JP4230866B2 - Image forming apparatus and image forming method - Google Patents

Description

  The present invention relates to image processing (pseudo halftone processing and gradation correction) mainly in an image forming apparatus using an electrophotographic process such as an electrostatic copying machine or a laser printer.

Conventionally, in a full-color image forming apparatus, a plurality of color materials (toners) are used, and a full-color image is realized by overlapping and forming an image of a color corresponding to each color material.
FIG. 34 is a schematic view of an image forming apparatus using an electrophotographic process. The photoreceptor drum 1 is formed by applying a photoreceptor on the surface of a conductor, and rotates in the direction of the arrow in the figure. The image forming apparatus forms an image in the following procedure.

(1) The charging unit 2 charges the surface of the photoreceptor to a desired potential.
(2) The exposure unit 3 performs optical writing on the photoconductor to form an electrostatic latent image corresponding to a desired image on the photoconductor.
(3) The developing unit 4 develops the electrostatic latent image formed by the exposing unit with toner to form a toner image on the photoreceptor.
(4) The transfer unit 5 transfers the toner image on the photoreceptor onto a recording sheet 6 such as paper conveyed by a conveyance unit (not shown).
(5) The cleaning unit 7 cleans the toner that is not transferred onto the recording sheet by the transfer unit 5 but remains on the photosensitive member.
(6) The recording sheet 6 to which the toner image is transferred by the transfer unit 5 is conveyed to the fixing unit 8. In the fixing unit 8, the toner is heated and fixed on the recording sheet.

  Since the photosensitive drum rotates in the direction of the arrow in FIG. 34, a desired image is formed on the recording sheet by repeating the steps (1) to (6).

  In many cases, an exposure unit in an image forming apparatus using an electrophotographic process is a method of performing light modulation by making an LD (laser diode) correspond to an output image. Laser light emitted from the LD forms an image on the photosensitive member via a collimating lens, an aperture, a cylindrical lens, a polygon mirror, and an f-θ lens. The polygon mirror is a rotating polygon mirror, and the laser beam scans (main scans) the photosensitive member by this rotation. On the other hand, since the photosensitive member is driven (sub-scanned) in a direction orthogonal to the laser scanning direction by the photosensitive member driving means, the photosensitive member is exposed by laser light to form an electrostatic latent image corresponding to a desired image. It can be formed two-dimensionally on the photoreceptor.

  FIG. 35 is a block diagram showing image processing steps. The image input unit corresponds to a scanner such as a digital copying machine, and sends document reading input image data to the image processing unit. The image processing unit performs various types of image processing such as filter processing, gradation correction processing (γ conversion processing), and pseudo halftone processing on the input image data, and the processing result (output image data) is a video signal processing unit. Hand over to. The video signal processing unit converts the output image data into an image signal, and drives the LD at a predetermined timing. In an image forming apparatus equipped with a plurality of LDs, the video signal processing unit distributes image signals for each LD to be used.

  The above is a monochromatic electrophotographic image forming apparatus. In the case of full color, the tandem has four electrophotographic process units corresponding to CMYK colors (the above-described explanatory diagram corresponds to one process unit). A type electrophotographic apparatus has been proposed. In this apparatus, toner images of CMYK colors are transferred onto a belt-shaped intermediate transfer member and superimposed, and thereafter, toner images for four colors are collectively transferred onto a recording sheet such as paper. The toner image on the recording sheet is fixed on the recording sheet by overheating and pressurizing in the fixing device, and is discharged outside the image forming apparatus.

  In addition to the above-mentioned configuration, a full-color image forming apparatus using a direct transfer method that superimposes CMYK color component toner images sequentially on a recording sheet is proposed. Has been. A revolver-type full-color image forming apparatus is also proposed in which a CMYK color developer is rotatably supported with respect to a single photosensitive drum, and is arranged so as to sequentially face each other.

  The image data input to the image forming apparatus has multi-value data such as 8 bits per pixel in a gradation image such as a photograph. In contrast, in an image forming apparatus (including an electrophotographic system) that forms an image (so-called hard copy) on paper, the number of gradations that can be expressed per pixel is substantially very small. In order to solve such a problem, in a hard copy device, the resolution is improved to 600 dpi, 1200 dpi, etc., and a plurality of pixels are used to modulate the image density in an area to produce a pseudo halftone image. indicate. Image processing performed in the step of converting the input image data into a pseudo halftone image is pseudo halftone processing.

  As the pseudo halftone process employed in the conventional full-color image forming apparatus, the following two pseudo halftone processes have been used. (A) Pseudo halftone processing method in which each color of CMYK called “same angle line screen” has the same angle and the same number of lines, (b) Each color of CMYK called “screen with rotation” has a different screen angle. This is a pseudo halftone processing method having different numbers of lines (for example, see Patent Documents 1 to 4).

  The pseudo halftone process (a) called “same angle screen” has a CMYK color toner formation position that coincides particularly in the formation of secondary and tertiary colors during full color image formation. On the transfer member or on the recording sheet, the toner overlaps the toner. For this reason, there is a problem that transfer dust is likely to occur when the toner is already transferred onto the toner on the intermediate transfer member or onto the toner on the paper. Occurrence of such transfer dust causes a phenomenon such as coloring in an achromatic image (gray image) or coloring by transfer dust toner in an image edge portion (image boundary portion).

  In particular, in the case of a tandem type full-color image, it is difficult to align the CMYK color toner images in the main scanning direction, so that the overlapping position of the CMYK colors in the main scanning direction is not constant and varies. . This deviation in the writing position in the main scanning direction is caused by a slight difference in clock signals (pixel clocks) for each CMYK color, a writing laser wavelength difference, a scanning lens difference, and the like. The problem is that the amount of deviation is different at different locations. Due to the deviation of the writing position, the problem that the color of the output image is visually different becomes remarkable.

  For the above-mentioned problem, (b) pseudo halftone processing called “rotated screen” is effective. In this method, since the screen angles of the CMYK colors are set to be different from each other, the overlapping of the toner images of the respective colors is not constant but random. Therefore, the above-described coloring in the achromatic image by the transfer dust and the writing position deviation are performed. It becomes possible to solve the problem of the change in color due to.

  However, in the above-mentioned (b) “screen with rotation” pseudo halftone process, the pseudo halftone process applied to each color of CMYK often has a different number of lines. The difference in the number of lines for each color of CMYK means that if the difference in the number of lines is about 15%, the difference in resolution for each color cannot be discerned by human eyes. It is not supposed to be.

JP-A-6-98184 Japanese Patent Laid-Open No. 7-212605 JP-A-9-284553 JP 2002-262113 A

  However, in the experiments conducted by the inventors, when the number of lines for each color of CMYK is different, the Raw γ characteristic (the characteristic of the reflection density of the output image with respect to the input data without gradation correction) is not small. It became clear that it was different. This proves to be a problem when 3C gray, which is the tertiary color, is reproduced with high accuracy (increase in gray balance), particularly in the highlight region (brightness 90 to 70). In order to maintain 3C gray satisfactorily in an achromatic state, it is necessary to match the γ characteristics (reflection density characteristics of the output image with respect to input data in a state in which gradation correction is performed) for each color of CMYK with considerable accuracy. For this purpose, it is highly desirable that the above Raw γ characteristics substantially match.

