 1006 performs this processing.
[0032]
Next, the operation of the image forming unit 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.
[0033]
The charging roller 102 charges the photosensitive member 101 to a uniform potential (for example, −700 V). A laser beam having a resolution of 600 dpi (dot per inch) formed by the laser scanner 103 is guided onto the photoconductor 101 by the folding 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 is 1 / e 2, which is 60 μm in the main scanning direction 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, which is the rotation direction of the photosensitive member 101 or the intermediate transfer member 106 and the traveling direction of the recording material 113.
[0034]
The one-component contact type developing device 105 as the developing means can be contacted and separated in the direction of the arrow in the figure. Among them, the yellow (Y) developing unit 105Y is first brought into contact with the other developing units, and the negatively chargeable Y toner is reversely developed by the action of an electric field of a power source (not shown) to be visualized on the photosensitive member 101. The The visualized Y toner has a primary transfer power source 108 and a primary transfer roller 107 on an intermediate transfer body 106 adjusted to an appropriate resistance by dispersing carbon in ETFE (ethylene tetrafluoroethylene copolymer). Thus, a bias having a polarity opposite to that of the toner is applied, and the toner is transferred by the action of the electric field. Further, the intermediate transfer member 106 is adjusted by a tension roller 118 so that an appropriate tension is applied. The untransferred toner on the photoconductor 101 is collected by a photoconductor cleaner 109 that is cleaned by bringing the blade into contact therewith, and then the photoconductor potential is reset by the charge eliminating lamp 110.
[0035]
The same operation is repeated for the magenta (M) developing unit 105M, the cyan (C) developing unit 105C, and the black (K) developing unit 105K by synchronizing the position of the intermediate transfer member 106 and the light emission timing of the exposure unit 103. Each color toner 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 body cleaner 119 are in a separated state.
[0036]
On the other hand, the recording material 113 such as paper or OHP is conveyed from the paper feeding cassette 112 to the registration roller pair 114 by the paper feeding means 111 and then synchronized with the full color image on the intermediate transfer member 106 and the driving roller 115. It is conveyed to a secondary transfer portion formed by a secondary transfer roller 116 that can be contacted and separated in the direction of the middle arrow. In the secondary transfer portion, the secondary transfer roller 116 contacts the intermediate transfer member 106 in synchronization with the recording material 113 to form and press the nip portion and calculate the voltage obtained from the primary transfer power source 108. The voltage determined in 121 is controlled to a constant voltage by the secondary transfer power source 117, and a full-color toner image is formed on the recording material 113 by the action of the electric field. Thereafter, the recording material 113 is fixed by the fixing unit 120 and discharged out of the apparatus.
[0037]
In the electrophotographic process section, a patch sensor 122 which is a density measuring means is further provided at a position facing the surface of the intermediate transfer body 106 downstream from the primary transfer roller. The patch sensor 122 is a sensor in which an LED and a photo sensor are combined. 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.
[0038]
Next, a first embodiment of the present invention will be described. In the first form, the area ratio is determined for each pixel of the image data, and pixels corresponding to the second pixel are arranged.
[0039]
Usually, image data often has 256 gradations for each color of RGB, and therefore, data converted from RGB to CMYK is usually converted to 256 gradations for each color. However, the number of gradations that can be obtained per pixel at the resolution of the image forming means is often not as high as 256 gradations, although it is multivalued.
[0040]
Therefore, as shown in FIG. 11, an image forming apparatus in which the area within a pixel is increased or decreased according to the value of each pixel, and pixel growth starts from the left end and can express m + 1 levels of gradations from 0 to m will be described as an example. To do.
[0041]
FIG. 12 shows how the pixels grow with respect to the input image data. In the growth process of FIG. 12, when the area ratio of the first pixel 1201 reaches 1-1 / 2na which is the first predetermined value, the size of the first pixel 1201 becomes constant, and the first pixel 1201 becomes constant. It shows how the second pixel 1202 located immediately below the pixel begins to grow instead.
[0042]
The first predetermined value 1-1 / 2na is a ratio when the maximum area ratio is 1. A represents the ratio of the size of one pixel to the diameter of the dots formed by the dot writing means of this 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.
[0043]
For example, when the maximum area ratio of one pixel is 5 m, n = 5, and each pixel can have a maximum size of five pixels. The relationship between the sizes of a and n is shown in FIG.
[0044]
In general, dots formed by dot writing means have a distribution in which the central portion of the dot has a higher density than the peripheral portion as shown in FIG. That is, the outline of the dot is not clear around the dot. For this reason, when two dots are adjacent to each other, the density distribution between the dots changes depending on the interval between the dots.