  Furthermore, depending on the type of pseudo halftone processing (such as a line-type growth method in which growth in the XY direction is prioritized), the reflection density increases rapidly with respect to input data such as gradation skip and pseudo contour. In addition, a phenomenon that the appearance of a portion where the reflection density hardly changes is noticeable. In such a case, when the number of lines in the pseudo halftone process is different for each color of CMYK, the place where the above-described gradation jump occurs is different for each color of CMYK. For this reason (the input data corresponding to the location where the gradation skip occurs for each CMYK color is different), in 3C gray that mixes the CMY colors at substantially the same ratio, the input data is simply adjusted to the same value. A phenomenon occurs in which the reflection density of CMYK does not necessarily match at a place where the gradation skip occurs. As a result, the color change is perceived in such a place as the gray balance is lost.

  The gray balance is characterized by a very strict tolerance due to visual characteristics. For this reason, in order not to be recognized as an abnormal image, it is necessary to maintain the gray balance with extremely high accuracy.

  In the full-color image forming apparatus, tone correction (γ conversion) is performed on input image data of each color of CMYK, and then an image is output. This gradation correction is not only correction for color correction but also so-called process control to reduce the influence of changes in toner density and toner charge amount in the development unit due to long-term use. It is also done from the necessity of the general. In particular, in gradation correction for characteristic changes in the latter electrophotographic process, patches of different patterns with different densities are set on the photosensitive member or intermediate transfer member for every fixed number of prints in the image forming apparatus. It is often performed by a method of forming and reading the reflectance of the toner image with a sensor. The reflectance data detected in this way is converted according to the reflection density on the actual recording sheet, fitted with a polynomial, and an approximate curve is obtained. A lookup table calculated from the approximate curve for correction conversion is created and stored. At the time of image output, the input image data is subjected to gradation correction conversion (γ conversion) based on the above lookup table, and then subjected to pseudo halftone processing to generate output image data. I am doing so.

  As described above, the gradation correction based on the characteristic change of the electrophotographic process is performed based on the reflectance from the toner image or the like on the intermediate transfer body while the main body is in operation, so that the reflectance is about 10 patches at most. A look-up table for tone correction conversion is determined based on information from different patches. When performing gradation correction processing (γ conversion) for each color of CMYK based on information from such a small number of patches, it is difficult to make the CMYK γ characteristics coincide with each other in detail. (The reflectance between the patches to be detected is an estimated value (complementary value) calculated from the above approximate expression, and therefore the actual Raw γ characteristic cannot be sufficiently reflected. (If the conversion characteristics cause gradation jumps or sudden gradation changes, the above-mentioned polynomial fitting may not be sufficient.)

  For this reason, when using pseudo halftone processing with different numbers of lines for each color of CMYK, the Raw γ characteristics differ for each color of CMYK, and in particular, gradation skipping as pointed out in the previous section occurs, giving a specific input / output relationship. Is generated for different input values for each color of CMYK, so that the reflection density of each color of CMYK is sufficiently matched even when the above-described gradation correction is performed. However, it is difficult to maintain the gray balance in the highlight portion.

  The present invention has been made in view of the above problems in the prior art, and provides an image forming apparatus and an image forming method capable of reproducing an image with high gray balance and high accuracy even in a 3C gray highlight region. For the purpose.

In order to solve the above problems, cyan (C), magenta (M), yellow (Y), black electrophotographic system performing full-color image formation by superposing a toner image corresponding to the four color components (K) In the image forming apparatus, pseudo halftone processing by line dithering is performed from input image data in the CMYK color space, and an angle α ° (the main scanning direction is the angle 0 ° direction and the counterclockwise direction is the direction of the pixels) From a periodic structure in which a vector A indicating the direction of an angle (positively defined) and a vector B for determining the number of lines are included, and the vector A and the vector B are not orthogonal and have different lengths Of the CMYK colors, the screen angle of the first color is α °, the screen angle of the second color is −90 + α °, the screen angle of the third color is −α °, A pseudo halftone processing device for generating image data in which the screen angle of the color of the color is 90-α °, the angle α is in the range of 25 ° to 32.5 °, and the number of lines of each color of CMYK is the same. An optical writing device that performs optical writing based on image data output from the pseudo halftone processing device, and a developing device that develops the toner image and forms an image corresponding to the optical writing, The image forming apparatus may be configured to include (Configuration A) .

In order to solve the above problems, cyan (C), magenta (M), yellow (Y), black electrophotographic system performing full-color image formation by superposing a toner image corresponding to the four color components (K) In the image forming apparatus, pseudo halftone processing by an error diffusion method having a line dither threshold matrix is performed from input image data in the CMYK color space, and an angle α ° (the main scanning direction is an angle 0 ° direction) And the vector A indicating the direction of the counterclockwise direction (positive angle) and the vector B for determining the number of lines, and the vector A and the vector B are not orthogonal and have a long length. Based on a dither matrix having a periodic structure that has a different relationship, the screen angle of the first color of the CMYK colors is α °, the screen angle of the second color is −90 + α °, and the third Has a screen angle of -α °, a screen angle of the fourth color of 90-α °, the angle α is in the range of 25 ° to 32.5 °, and the number of lines of each color of CMYK is the same. A pseudo halftone processing device that generates image data; an optical writing device that performs optical writing based on image data output from the pseudo halftone processing device; and the toner image corresponding to the optical writing And a developing device that forms an image by developing the image (Structure B) .

Oite before outs imaging equipment, may the halftone processing unit number of lines in the CMYK color image data generated is to be 141 lpi or more.

The image forming apparatus according to the first aspect of the present invention, which is provided to solve the above-described problem, provides a toner image corresponding to four color components of cyan (C), magenta (M), yellow (Y), and black (K). In an electrophotographic image forming apparatus that forms a full-color image by superimposing, pseudo-halftone processing is performed from input image data in the CMYK color space by a line dither method, and an angle of 26.6 ° (main) A vector A indicating the direction of the scanning direction is an angle of 0 ° and the counterclockwise direction is defined as positive) and a vector B for determining the number of lines. The screen angle of the first color of the CMYK colors is 26.6 ° and the screen angle of the second color is based on a dither matrix having a periodic structure that is not orthogonal to B and has a different length. The angle is -63.4 °, the screen angle of the third color is angle -26.6 °, the screen angle of the fourth color is angle 63.4 °, and the number of lines of each color of CMYK is 191.7 lpi A pseudo halftone processing device that generates certain image data; an optical writing device that performs optical writing based on image data output from the pseudo halftone processing device; and the toner corresponding to the optical writing And a developing device for developing the image to form an image.