[0045]
Accordingly, FIG. 15 shows the density distribution when the first pixel and the adjacent third pixel adjacent to each other in the growth direction of the first pixel have a 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 in which dots are not written is 1 / 2na. 1 / 2na corresponds to a ratio of 1/2 of the dot diameter to the maximum area ratio, and the dot 1502 and the dot 1503 overlap each other by a half of the dot diameter.
[0046]
Accordingly, when the overlapping portion becomes larger, the density is sufficiently increased even in the area 1501 where writing is not performed, that is, the dot formed by the first pixel and the dot formed by the third pixel are connected to form one dot. End up.
[0047]
Therefore, the growth of the first pixel is stopped at the first predetermined value 1-1 / 2na, and the gradation is ensured by growing the second pixel.
[0048]
Further, since the first predetermined value is a ratio to the maximum area ratio, the maximum value is 1, but when it is 1, the dot 1502 and the dot 1503 are completely overlapped, so that the first predetermined value is not suitable. The predetermined value of must be set to less than 1.
[0049]
Further, with respect to the first predetermined value, in the case of an image forming unit configured using an image exposure unit capable of pulse width modulation as a dot writing unit, a pulse required for exposing the entire area of the first pixel. When the width is 1, it is desirable to set it in the range of 0.3 or more and less than 1 in terms of pulse width.
[0050]
For example, the configuration of the image forming apparatus described with reference to FIG. 1 includes 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.
[0051]
On the other hand, the shape of the pixel has a rectangular shape with a unit length obtained from a value such as “dpi”, which is a measure of the resolution of the image forming apparatus, as one side. Therefore, for example, as in the image forming unit having a resolution of 600 dpi described with reference to FIG. 1, the relationship as shown in FIG. 16 is used at the minimum so that the beam diameter is larger than the pixel size in order to write the entire pixel. Is common.
[0052]
Accordingly, 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 image is expressed by the area ratio of one pixel unit where n = 1. In this case, the pulse width necessary for exposing the entire area of the first pixel is 1, that is, the pulse width necessary for exposing one pixel is 1, and the first predetermined value is 1 in terms of pulse width. -1 / 2na = 0.3.
[0053]
Further, in the image forming apparatus designed so that the spot diameter 1701 in the growth direction of the pixel is smaller than the pixel size 1702, that is, a is larger as shown in FIG. It is desirable to set in the range of 0.3 or more and less than 1 in terms of pulse width.
[0054]
Next, a second embodiment will be described. This embodiment is an example in which the image forming means having a resolution of 600 dpi in FIG. 1 is used, and when each pixel has a maximum area ratio in order to secure more gradation, the size becomes 2 pixels or more.
[0055]
In the previous embodiment, a value of 0 to m per pixel was taken to express the density of m + 1 gradations. However, since m cannot generally be taken as much as 255, by expressing the gradation by increasing the size of the pixel, it is possible to express more gradations and improve the image quality.
[0056]
That is, as shown in FIG. 18, if configured so that the growth of pixels can exceed n pixels and grow to the size of n pixels, m + 1 gradations from 0 to m can be expressed in one pixel. The pixel takes a value of 0 to nm and can express gradation of nm + 1 level.
[0057]
However, increasing the value of n in order to increase the number of gradations increases the size of each pixel, thus simultaneously reducing the resolution. Therefore, there is a good n range in which both the number of gradations and the resolution are compatible.
[0058]
FIG. 19 shows the resolution, the number of gradations, and the value of the first predetermined value when n is 1 to 10. For the first predetermined value, as in the previous embodiment, when the pulse width necessary to expose the entire area of the first pixel, that is, the pulse width necessary to expose the entire n pixels is 1, It is a value represented by 1-1 / 2na in terms of pulse width. Here, a is 42 μm / 60 μm = 0.7, and although not described in FIG. 1, the number of gradations m + 1 that can be expressed in one pixel is 33, that is, m = 32.
[0059]
As shown in FIG. 19, if n is set to 8 or more, gradations of 257 gradations or more can be expressed, so that the input image data can be sufficiently reproduced even with 256 gradations. However, in this case, the resolution is 75 dpi or less, which is not preferable. Usually, when expressing an image with gradation such as a photograph, it is desired to secure at least 100 dpi.
[0060]
Therefore, in order to keep both resolution and gradation good, the first predetermined value is desirably set to a value of 0.75 to 0.9 in terms of pulse width.
[0061]
Next, a third embodiment for realizing the present invention will be described in detail. The third form is a form using a density pattern method, which is generally used when a plurality of pixels are handled as one block and gradation is expressed by the size of the area ratio in the block.
[0062]
In this embodiment, as shown in FIG. 20, in the system in which the area in the pixel increases or decreases depending on the value of each pixel, the pixel growth starts from the center and each pixel can take 97 gradations from 0 to 96. Further, an image forming apparatus having an area of one pixel at 32 and a pixel area of three pixels at the maximum 96 will be described as an example.
[0063]
In this image forming apparatus, in order to ensure sufficient gradation, gradation is expressed by the area ratio in the block with 3 × 3 9 pixels as one block. Since each pixel can have an area ratio of 33 steps (0 to 32), the entire block can have an area ratio of 289 steps (32 × 9 + 1).
[0064]
Therefore, as shown in FIG. 21, processing is performed using a density pattern method in which one of the 289-level gradation block patterns is associated with each of the input 256-level values. Therefore, as the relationship of the output density with respect to the input, an arbitrary gradation curve can be obtained by changing the correspondence relationship of FIG.
[0065]
FIG. 22 shows how the area ratio of the block is increased.
[0066]
First, in the block 2201 showing density 0, all of the pixels in the block start from 0, and in step 1, the pixel 2202 located at the center (X position 2, Y position 2) of the block 2201 grows and has the maximum value. In 96, a dot 2205 for three horizontal pixels is grown.
[0067]
Next, in step 2, the pixel 2203 located immediately above the pixel 2202 that has already grown to the maximum value (X position 2, Y position 1) grows to become a dot 2206.
[0068]
When the dot 2206 grows up to the size of three horizontal pixels, the pixel 2204 positioned immediately below the dot 2202 (X position 2 and Y position 3) grows to become a dot 2207. When the dot 2207 grows up to the size of three horizontal pixels, all the pixels in the block 2201 are filled and the maximum density is reached (step 3). The input value at this time is 255.
[0069]
When the growth of the pixels as described above is observed macroscopically, as shown in FIG. 23, when the density is low, the points 2301 exist at positions away from each other, and each grows in the horizontal direction as the density increases. When each point grows in a horizontal direction to some extent, adjacent points are connected at a certain place to form a single horizontal line 2302. The steps so far correspond to step 1.
[0070]
When the concentration is further increased, bump-like points 2303 are generated at positions separated from each other adjacent to the upper portion of the horizontal line 2302 and grow in the horizontal direction. When the point 2303 grows in a horizontal direction to some extent, the adjacent points 2303 are connected to each other in the same manner as in Step 1 to form a horizontal line. At this time, since two horizontal lines continuous in the Y direction are formed, the situation is the same as when the white line 2304 is present in the solid image. The steps so far correspond to step 2.
[0071]
In the last step 3, points 2305 are generated so as to fill the white lines 2304 left in step 2, and each grows in the horizontal direction. Finally, all the white lines are filled to form a solid image 2306.
[0072]
FIG. 24 shows a gradation curve 2401 representing the output density with respect to the input value. This gradation curve 2401 is the result of associating the aforementioned block pattern with each input value, and the block pattern is associated with the input value so that the density is proportional to the luminance.
[0073]
When an image is actually processed, the gradation of the highlight portion (low density portion) has a high degree of influence on the image quality, and it is necessary to sufficiently ensure the gradation of the low density region 2402 in FIG. In particular, human skin color, which is easily noticeable to human eyes, cannot be expressed satisfactorily unless the tone of the magenta component is sufficiently secured. Since the input value of the flesh-colored magenta component generally indicates an intermediate portion, it is necessary to ensure sufficient gradation in the intermediate density region 2403 in FIG.
[0074]
When the above-described gradation expression method is used, switching from Step 1 to Step 2 is included in the intermediate density region 2403 when the step described with reference to FIG. 22 corresponds to the gradation curve 2401 of FIG.
[0075]
Switching from step 1 to step 2 means a state at the moment when adjacent points are connected to form a horizontal line. And, as can be seen from the description in the previous embodiment, the phenomenon in which the points are connected to each other occurs during Step 1 before switching to Step 2 rather than to occur at the moment of switching from Step 1 to Step 2. There are many things.
[0076]
Therefore, FIG. 25 shows an enlarged view of two contact points and a block pattern of both states for the state B immediately before the state A just before the adjacent points are connected in step 1. From FIG. 25, it can be seen that when the value of the pixel at the center of the block is 81, a horizontal line is formed by connecting to the adjacent point.
[0077]
In terms of data, it is ideal to form a horizontal line when the value of the pixel at the center of the block reaches 96, which means that the point growth ended earlier than expected. This indicates that there is a density jump between the density indicated by the block pattern having the pixel value of 80 in the center of the block and the density indicated by the block pattern having the value of the block center of 81.