According to a second aspect of the present invention, there is provided an image forming apparatus according to a second aspect of the present invention, wherein a toner image corresponding to four color components of cyan (C), magenta (M), yellow (Y), and black (K) is provided. In an electrophotographic image forming apparatus that forms a full-color image by superimposing, pseudo halftone processing is performed from the input image data in the CMYK color space by an error diffusion method having a line dither threshold matrix, and a series of pixels As a vector A indicating the direction of the angle 26.6 ° (the angle defined by the main scanning direction being the angle 0 ° direction and the counterclockwise direction being positive) and the vector B determining the number of lines. Based on a dither matrix having a periodic structure in which the vector A and the vector B are not orthogonal and have different lengths, the screen angle of the first color of the CMYK colors is an angle of 26. .6 °, second color screen angle is −63.4 °, third color screen angle is −26.6 °, fourth color screen angle is angle 63.4 °, In addition, a pseudo halftone processing apparatus that generates image data in which the number of lines of each color of CMYK is 191.7 lpi, and an optical writing apparatus that performs optical writing based on the image data output from the pseudo halftone processing apparatus, And a developing device that develops the toner image to form an image corresponding to the optical writing.

According to a third aspect of the present invention, there is provided an image forming apparatus according to a third aspect of the present invention, wherein the first to fourth colors of the image data generated by the pseudo halftone processing apparatus according to the first or second aspect of the present invention are provided. Of these, M and C colors are assigned to two colors having a screen angle difference of 90 °.

The image forming apparatus according to the invention of claim 4 provided in order to solve the above problems, in the invention of claim 1 to 3, wherein the pseudo-halftone processing unit, the number of gradations per pixel located at a resolution 600dpi A pseudo halftone process of 2 bits or more is performed.

The image forming apparatus according to the invention of claim 5, provided in order to solve the above problems, in the invention of claim 1 to 3, wherein the pseudo-halftone processing unit performs a pseudo-halftone processing resolution is more than 1200dpi It is characterized by that.

The image forming apparatus according to the invention of claim 6, provided in order to solve the above problems, claimed in the invention of claim 1 to 5, and detects the toner image formed on the basis of the preset pattern, the detection result And a gradation correction processing device for performing gradation correction processing on the input image data based on the above.

The image forming apparatus according to the invention of claim 7 provided in order to solve the above problems, in the invention of claim 6, wherein the gradation correction processing unit, approximates the detection result of the toner image in the polynomial approximation conversion A gradation correction process is performed by creating a lookup table.

In order to solve the above problems, cyan (C), magenta (M), yellow (Y), black electrophotographic system performing full-color image formation by superposing a toner image corresponding to the four color components (K) In the image forming method, from input image data in the CMYK color space, a vector indicating a direction of an angle α ° (an angle defined as a main scanning direction is an angle of 0 ° and a counterclockwise direction is positive) as a series of pixels. Based on a dither matrix composed of a periodic structure in which A and vector B determine the number of lines, and the vector A and vector B are not orthogonal and have different lengths, The screen angle of the first color is the angle α °, the screen angle of the second color is the angle −90 + α °, the screen angle of the third color is the angle −α °, and the screen angle of the fourth color is the angle 90−α °. And the corner Image data in which the degree α is in the range of 25 ° to 32.5 ° and the number of lines of each color of CMYK is the same is generated by the pseudo halftone process using the line dither method, and is generated by the pseudo halftone process. The image forming method may be characterized in that optical writing is performed based on image data, and the toner image is developed to form an image corresponding to the optical writing (Configuration C) .

In order to solve the above problems, cyan (C), magenta (M), yellow (Y), black electrophotographic system performing full-color image formation by superposing a toner image corresponding to the four color components (K) In the image forming method, from input image data in the CMYK color space, a vector indicating a direction of an angle α ° (an angle defined as a main scanning direction is an angle of 0 ° and a counterclockwise direction is positive) as a series of pixels. Based on a dither matrix composed of a periodic structure in which A and vector B determine the number of lines, and the vector A and vector B are not orthogonal and have different lengths, The screen angle of the first color is the angle α °, the screen angle of the second color is the angle −90 + α °, the screen angle of the third color is the angle −α °, and the screen angle of the fourth color is the angle 90−α °. And the corner Image data in which the degree α is in the range of 25 ° to 32.5 ° and the number of lines of each CMYK color is the same is generated by pseudo halftone processing using an error diffusion method having a line dither type threshold matrix, An image forming method may be provided in which optical writing is performed based on image data generated by pseudo halftone processing, and the toner image is developed to form an image corresponding to the optical writing. (Configuration D) .

Oite before outs imaging how, may the halftone screen ruling of the CMYK colors of the image data generated by the processing is to be 141 lpi or more.

According to an eighth aspect of the present invention, there is provided an image forming method according to an eighth aspect of the present invention, wherein a toner image corresponding to four color components of cyan (C), magenta (M), yellow (Y), and black (K) is obtained. In an electrophotographic image forming method in which a full color image is formed by superimposing, from an input image data in the CMYK color space, an angle of 26.6 ° (main scanning direction is an angle of 0 °, counterclockwise as a series of pixels) The vector A indicating the direction) and the vector B determining the number of lines, and the vector A and the vector B are not orthogonal and have different lengths. Of the CMYK colors, the first color screen angle is 26.6 °, the second color screen angle is −63.4 °, and the third color screen angle is based on a dither matrix having a periodic structure. Image data having an angle of −26.6 °, a screen angle of the fourth color of 63.4 °, and the number of lines of each CMYK color of 191.7 lpi is generated by pseudo halftone processing using a line dither method. Then, optical writing is performed based on the image data generated by the pseudo halftone process, and the toner image is developed to form an image corresponding to the optical writing.

According to a ninth aspect of the present invention, there is provided an image forming method according to a ninth aspect of the present invention, wherein a toner image corresponding to four color components of cyan (C), magenta (M), yellow (Y), and black (K) is provided. In an electrophotographic image forming method in which a full color image is formed by superimposing, from an input image data in the CMYK color space, an angle of 26.6 ° (main scanning direction is an angle of 0 °, counterclockwise as a series of pixels) The vector A indicating the direction) and the vector B determining the number of lines, and the vector A and the vector B are not orthogonal and have different lengths. Of the CMYK colors, the first color screen angle is 26.6 °, the second color screen angle is −63.4 °, and the third color screen angle is based on a dither matrix having a periodic structure. Error diffusion with a line dither type threshold matrix for image data in which the angle of −26.6 °, the screen angle of the fourth color is an angle of 63.4 °, and the number of lines of each CMYK color is 191.7 lpi The image is generated by pseudo halftone processing according to the method, optical writing is performed based on the image data generated by the pseudo halftone processing, and the toner image is developed corresponding to the optical writing to form an image. It is characterized by that.

The image forming method according to the invention of claim 10 provided in order to solve the above problems, according to claim 8 or 9 invention, among the first to fourth color image data generated by the pseudo-halftoning The M color and the C color are assigned to two colors having a screen angle difference of 90 °.

The image forming method according to the invention of claim 11 provided to solve the above-mentioned problem is that, in the inventions of claims 8 to 10 , the pseudo halftone processing has a resolution of 600 dpi and the number of gradations per pixel is 2 bits. The process is performed as described above.

The image forming method according to the invention of claim 12, provided in order to solve the above problems, in the invention of claim 8 to 10, wherein the pseudo-halftone processing, characterized in that the resolution is carried out the processing above 1200dpi .

The image forming method according to the invention of claim 13, provided in order to solve the above problems, in the invention of claim 8 to 12, detects the toner image formed on the basis of the preset pattern, the detection result Based on the above, gradation correction processing is performed on input image data.

The image forming method according to the invention of claim 14, provided in order to solve the above problems, in the invention of claim 13, wherein the gradation correction process, is created by the detection result of the toner image by approximating the polynomial approximation In other words, the tone correction processing is performed using a conversion lookup table.

In the inventions of the configurations A, B, C, and D , “in the four pseudo-halftone processes applied to the CMYK color components, an angle α ° (the main scanning direction is the angle 0 ° direction and the counterclockwise direction) A vector A indicating the direction of the rotation) and a vector B for determining the number of lines, and the vector A and the vector B are not orthogonal and have different lengths. Based on a dither matrix having a certain periodic structure, the screen angle of the first pseudo halftone process is set in the direction of the angle α ° with respect to the angle 0 ° direction, and the screen angle of the second pseudo halftone process is the angle. -90 + α °, the screen angle of the third pseudo halftone process is set to an angle of -α °, the screen angle of the fourth pseudo halftone process is set to an angle of 90-α °, and the angle α is 25 ° to 32.5. ° range ”,“ CMYK for each color Similar LPI halftone processing are completely identical "is characterized.

By adopting such a configuration, compared to a pseudo halftone processing method in which the number of CMYK lines to which a screen angle difference is added, which is one method of a conventional pseudo halftone processing method, which is given a screen angle difference, is different. The γγ curve (relationship of the reflection density of the output image with respect to the input data in a state where gradation correction is not performed) for each color of CMYK is not different. Therefore, it is possible to obtain a Raw γ curve that is substantially the same for each color of CMYK. This is a very great advantage when forming 3C black that overlaps CMY at approximately this ratio. 3C black has a problem that adjustment for maintaining an achromatic color called gray balance is very difficult. This is because a change in gray portion (a change from an achromatic color to a chromatic color) is perceived very sensitively in terms of visual characteristics. For this reason, even when compared with changes in other colors (chromatic colors), the color change in the gray portion is a big problem. In order to deal with such a problem, in the image forming apparatus having the configuration A , the number of lines of the pseudo halftone processing applied to the CMY image data is completely the same, so that the Raw γ characteristics of the CMY plate are substantially matched. Can be done. As a result, when 3C black is formed in which CMY colors are superposed at substantially the same ratio, the gray balance particularly in the highlight portion (lightness range of 90 to 70) can be maintained well.

  Further, in the pseudo halftone process applied to each color component of CMYK, if the configuration is such that the screen angle difference between any two colors is 25 degrees or more, the conventional pseudo halftone is performed. Another pseudo-halftone processing method called “same angle line screen”, which is a CMYK color method, in which all colors have the same angle and the same number of lines, is pointed out in the section of the prior art. An image forming apparatus that does not cause a problem can be realized. Specifically, (a) since the toner formation positions are the same for each color of CMYK, transfer dust is generated during transfer as the toner adhesion amount increases, and this portion is visually colored. The problem of being perceived as an abnormal image, (b) Especially in a tandem machine, since the positional accuracy of optical writing in the main scanning direction is lowered, the overlap of CMYK is not the same even in one sheet, and the corresponding color The problem of being perceived as a change in the number of occurrences does not occur.

  Further, as described above, if the screen angle difference between any two colors is 25 degrees or more, secondary colors called red moire (red (R), blue (B), green ( G), a color expressed using two color components of CMYK color components, etc.), a tertiary color (such as 3C black), and the like. It can be completely eliminated.

In addition, according to experiments conducted by the inventors, this color moire has a different degree of occurrence depending first on the screen angle, and it is dragged to the plate with the lowest number of lines of color components to be superimposed. There is a tendency to end up. In other words, the low frequency interference pattern is dependent on the plate with the lowest number of lines. As a result, from the viewpoint of color moire, if only one component of the CMYK color components has a low number of lines, the color moire can be sufficiently reduced even if the number of other components is increased. is not. Since the Y component and the like are formed in a region having a high brightness, the structure of the pseudo halftone process is not noticeable. For this reason, the number of lines of the Y component may be lowered as compared with other color components. However, in such a case, the color moiré is dragged by the Y component having the lowest number of lines, and an interference pattern appears. Therefore, it is sufficiently efficient that the number of lines differs for each color component of CMYK. That's not true. On the other hand, in the configuration A , since the number of lines of each color component of CMYK is completely the same, it can be seen that the combination can reduce the generation of color moiré most efficiently. (Especially in the electrophotographic method, it is desirable to perform pseudo halftone processing with a low number of lines when aiming at good gradation expression. For this reason, considering the balance with gradation reproduction, the above color moiré is reduced. (It is desirable to keep the number of lines as low as possible within the range to be eliminated.)

Here, in the image forming apparatus in which each pixel is formed in a mesh shape, such as the LD raster method or the LED writing method, by using the configuration of the configurations A, B, C, and D , the above-described configuration is used. Since the CMYK color component pseudo-halftone process has the screen angle relationship, the number of lines of each CMYK color can be completely matched. Further, by limiting the angle α within the above range, the screen angle difference between any two colors can be set to 25 degrees or more.

In the first, second, eighth , and ninth aspects of the invention, in the four pseudo-halftone processes applied to the CMYK color components, an angle of 26.6 ° (the main scanning direction is the angle 0 ° direction) The vector A indicates the direction of the counterclockwise direction (an angle defined as positive) and the vector B that determines the number of lines. The vector A and the vector B are not orthogonal and have different lengths. Based on the dither matrix having the related periodic structure, the screen angle of the first pseudo halftone process is set in the direction of the angle 26.6 ° with respect to the angle 0 ° direction, and the second pseudo halftone process is performed. “The screen angle is −63.4 °, the screen angle of the third pseudo halftone process is −26.6 °, and the screen angle of the fourth pseudo halftone process is the angle 63.4 °”, “CMYK The number of pseudo-halftone processing lines for each color is 19 .7 is a lpi "it is a feature.

Further, in addition to the configuration requirements of the inventions of configurations A, B, C, and D , the number of lines of pseudo halftone processing may be 141 lpi or more.
According to experiments conducted by the inventors, it is apparent that the object of the present invention can be achieved without generating color moire in two colors and tertiary colors if the number of lines is 141 lpi or more. Became. On the other hand, when the number of lines in the pseudo halftone process is lower than 141 lpi, the color moire is not perceived, but the structure of the pseudo halftone process in K color or M color with strong brightness contrast. Has been perceived. For this reason, for the number of lines lower than 141 lpi, the periodic structure due to pseudo halftone processing in K color or M color single color is perceived, so the effect of eliminating color moiré is not fully utilized. Think of it as an area. The periodic structure is perceived in the K color and the M color in the edge portion, contour portion, fine line, and periodic image of the image. Due to these adverse effects, it is considered that the image quality cannot be sufficiently improved by eliminating the color moire.

In the present invention of claim 3 and 10, in addition to the configuration requirements of the claims 1, 2, 8 and 9, the first to fourth components of the pseudo halftone process, two pseudo intermediate angle 90 degrees The feature is that M color and C color are assigned to the tone processing.