[0078]
This phenomenon occurs when the potential gradients for forming the respective points on the photosensitive member are too close to each other so that they influence each other and become one. FIG. 26 shows a potential gradient corresponding to each of A and B in FIG.
[0079]
Therefore, as shown in FIG. 27, A ′, A ″, A ′ ″, A ′ ″ between a block pattern having a pixel value of 80 in the center of the block and a block pattern having a pixel value of 81 in the center of the block. Insert block pattern as shown in '.
[0080]
The inserted block pattern is a second pixel at the upper right (X position 3, Y position 1) of the block pattern in which the value of the pixel at the center of the block corresponding to the first pixel in which the dots are not connected is 80. This is a pattern in which very small values corresponding to are arranged and increased.
[0081]
The density gradation is obtained by making the block patterns of 293 steps (289 + 4) including these four block patterns A ′, A ″, A ′ ″, A ″ ″ of FIG. 27 correspond to 256 input values. When the first pixel reaches 80 corresponding to the first predetermined value, the first pixel takes a constant value, and the second pixel increases in value.
[0082]
Here, the first predetermined value 80 corresponds to 0.83 when the pulse width at 96 is 1, since the maximum value that the pixel can take is 96, 0.75 to 0.9. Is set in the range. That is, the value is set such that 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 or connected.
[0083]
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 influence each other, and the dots formed by the first pixel 2801 and the third pixel 2803 It is possible to express the intermediate density of the patterns expressed by A and B in FIG. 25 by slightly changing the interval between them, thereby preventing the density jump.
[0084]
If the position of the second pixel is a direction different from the growth direction of the first pixel, for example, the effect of controlling the potential gradient of the first pixel is just below or directly above the first pixel. Although it can be expected, the dots tend to be connected in the vertical direction and the texture tends to occur. Therefore, in order to prevent unnecessary texture and obtain more control effect, the first pixel and the second pixel may be arranged so that the respective pixel centers are in an oblique positional relationship as in this embodiment. desirable. In the present embodiment, the position of the second pixel is the upper right, but of course, it may be the lower right, the upper left, and the lower left.
[0085]
After the second pixel reaches the second predetermined value and the dots formed by the first and second pixels are smoothly connected, the first pixel exceeds the first predetermined value. By increasing again, a higher density gradation can be expressed. At this time, it is desirable that the second pixel, which has finished the purpose of controlling the potential gradient formed by the adjacent first pixel and third pixel, be reduced to a value equal to or smaller than the second predetermined value.
[0086]
Further, in order to prevent the unnecessary second pixel from adversely affecting the image, the second pixel is reduced to 0 after the second pixel reaches 6 in this embodiment. After reaching the second predetermined value, the area ratio is preferably zero.
[0087]
In the above description, the 3 × 3 block is described. However, the present invention is not limited to this size, and the same applies when the block size is other than 3 × 3.
[0088]
Next, a fourth embodiment of the present invention will be described. The fourth form is a form using a multi-value dither method.
[0089]
FIG. 29 shows the flow of processing of the fourth embodiment.
[0090]
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 into a temporary converted value 2903 having multiple values by the multi-value dither processing means 2902. Similarly, the number of gradations of the temporary conversion value 2903 is more than the number of gradations of the output value 2905 that takes multiple values, and is converted into the output value 2905 by the conversion processing means 2904.
[0091]
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 converts the temporary conversion value 2903 exceeding 96 into a value within 96 by converting the remainder obtained by dividing the temporary conversion value 2903 by 96 into the output value 2905.
[0092]
The dither matrix used by the multi-value dither processing means 2902 when the third embodiment is performed in this embodiment is shown in FIGS.
[0093]
In the multi-value dither processing means 2902, there are 226 dither matrices for converting 256-value input into 227 values. Here, the size of each matrix is 3 × 3 pixels. Each of the 226 dither matrices corresponds to the value after the multi-value dither processing, and the numerical value in each matrix corresponds to the input value.
[0094]
That is, when the input value is greater than or equal to the numerical value in the matrix, the pixel value at that position is output as the output value.
[0095]
At the same time, when the input value is equal to or greater than the numerical value in the matrix, the larger matrix value becomes effective.
[0096]
That is, when attention is paid to the lower right pixel in the matrix of FIGS. 30 to 38, when the input is 139, the output of this pixel is 1 because it is equal to or greater than the lower right value of the matrix of 1. Next, when the input is 140, the output is 2 because it is equal to or greater than the value of the matrix of 2.
[0097]
When the input is 150, the value of the matrix of 97 becomes 141 or more, so 97 is output.
[0098]
When the input reaches 254, the matrix value 253 of 129 and the matrix value 253 of 226 become equal to or greater than 253. In this case, the result of the matrix corresponding to the larger value is valid, and the output is 226.
[0099]
Since all other matrix values are 255, they are effective only when the input value is 255. However, when the input is 255, the value of the matrix of 226 is also effective, so that the output is the largest 226 after all.
[0100]
Therefore, it can be seen that the lower right pixel in the matrix takes one of four outputs 1, 2, 97, and 226 for input values 0 to 255.
[0101]
In FIG. 30, numerical values with [] are arranged beside values exceeding 96 as temporary conversion values of each dither matrix. This indicates an output value finally outputted through the conversion processing means 2904, that is, a value of 0 to 96.
[0102]
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, when the temporary conversion value is 97 or 194, the final output is 0. It can be seen that the final output value when converted to the maximum value 226 is 32.
[0103]
Next, it is confirmed that the processing using the dither matrix of FIGS. 30 to 38 is the same as the content of the third embodiment.
[0104]
A process in which 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 gradations is seen on the dither matrix in FIGS.
[0105]
First, the gradation at which the value of the first pixel is 80 is 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 when the input value is 100 or more.
[0106]
Further, from FIG. 30, when the input value is 94, the upper right pixel value corresponding to the second pixel is 1, and thereafter 95 is 2, 96 is 3, 97 is 4, 98 is 5, 99 is 6.
[0107]
Then, the value 97 of the second pixel when 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.
[0108]
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 lower values are provided, and the number of dither matrices used in the multi-value dither processing is determined by the number of gradations of the output value. By setting the value to exceed the value, it is possible to realize the same processing as the gradation processing realized by the density pattern method, and it is possible to realize processing with higher resolution.
[0109]
Next, a fifth embodiment of the present invention will be described. The fifth form is applicable to a case where it is difficult to express a small area ratio due to engine characteristics or the formation of a potential gradient is unstable due to environmental fluctuations.
[0110]
First, the characteristics of the image forming means used in the following description will be described.
[0111]
Here, the image forming means suppresses the gradation to 7 values (0, 1, 2, 3, 4, 5, 6) per pixel in order to stably express the dots. As shown, as the value of each pixel increases, the area of dots 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 photosensitive member and the dot shape formed by the latent image are the beams scanned by the exposure unit. Due to the influence of the spot diameter and the sensitivity of the photosensitive member, a rounded dot shape as shown in FIG. 40 is obtained.
[0112]
Thus, FIG. 41 shows how dots are formed when there is pixel data at positions close to each other. In FIG. 41, I is when there is pixel data arranged side by side, II is when there is pixel data at an oblique position, and III is when there is pixel data arranged vertically.
[0113]
When the pixel data of 6 and the pixel of 3 are arranged in the state I, the dots formed by each of them are connected to form one dot as in a. However, when 6 pixels and 2 pixels are arranged, each dot forms an independent dot as in b.
[0114]
In the state II, if the pixel data is 4 and 3 pixels are aligned, the dots formed by each are connected as in a to form one dot, and the 3 pixels and 3 pixels are aligned. If so, independent dots are formed.
[0115]
In the state of III, if the pixel data is 6 and 2 pixels are arranged, one dot is formed as shown in a. However, if 6 pixels and 1 pixel are arranged, independent dots are formed. Form.
[0116]
In this way, the dots may be connected or not connected depending on the value of the pixel data, which may cause a density jump.
[0117]
Therefore, image formation is performed by applying the dither matrix according to the present invention shown in FIG.
[0118]
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.
[0119]
That is, regarding the pixel 4201 in FIG. 42, when the input value for the pixel 4201 is 5, the output of the pixel 4201 is 1 because it is larger than 4 of the 1 matrix, and when the input value is 12, 9 of the 2 matrix. The output value is 2.
[0120]
Further, when the input value is 15, the output value is 3 because it exceeds 13 of the matrix of 3, and when the input value is 40, it exceeds 31 of the matrix of 4, so that the output value is 4, and 5 when the input value is 70. When the input value is 255, the output value exceeds 6, so that the output value is 6, that is, a solid pixel.
[0121]
Here, looking at the structure of each matrix in FIG. 42, one matrix is a block group composed of repetitions of a dither matrix composed of 2 × 2 pixel blocks shown in FIG. The matrices 4 and 4 are block groups each including nine types of 2 × 2 pixel blocks shown in FIGS. 44, 45, and 46, respectively.
[0122]
Further, the matrix of 5 and the matrix of 6 are a block group constituted by repeating one type of block shown in FIGS. 47 and 48, respectively.