  In a full-car image forming apparatus that creates an image by superimposing CMYK color toners, particularly in a dry electrophotographic method using toner mainly composed of a powdered resin, secondary colors (red (R), In the reproduction of green (G) and blue (B), there is a drawback that it is difficult to reproduce a color with high brightness and high saturation. This is due to the fact that the ideal secondary color is not realized in the part where the toner is superimposed on the toner during color superimposition (in the toner overlapping part, the melting at fixing is performed). This is considered to be caused by the deterioration of the state compared to the case of only one color and the increase in the thickness of the toner layer. In other words, compared to the case where the secondary color is reproduced without forming the overlapping portion of the toner, the case where the secondary color is reproduced by superimposing the toner has a greater decrease in lightness and a visually turbid color. Will be perceived as.

In order to maintain the lightness of such secondary colors at a high level, it is desirable to superimpose monochromatic components so that the overlap of each color is minimized for the above reasons. From this point of view, line-type pseudo halftone processing Then, it is most desirable that the screen angle difference is 90 degrees. In addition, a hard copy output device such as an electrophotographic method has a disadvantage that a color reproduction range of blue (B) is lower than that of a display device such as a CRT display. For this reason, it is desirable to make the blue color reproduction range as wide as possible. In Claims 3 and 10 , since the screen angle difference between the C color and the M color forming the blue color is set to 90 degrees, it is possible to prevent a decrease in the brightness of the blue color and to improve the color reproduction range. The effect of being able to keep is acquired.

The pseudo halftone process may be a dither method.
By performing pseudo halftone processing by the dither method, it becomes possible to perform pseudo halftone processing at high speed while maintaining a level of image quality that does not cause any practical problems. In the present invention, it is possible to obtain an effect that pseudo halftone processing can be performed at a high speed (much faster than the dither threshold type error diffusion method described below).

Further, in claim 1, 8, is characterized halftoning is line screen type dither method.

The single-line type has a feature that the toner adhesion area can be widened as compared with the dither processing of the one-pixel priority growth type. This has the advantage that the thickness of the toner layer adhering to the toner adhering region can be relatively reduced for the reason pointed out in the third aspect . In the line type, a toner image having a relatively low layer (a toner layer having a small thickness) is formed as compared with the one-pixel priority growth type. When the toner layer is formed thin, there is no adverse effect such as a decrease in lightness of the secondary color caused by the deterioration of fixing and melting due to an increase in toner thickness, as pointed out in the third aspect . For this reason, it becomes possible to avoid the problem of giving the impression that the output image is cloudy.
In the first and eighth aspects, for the reasons described above, an effect can be obtained in which an output image maintaining a high brightness can be obtained particularly in a secondary color.

  Alternatively, the pseudo-halftone process may be an error diffusion method having a dither threshold matrix.

  In error diffusion processing with a dither-type threshold matrix, compared to the dither method, the shakiness of the edge portion is improved, and extremely high light images (images of thin and thin characters, low contrast images, etc. are called) )) Is improved. On the other hand, there are problems such as color moiré that does not appear in the normal error diffusion method, and dithering methods in which the Raw γ characteristics due to the difference in the number of lines of the dither threshold are different for each CMYK color.

Such a problem caused by the difference in the color moire and the number of dither threshold lines can be solved for the reason described in the configuration A. As a result, the problems that cannot be solved by the above-mentioned normal dither method (such as rattling of the edge portion) are solved, and furthermore, the problems caused by the difference in the number of lines of color moire and dither threshold are solved. The device can be realized.
According to the second and ninth aspects of the invention, pseudo halftone processing is performed by the error diffusion method having the dither type threshold matrix according to the first aspect, and error diffusion having the dither type threshold matrix is performed according to the eighth aspect. The effect of combining the effects of the invention of performing pseudo halftone processing by the method can be obtained.

According to the fourth and eleventh inventions, according to the experiment conducted by the inventor, when the pseudo halftone process has a resolution of 600 dpi and is applied to a pseudo halftone process of 2 bits or more per pixel, the configuration A It has been clarified that the described effects are sufficiently exerted.

  On the other hand, it has been clarified that the above-mentioned effects are not sufficiently exhibited with a resolution of 600 dpi / 1-bit dither. In a dither process such as a resolution of 600 dpi / 1 bit, a density change at a low frequency occurs due to the basic number of lines. This is because in the case of dithering with a small number of bits per pixel (for example, 1 bit), a multi-gradation number such as 256 gradations is expressed by combining a plurality of basic units (unit cells). It is. (The dither matrix in which a plurality of basic units are combined is called a super matrix (mother matrix), and the basic unit is called a sub-matrix.) This is because the number of gradations that can be expressed by only the basic unit is 256 gradations or less. Such a need arises. For this reason, a difference in density occurs in each basic unit, and this density change occurs across a plurality of basic units. Therefore, in addition to the number of dither lines, a low line number (low frequency) component appears.

In addition, the low line number component described above has a lower frequency than the basic line number of dither in the spatial frequency, and the dither screen angle in the direction (corresponding to the dither screen angle). Does not match. For this reason, even when the dither screen angle is set as in claim 1 (claim 8 ) or claim 2 (claim 9), since the angle is different for the low line number component, CMYK As a result of the interference of the low linearity components of each color component, a problem has been revealed that a visual periodic structure similar to that of color moire is perceived.

  According to the results of experiments conducted by the inventors, if 600 dpi / 4-bit dither and 600 dpi / 2-bit dither are used, the contribution of the low line number component due to the formation of the sub-matrix of the above-mentioned dither is not large, so color moiré is reduced. The object of the present invention that can be solved can be achieved. On the other hand, with 600 dpi and 1-bit dither, the phenomenon such as the color moire due to the low line number component as described above cannot be ignored, and the effect of eliminating the color moire which is the object of the present invention is not sufficiently exhibited. The effect will be halved.

In the fifth and twelfth inventions, when the resolution is changed to 1200 dpi for the same reason as the fourth and eleventh inventions, an experiment similar to the experiment described in the fourth and eleventh aspects is performed, and the resolution is 1200 dpi. If the number of lines is the same, the number of gradations that can be expressed in the basic unit (unit cell) is about four times, so even if the number of gradations per pixel is smaller than 600 dpi. The influence of the interference of the low line number component is reduced.
Further, it has been clarified that when the resolution is 1200 dpi or more, the effect of eliminating the color moire is sufficiently exhibited when the number of gradations per pixel is 1 bit or 2 bits.

According to the sixth and thirteenth aspects of the present invention, when gradation correction processing is performed on input data, a toner image formed based on a preset pattern is detected, and gradation correction processing is performed based on the detection result. It is a feature to perform.

  In an electrophotographic image forming apparatus, gradation correction is performed by the method described in the section of the prior art. As one of them, a preset toner image is detected, and the detection result is reflected on the output image. A method of estimating and calculating the relationship (Rawγ characteristic) between the input data and the reflection density of the output image in correspondence with the density is used. Based on this result, a target output image is obtained by performing gradation correction on the input data. When calculating the Raw γ characteristics, the input / output relationship is calculated from an approximate curve by the least square method or the like, or the estimation is performed by the interpolation method. Making a decision.