[0123]
As shown in FIG. 49, these matrices are configured such that each pixel in a 2 × 2 pixel block has a priority and grows according to that priority. Also, as shown in FIG. 50, in the block group of 4 × 4 blocks, each block is prioritized and is grown according to the priority.
[0124]
The dot growth process when the matrix of FIG. 42 is applied will be described in detail below. Here, consider the case of processing image data composed of pixels having uniform values. 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”.
[0125]
First, when the pixel value is from 0 to less than 4, the output value of all the pixels is 0. When the pixel value is 4 or more, the output value of the pixel 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) in the entire block group ), (5, 3), (7, 3), (1, 5), (3, 5), (5, 5), (7, 5), (1, 7), (3, 7), The output value of the pixels (5, 7) and (7, 7) is 1.
[0126]
Next, when the input value becomes 9 or more, the output value of the pixel becomes 2.
[0127]
When the input value becomes 13 or more, the output value becomes 3, and the output of the pixels at the X position 2 and the Y position 2 having the second priority in the block becomes 1.
[0128]
Thereafter, the priority level 1 pixel remains at the output value 3, the priority level 2 pixel becomes 2 when the input value is 18 or more, and becomes 3 when the input value is 22 or more.
[0129]
So far, each pixel has formed a single dot.
[0130]
Next, when the input value becomes 27 or more, the output value of the first priority level pixel becomes 4, but as described above, the combination of the priority level 1 pixel and the priority level 2 pixel is 4 or 3. Dots formed by each pixel are connected.
[0131]
Therefore, when the dither matrix of FIG. 42 is used, the output of the pixel of priority 1 becomes 4 only in the block of priority position 1 X position 1 and Y position 1 according to the priority order of the blocks in the block group, and the other blocks The previous state, that is, the pixel with priority 1 and the pixel with priority 2 remain at 3.
[0132]
Thereafter, pixels of priority 1 in each block grow to 4 in order according to the priority in the block group.
[0133]
FIG. 51 shows a table summarizing dot growth when the input values are similarly viewed up to 255.
[0134]
In the table, the gradation column indicates the order of each gradation that can be expressed by the dither matrix in FIG. The input value indicates an input value for outputting each gradation. When the input value is equal to or greater than this value, the corresponding gradation is output. The arrangement column shows how the blocks configured 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 column. If the value is a number, the corresponding block in the block count is filled with the pixel value shown in the following column, and the other blocks are the previous ones. It shows that there is no change in the pixel value filled with gradation.
[0135]
From FIG. 51, it can be seen that 57 gradations from 0 to 56 can be expressed by the dither matrix of FIG.
[0136]
In addition, the gradation surrounded by a thick frame in FIG. 51 indicates a combination of pixel values in which the dots formed by each pixel described above are connected to each other. It can be seen that the pixel values are filled for each block.
[0137]
That is, even if the dots are connected to each other and the density jump occurs, this is an extreme phenomenon, and when viewed macroscopically, the density jump does not occur, so that it is possible to form an image with excellent gradation.
[0138]
In the above description, the image forming apparatus shown in FIG. 10 has been described. However, since the configuration of the apparatus changes depending on the purpose of the apparatus, the configuration is not limited to that of FIG.
[0139]
For example, in FIG. 52, the RGB data is converted from CMYK data by the conversion processing means 5203 in the image data output apparatus 5201, the gradation processing means 5204 performs gradation processing, and the image data is transferred to the image forming apparatus 5202. In the image forming apparatus 5202 to be output, the buffer 5205 performs decompression processing, sends the data to the image forming unit 5206 as it is, and prints it.
[0140]
53, only RGB → CMYK conversion is performed by the conversion processing unit 5303 in the image data output device 5301. In the image forming device 5302, gradation processing is performed on the data from the buffer 5304 by the gradation processing unit 5305. In this configuration, the data is output at 5306.
[0141]
The present invention can also 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 as in the configuration of FIG.
[0142]
Further, in this embodiment, a printer using an electrophotographic method has been described as an example. However, an image forming apparatus that is different in the shape of dots formed with respect to the area ratio of pixels to be output, particularly affected by adjacent pixels. Needless to say, the present invention can be applied to, for example, an ink jet printer.
[0143]
【The invention's effect】
As described above, according to the image forming apparatus of the present invention, Determine the area ratio for each pixel, After the area ratio of the first pixel has reached the first predetermined value, the area ratio of the first pixel is constant, and the second pixel is adjacent in a direction different from the growth direction of the first pixel. As a result, the density jump that occurs when the first pixel continues to grow is suppressed and the image quality is improved.
[0144]
Furthermore, a first predetermined value is set to a value at which a dot formed at the first pixel position and a dot formed at a third pixel position adjacent to the first pixel in the growth direction are close to each other or connected. This makes it possible to finely control the dot interval using the second pixel from the state where the dots formed by the first pixel and the third pixel are not connected to each other, so that the density jump when connecting There is an effect to prevent.
[0145]
In particular, when the image forming means is provided with a 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 1 When the ratio of the size of the pixel formed by the pixel having the maximum area ratio to the pixel size is n and the maximum area ratio is 1, the first predetermined value is 1-1 / 2 na or more and less than 1 By setting to, 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 distance between the dots can be controlled. There is an effect to prevent.
[0146]
Even when the image forming apparatus includes an image exposure unit capable of pulse width modulation and an image carrier on which an image corresponding to the image exposure from the image exposure unit is formed, the first predetermined value is set as the image exposure unit. Is set to a value not less than 0.3 and less than 1 in terms of pulse width, assuming that the pulse width required to expose the entire area of the first pixel is 1, thereby forming a third pixel adjacent in the growth direction. Do not cause density jumps because they are not connected to dots.
[0147]
Then, by setting the first predetermined value to 0.75 to 0.9 in terms of pulse width, it is possible to ensure resolution and gradation without causing a density jump.
[0148]
In addition, by arranging the second pixel center obliquely with respect to the first pixel center, the control efficiency of the first pixel by the second pixel is improved, density jump is easily suppressed, and unnecessary texture is generated. Has the effect of making it difficult.
[0149]
Then, after the area ratio of the second pixel reaches the second predetermined value, the area ratio of the second pixel is decreased to the second predetermined value or less, and the area ratio of the first pixel is decreased to the first value. Since the density is increased beyond a predetermined value, it is possible to express higher density while preventing density jump.
[0150]
Furthermore, after the area ratio of the second pixel reaches the second predetermined value, the area ratio of the second pixel is set to 0, so that it is possible to eliminate image degradation due to unnecessary dots.
[0151]
The gradation processing means performs multi-value dither processing, and stores a plurality of dither matrices corresponding to each area ratio of the pixels, and stores the image density of each pixel based on image information in the dither matrix. Multi-value dither processing means for reading out the largest area ratio among the area ratios corresponding to the dither matrix that is equal to or greater than the threshold value of the input density, and the number of gradations of the obtained multi-value dither processing means. Since the conversion means for converting to the following values is provided, it is possible to perform processing with higher resolution while preventing density jumps.
[0152]
Furthermore, since the number of dither matrices stored in the storage means exceeds the number of gradations that each pixel can take, the dots once generated in the low density part can be eliminated in the high density part, Image degradation due to unnecessary dots can be eliminated.
[0153]
In this way, the distance between the pixels is so close that the potential gradients on the photoconductors influence each other, and the dot area formed on the medium does not increase unexpectedly. This has the effect that an excellent image forming apparatus can be constructed.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of an image forming unit of an image forming apparatus of the present invention.
FIG. 2 is a diagram showing the priority order of pixels that grow in a block of a plurality of pixels.
FIG. 3 is a diagram illustrating 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 a connection of dots.
FIG. 7 is a diagram showing an abnormal increase in dot area.
FIG. 8 is a diagram showing a density jump generation region.
FIG. 9 is a diagram 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 showing a relationship among 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 spot diameter and pixel size.
FIG. 18 is an explanatory diagram of pixel growth.
FIG. 19 is a relationship diagram between resolution and a first predetermined value.
FIG. 20 is an explanatory diagram of pixel growth.
FIG. 21 is a diagram showing a relationship between an input value and a block pattern.
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 a potential gradient before and after connecting adjacent points.
FIG. 27 is a diagram showing a block pattern in which second pixels are arranged.
FIG. 28 is a diagram showing that the dot interval is changed by the second pixel.
FIG. 29 is a diagram showing the flow of processing of 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 multi-value dither processing means.
FIG. 32 is a diagram showing a dither matrix used in multi-value dither processing means.
FIG. 33 is a diagram showing a dither matrix used in 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 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 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 diagram for explaining 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 the dither matrix.
FIG. 44 is a diagram showing nine blocks constituting the dither matrix.
FIG. 45 is a diagram showing nine blocks constituting a dither matrix.
FIG. 46 is a diagram showing nine blocks constituting the dither matrix.
FIG. 47 is a diagram showing one block constituting the dither matrix.
FIG. 48 is a diagram showing one block constituting the dither matrix.
FIG. 49 is a diagram showing the priority of growth in a block.
FIG. 50 is a diagram showing a priority order of growth within a block county.
FIG. 51 is a diagram showing all gray levels that can be expressed in a fifth form;
FIG. 52 is a diagram showing an image forming apparatus according to another embodiment.