  At this time, ideally, the above-described gradation correction may cause the relationship between the input data and the reflection density of the output image (γ characteristic after gradation conversion) to match the target γ characteristic (target γ characteristic). it can. However, in practice, since correction of all input data is determined based on the results obtained from limited data as described above, the values of the points derived by estimation without actually collecting data are true. Does not necessarily match the value of. In other words, the input / output relationship can be matched with the target γ characteristic in the rough part, but the fine part cannot be matched completely. Also, due to such circumstances, it is not possible to completely perform such a change in a gradation jump or a sudden change in the gradation so as to match the target γ characteristic.

In the invention of claim 6, 13, claims 1, 2, 8 and 9 of the invention Like, since the number of lines of the pseudo-halftone processing are perfectly matched for respective CMYK color components are substantially same Rawγ characteristics I already explained that you can. This brings about a great effect in the case where gradation correction is performed as described above. In other words, as explained in the previous paragraph, even when performing gradation correction, it is almost impossible to make the γ characteristic after gradation conversion completely match the target γ characteristic due to restrictions on the number of data collected. Is possible. At this time, if the number of lines in the pseudo halftone process is completely the same, even if the converted γ characteristic cannot be matched with the target γ characteristic as described above, the amount of deviation between the two (target The deviation of the γ characteristic after gradation conversion from the γ characteristic is substantially the same for each color component of CMYK. This means that when the colors of CMYK after gradation conversion are overlapped, the gray balance is not lost (gray coloring). This effect is particularly significant in gray balance in the highlight portion (brightness 90 to 70 region). In the electrophotographic process, it has been clarified that the input / output relationship (Rawγ characteristic) in the highlight part becomes a curve showing a rapid density change, and the gray balance in this part is maintained well. It is because it becomes possible to do.

On the other hand, when the number of lines in each color of CMYK is different, the difference between the γ characteristic after gradation conversion and the target γ characteristic when gradation correction is performed as described above is different for each CMYK color component. Different. In particular, in a place where the concentration change is abrupt, such a problem becomes conspicuous because the target and the γ characteristic cannot be sufficiently matched. Furthermore, when the Raw γ characteristic has a portion that changes abruptly, such as a highlight portion, and the RAW γ characteristic is different for each color of CMYK, the function form of the approximate expression may be greatly different for each CMYK. In some cases, the difference between the γ characteristic after gradation conversion and the target γ characteristic becomes larger than that before gradation conversion. Thus, when the number of lines of CMYK colors is different, there remains a problem that it is difficult to maintain gray balance.
In the inventions of claims 6 and 13 , such a problem does not occur, and the effect that the gray balance in the highlight portion can be maintained well is obtained.

According to the seventh and fourteenth aspects of the present invention, approximation by polynomial approximation is performed in the step of gradation correction, and a conversion lookup table or the like is determined using this polynomial approximation formula or the like. Compared to other methods, the derivation of the approximate expression by polynomial approximation can be performed by simple calculation, which has the advantage of reducing the calculation load. However, when fitting with actual γ characteristics by polynomial approximation, the amount of deviation is large, and it is not sufficient to match the γ characteristics after gradation conversion with the target γ characteristics.

Therefore, for the same reason as the invention of claim 6 and 13, in the invention of claim 7, 14, the difference between the γ characteristics and the target γ characteristics after the gradation conversion for each CMYK colors even when using the polynomial approximation Since they substantially match, the gray balance can be maintained well. Further, there is an advantage that a conversion lookup table for gradation conversion can be derived at high speed.

A first embodiment of an image forming apparatus according to the present invention will be described below.
FIG. 1 is a schematic view of an image forming apparatus according to the present invention.
The image forming apparatus 10 is a full-color image forming apparatus that forms an image by superimposing four color component images of cyan (C), magenta (M), yellow (Y), and black (K) on a recording sheet. . Here, four image forming units 31 are arranged as shown in FIG. 1 corresponding to each color component of CMYK. Each color component image formed by each image forming unit 31 is sequentially transferred to a belt-like intermediate transfer member (intermediate transfer belt) 41 disposed in contact with the four image forming units 31. Since the intermediate transfer member 41 is rotated at a predetermined timing by a driving unit (not shown), each color component image is superimposed and formed on the intermediate transfer member 41 at a predetermined position. The respective color component images superimposed on the intermediate transfer body 41 are collectively transferred onto a recording sheet to become an image on the recording sheet.

  Each of the monochromatic image forming units 31 includes a photosensitive drum, a charger for charging the photosensitive drum to a desired potential, and image data for output (pseudo halftone processing is performed on the photosensitive drum charged to the desired potential. A laser optical unit 21 for writing the image data), a developing unit for developing the electrostatic latent image formed on the photosensitive drum by the writing by the laser optical unit 21 with toner corresponding to each color component, and a developing unit. Formed from a transfer device for transferring the toner image developed on the photosensitive member onto the intermediate transfer member 41, and a cleaner for cleaning untransferred toner remaining on the photosensitive member without being transferred to the intermediate transfer member 41. Is done.

  A recording sheet such as paper is conveyed by a conveying means from a recording sheet bank (not shown) and then conveyed to the secondary transfer unit 43 at a predetermined timing by a registration roller 42. In the secondary transfer unit 43, the toner image (four-color component toner image) on the intermediate transfer member 41 is transferred to a desired position on the recording sheet. The recording sheet to which the toner image has been transferred is heated and pressurized by the fixing device 44 and discharged outside the apparatus.

  The laser optical unit 21 performs pulse width modulation (PWM) control corresponding to the output image data of each color component, and performs optical modulation driving of the laser. The output image data is subjected to color correction processing and gradation correction processing (γ conversion) on the input data, and then subjected to pseudo halftone processing. It is supposed to be transferred to.

In the optical unit 21, the writing optical system may be an LED writing optical system instead of the laser system.
The LED writing optical system has advantages such as a reduction in noise due to the absence of a driving unit such as a polygon mirror and a reduction in space by reducing the size of the optical unit itself, as compared with the laser system.

Next, description will be given of image processing performed when the output image data is created from the input image data.
Since the image forming apparatus of FIG. 1 is a so-called laser printer type, it is assumed that the input image data is a multi-value (8-bit) image from a personal computer or the like (in the case of a digital copying machine or the like, a scanner that reads a document is used). (It is assumed that input data is sent from this scanner unit.)

The input image data is emphasized by the MTF filter processing unit 12 in the image processing unit, and then the color correction / gradation correction processing unit (γ conversion) 13 performs color conversion from the RGB color space to the CMYK color space. Then, density control for realizing a preset gradation is performed. Next, pseudo halftone processing is performed by the pseudo halftone processing unit 14 so as to match the printer characteristics, and is delivered to the image output side (laser light modulation driving side) as output image data.
Since the MTF filter process, the color correction process and the γ correction process are the same as those in the prior art, detailed description thereof is omitted here.

Next, the dither method (multi-value dither method) which is a pseudo halftone process in the present invention will be described.
Here, the data after performing the pseudo halftone process is a 4-bit dither whose data is 4 bits (16 values). In the 4-bit dither, each pixel of 8-bit data that is an input image (each pixel is expressed by 256 gradations from 0 to 255) is converted into output image data expressed by 16 gradations from level 0 to level 15. . This conversion is performed by comparing the gradation value (256 gradations) of each pixel of the input image data with a threshold value set in advance to the 16 gradation levels so that each pixel of the input data is level 0 to level. It is determined which of 15 levels it belongs to. That is, the 4-bit dither matrix is formed by 15 matrices in which threshold values are described. The calculation of the output image data by the dither method may be a conventionally known method, for example, the method described in paragraphs 0036 to 0040 of Japanese Patent Laid-Open No. 2000-297883.

Next, the dematrix in the image forming apparatus of the present invention will be described.
2 and 3 show the cyan dither matrix. This dither matrix has a matrix size of 14 × 14 and a line of one pixel connected in the 63.4 degree direction grows from level 0 to level 15, and then the adjacent line grows from level 0 to level 15, This is a dither matrix of a growth method in which this is repeated (here, the angle indicating the series of pixels is an angle calculated counterclockwise with the main scanning direction (rightward in the horizontal direction in this document) being 0 degrees. ).

  In the description with reference to FIGS. 2 and 3, the matrix size is exemplified as a size of 7 × 7. This is because the minimum cycle unit is 7 × 7 in the dither matrix of the first embodiment. In practice, dither processing is performed using a dither matrix having a matrix size of 14 × 14. However, by increasing the matrix size, the gradation steps of the output image data after dither processing are as uniform as possible (the dither structure is the same). The purpose is to be a growth form that is as invisible as possible. In the following, when the dither matrix is exemplified, the explanatory diagram becomes large, and therefore, the dither matrix in the minimum cycle unit is exemplified.

  4 and 5 show a magenta dither matrix. The matrix size is a dither matrix that is a growth method connected in the direction of −26.6 degrees with a matrix size of 14 × 14. The dither matrix of the yellow plate is shown in FIGS. 6 and 7, and the matrix size is 14 × 14 and is a dither matrix that is a growth method connected in a 26.6 degree direction. The dither matrix of the black plate is shown in FIGS. 8 and 9, and the dither matrix is a growth method in which the matrix size is 14 × 14 and is continuous in the −63.4 degree direction.

  The CMYK dither matrix has the same number of lines and is 191.7 line / inch. When the number of lines is 191.7 lpi, the basic unit (unit cell) of the dither matrix is 7 pixels because of the periodic structure of dither. For this reason, if the matrix size is 14 × 14 dither matrix, 28 basic units are included. That is, it is a dither matrix having a structure including 28 sub-matrices in a 14 × 14 matrix.

  FIG. 10 is a diagram showing the screen angle of the dither matrix for each of the CMYK colors. As is apparent from FIG. 10, in the present embodiment, it can be seen that the condition that the screen angle difference between any two CMYK dither matrices is 25 degrees or more is satisfied. Here, the combination with the smallest angle difference is a combination of the C version and the Y version, and the M version and the K version, and the angle difference of 36.8 degrees is the combination with the smallest angle difference. It can be seen that the above conditions are satisfied.

  The CMYK dither matrix is a multi-line dither matrix. The line type is one form of pseudo halftone processing, also called a line screen, and is a pseudo halftone processing method in which a series of pixels gradually grow.

  Note that when the dither matrix is created, the screen angle may be changed to an angle other than that shown above. Also, the number of dither gradations and resolution may be changed.

  Further, the image forming system to which the present invention is applied may be the tandem type direct transfer system shown in FIG. 11 or the revolver type image forming system shown in FIG.

A second embodiment of the image forming apparatus according to the present invention will be described.
Here, except for the dither matrix, other configurations are the same as those in the first embodiment.

  FIGS. 13 and 14 show a C-size dither matrix according to the second embodiment. It can be seen that the growth method is different from that of the C version dither matrix (FIGS. 2 and 3) of the first embodiment. In the first embodiment, when a row of pixels connected in the 63.4 degree direction expresses a larger gradation value, a growth type (line-type growth) in which the pixels connected in a row grow simultaneously. Dither matrix.

  In contrast, the second embodiment (FIGS. 13 and 14) is a growth type dither matrix in which the growth of the next pixel starts after the growth of one pixel is completed. At this time, all the basic units (unit cells, which are formed from the pixels of the seventh embodiment) are of a growth type that grows almost simultaneously. In the present invention, such a growth type is hereinafter referred to as “one-pixel priority type growth”. In the present invention, the other MYK dither matrices are the same one-pixel preferential growth type dither matrix as in FIGS. 13 and 14, and the respective screen angles are the same as the dither matrix in the first embodiment. The M version is a dither matrix connected in the direction of −26.6 degrees, the Y version is in the direction of 26.6 degrees, and the K version is in the direction of −63.4 degrees (the CMYK version is not shown).

Next, a third embodiment of the image forming apparatus according to the present invention will be described.
The present invention is an image forming apparatus that performs pseudo halftone processing (dither threshold error diffusion processing) by an error diffusion method having a dither type (line dither type) threshold matrix. Here, only the configuration related to the pseudo halftone processing is different from that of the first embodiment, and the other configuration is the same as the configuration and operation.
In a generally well-known error diffusion process, the threshold value is a constant value, whereas in the dither threshold error diffusion process, there is a difference in the threshold value, and a dither-type threshold matrix is used as the threshold value. For example, it is also possible to use the dither matrix shown in the first embodiment as it is as a threshold value and use it as a threshold matrix for dither threshold error diffusion processing.

  In the image forming apparatus according to the present invention, the pseudo halftone processing unit 14 in FIG. 1 becomes the error diffusion processing unit 34, and the error diffusion processing unit 34 includes an addition unit 341, a quantization unit 342, and an error calculation unit as shown in FIG. 343, an error storage unit 344, and a correction value calculation unit 345.

  FIG. 16 is a diagram showing a weight matrix used in the calculation in the correction value calculation unit 343 shown in FIG. Here, the numerical values shown in FIG. 16 mean the error weight in the corresponding pixel, and the mark * represents the target pixel. Note that various configurations are possible for the weight matrix, and the configuration is not limited to the configuration shown in FIG. 16 (weight matrix A). The weight matrix is stored in advance in a storage unit built in the correction value calculation unit 343.

Next, the pseudo halftone processing step in the present invention will be described in detail. In the present invention, dither threshold value pseudo halftone processing is performed by an error diffusion method to generate output image data.
That is, in FIG. 15, the correction value calculation unit 343 reads the error e _{x + j, y + j} of the quantized pixel from the error storage unit 344 and reads the weight matrix corresponding to this error from the built-in storage unit. . Then, the error e _{x + j, y + j} and the corresponding weight matrix are integrated for each pixel as described below, the product for each pixel is added, and the correction value E is calculated by dividing by the sum of the weights of the weight matrix. _{x and y} are calculated.

Here, the correction value E _{x, y} can be expressed by the following equation when developed using the weight matrix shown in FIG. 16 as an example.

Next, the input image data (the data after the filter processing and the color / gradation correction processing and the 8-bit data in the first embodiment) d _{x, y} and the correction value E _{x, y} are added by the adder 341 to perform correction. Pixel data D _{x, y} is output.