FIG. 53 is a diagram showing an image forming apparatus according to another embodiment.
[Explanation of symbols]
100: Image forming means
101 ... Photoconductor
102 ... Charging roller
103 ... Laser scanner
104 ... Folding mirror
105Y ... Yellow developer
105M ... Magenta developer
105C ... Cyan developer
105K ... Black developer
106: Intermediate transfer member
107: Transfer roller
108: Power supply for primary transfer
109 ... Photoconductor cleaner
110 ... Static elimination lamp
111: Paper feeding means
112 ... paper feed cassette
113 ... Recording material
114 ... Registration roller pair
115: Driving roller
116: Secondary transfer roller
117 ... Power supply for secondary transfer
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 ... Improvement area
802 ... Density jump area
1001 ... Image forming apparatus
1002 ... Image data output device
1003 ... Buffer
1004 ... Conversion processing means
1005 ... selector
1006 ... gradation processing means
1201... First pixel
1202 ... Second pixel
1501 ... Area where dots are not written
1502, 1503 ... dot
1701 ... Spot diameter in the growth direction of pixels
1702 ... Pixel size
2201 ... Block
2202, 2203, 2204... Pixel
2205, 2206, 2207 ... dots
2301 ... Point
2302 ... Horizontal line
2303 ... Hump-shaped point
2304 ... White line
2305 ... dot
2306: Solid image
2401 ... gradation curve
2402 ... Low concentration region
2403: Intermediate density region
2801: First pixel
2802 ... Second pixel
2803: Third pixel
2901: Input value
2902: Multi-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 apparatus
5203 ... Conversion processing means
5204 ... gradation processing means
5205: Buffer
5206 Image forming means
5301: Image data output device
5302 ... Image forming apparatus
5303 ... Conversion processing means
5304: Buffer
5305 ... gradation processing means
5306 ... Image forming means

Claims (10)

An image forming apparatus comprising: gradation processing means for determining an area ratio of each pixel based on image information and generating an image signal; and image forming means for forming an image based on an image signal from the gradation processing means. The gradation processing means has means for determining the area ratio for each pixel, and after the area ratio of the first pixel reaches the first predetermined value, the area ratio of the first pixel is constant. And an area ratio of second pixels adjacent to each other in a direction different from the growth direction of the first pixels is increased.   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 third adjacent to the growth direction of the first pixel. 2. The image forming apparatus according to claim 1, wherein a dot formed at the third pixel position by the image forming unit is set to a value that is close to or connected according to a pixel area ratio.   The image forming means has dot writing means for forming dots based on the area ratio of each pixel, and a ratio of the size of one pixel to the diameter of the minimum dot 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 n is n, the first predetermined value is set to 1−1 / 2 na or more and less than 1 when the maximum area ratio is 1. The image forming apparatus according to claim 1.   The image forming means includes an image exposure 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 exposure means is formed, and the first The predetermined value is set to a value not less than 0.3 and less than 1 in terms of pulse width, where 1 is a pulse width necessary for the image exposure means to expose the entire area of the first pixel. The image forming apparatus according to claim 3. An image forming apparatus comprising: gradation processing means for determining an area ratio of each pixel based on image information and generating an image signal; and image forming means for forming an image based on an image signal from the gradation processing means. The gradation processing means has means for determining the area ratio for each pixel, and after the area ratio of the first pixel reaches the first predetermined value, the area ratio of the first pixel is constant. And the area ratio of the second pixel adjacent in a direction different from the growth direction of the first pixel is increased, and the image forming means is an image exposure 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 exposure means is formed, and the first predetermined value is such that the image exposure means exposes the entire area of the first pixel. When the pulse width and 1 needed, 0.75 the pulse width converted An image forming apparatus characterized in that it is set to 0.9. The relative first pixel center, an image forming apparatus according to any one of claims 1 to 5, wherein the second pixel center are arranged obliquely. The gradation processing means 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, and 6. The image forming apparatus according to claim 5, wherein the area ratio of one 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. The gradation processing means performs multi-value dither processing and stores a plurality of dither matrices corresponding to each area ratio of the pixels, and stores the image density of each pixel based on image information in the dither matrix. Multi-value dither processing means for reading out the largest area ratio among the area ratios corresponding to the dither matrix that is equal to or greater than the threshold value of the input density, and the number of gradations of the obtained multi-value dither processing means. the image forming apparatus according to any one of claims 1 to 8 characterized in that it has a conversion means for converting the following values.   The image forming apparatus according to claim 9, wherein the number of dither matrices stored in the storage unit exceeds the number of gradations that each pixel can take.

Images