Next, the correction pixel data D _{x, y} is quantized by the quantization unit 342. For example, under the conditions in the first embodiment, the quantization number is 2, and the output image data O _{x, y} takes a value of 0 or 1. At this time, the quantization unit 342 determines the output image data O _{x, y} to be 0 or 1 by comparing the corrected pixel data D _{x, y} with the threshold Th. In the present invention (dither threshold error diffusion processing), the dither matrix described in the first embodiment is used as the threshold value matrix as the threshold value Th. As a result, dither-like error diffusion processing can be realized. Although the threshold matrix in the present invention is as described above, there is no problem even if another threshold matrix is used. In addition to the above-described dither matrix, a dither matrix having the dither matrix characteristics described in the present invention can be used as a threshold value matrix for error diffusion processing.

Further, in response to the result of quantization in the quantization unit 342, the error calculation unit 345 outputs the corrected pixel data D _{x, y} , output image data O _{x, y,} and a predetermined value B (first implementation). Since the quantization number is 2 in the condition of the form, the error e _{x, y} in the target pixel is calculated by the following equation according to 255).

e _{x, y} = D _{x, y} −O _{x, y} × B

Under the conditions of the first embodiment, the quantization number is 2, but another value may be used. In the normal multilevel error diffusion method, the value of B is a quantization number of the output image data O _{x, y} between 0 and the maximum value of the input image data (8 bits = 255 in the condition of the first embodiment). The value is equally divided according to the value, but may be an arbitrary value. In that case, a look-up table or the like in which a value corresponding to the output image image data O _{x, y} is previously determined and stored is used.

In this way, the calculated error e _{x, y} is newly stored in the error storage unit 344 and used later in calculating the correction value. In the error diffusion process, output image data (quantized data) is calculated by sequentially performing such a procedure on each pixel of the input image data.

Next, a fourth embodiment of the image forming apparatus according to the present invention will be described.
Here, in the image forming process, the configuration and operation are exactly the same as those in the first embodiment.
In the present invention, pseudo halftone processing is performed after tone correction is performed on an input image. In this tone correction, a gradation level based on detection data such as a toner image formed on an intermediate transfer member is used. The tone correction is different from the first embodiment.

  In the present invention, a toner image corresponding to a patch image having an image pattern stored in advance is formed on the intermediate transfer member, and the toner image is read by an optical sensor. Next, the output value from the optical sensor is converted into the reflection density value of the output image by using the corresponding relational expression between the output value of the optical sensor and the reflection density value of the output image. From this converted value, the relationship (Raw γ characteristic) with the reflection density value of the output image with respect to the input data at this time (without gradation correction) is found. Next, the inverse function of the Raw γ characteristic is calculated by a fourth-order polynomial approximation (least square method). Using the quartic polynomial determined in this way, a one-dimensional lookup table for gradation correction is created and stored from the target γ characteristic (target γ characteristic) stored in advance. At the time of image output, tone correction is performed using the lookup table created in this way, and then pseudo halftone processing is performed to output the image.

  Note that the apparatus configuration necessary for performing the gradation correction and the method for determining the conversion of the gradation correction may be a conventionally known technique. For example, Japanese Patent Laid-Open No. 9-230644 (detection from a toner image on a photoreceptor) ), And JP-A-9-233350 (detection from an output image).

An example in which the present invention is actually applied will be described below.
First, the toner used in this embodiment will be described. The toner used here is a so-called polymerized toner prepared by the following procedure.

(1) Synthesis of resin fine particle emulsion In a reaction vessel equipped with a stirrer and a thermometer, 683 parts of ion-exchanged water and 11 parts of sodium salt of ethylene oxide methacrylate adduct sulfate (Eleminol RS-30: manufactured by Sanyo Chemical Industries) When 83 parts of styrene, 83 parts of methacrylic acid, 110 parts of butyl acrylate and 1 part of ammonium persulfate were added and stirred at 400 rpm for 15 minutes, a white emulsion was obtained. The system was heated to raise the system temperature to 80 ° C. and reacted for 5 hours. Further, 30 parts of a 1% ammonium persulfate aqueous solution was added dropwise, and the mixture was aged at 80 ° C. for 7 hours, and the vinyl resin (styrene-methacrylic acid-butyl acrylate-methacrylic acid etherene oxide adduct sulfate sodium salt was added. Polymer) aqueous dispersion [fine particle dispersion 1] was obtained. The volume average particle diameter of [fine particle dispersion 1] measured by LA-920 was 0.09 μm. A portion of [Fine Particle Dispersion 1] was dried to isolate the resin component. The resin content Tg was 58 ° C.

(2) Preparation of aqueous phase 1000 parts of ion-exchanged water, 83 parts of [fine particle dispersion 1], 37 parts of a 48.5% aqueous solution of dodecyl diphenyl ether disulfonate (Eleminol MON-7: manufactured by Sanyo Chemical Industries), 90 parts of ethyl acetate was mixed and stirred to obtain a milky white liquid. This is referred to as [Aqueous Phase 1].

(3) Synthesis of low molecular weight polyester In a reaction vessel equipped with a cooling tube, a stirrer, and a nitrogen introduction tube, 229 parts of bisphenol A ethylene oxide 2 mol adduct, 529 parts of bisphenol A propylene oxide 3 mol adduct, terephthalic acid 208 , 46 parts of adipic acid and 2 parts of dibutyltin oxide, reacted at 230 ° C. for 8 hours at normal pressure, and further reacted for 5 hours at a reduced pressure of 10-15 mmHg, and then added 44 parts of trimellitic anhydride to the reaction vessel. The mixture was reacted at 180 ° C. and normal pressure for 2 hours to obtain [Low molecular polyester 1]. [Low molecular polyester 1] had a number average molecular weight of 2500, a weight average molecular weight of 6700, and a Tg of 43 ° C.

(4) Synthesis of prepolymer having isocyanate group In a reaction vessel equipped with a cooling pipe, a stirrer and a nitrogen introduction pipe, 682 parts of bisphenol A ethylene oxide 2-mol adduct and 81 parts of bisphenol A propylene oxide 2-mol adduct 283 parts of terephthalic acid, 22 parts of trimellitic anhydride and 2 parts of dibutyltin oxide were reacted at 230 ° C. at normal pressure for 8 hours, and further reacted at reduced pressure of 10-15 mmHg for 5 hours to obtain [Intermediate polyester 1]. It was. [Intermediate Polyester 1] had a number average molecular weight of 2,100, a weight average molecular weight of 9,500, Tg of 55 ° C., an acid value of 0.5, and a hydroxyl value of 51.
Next, 410 parts of [intermediate polyester 1], 89 parts of isophorone diisocyanate, and 500 parts of ethyl acetate are placed in a reaction vessel equipped with a cooling pipe, a stirrer, and a nitrogen introduction pipe, and reacted at 100 ° C. for 5 hours. Polymer 1] was obtained. [Prepolymer 1] had a free isocyanate weight% of 1.53%.

(5) Synthesis of ketimine In a reaction vessel equipped with a stirrer and a thermometer, 170 parts of isophoronediamine and 75 parts of methyl ethyl ketone were charged and reacted at 50 ° C. for 5 hours to obtain [ketimine compound 1]. The amine value of [ketimine compound 1] was 418.

(6) Preparation of pigment masterbatch Add 1200 parts of water, 540 parts of carbon black (Printex60 Dexa), 1200 parts of low molecular weight polyester 1 and mix with a Henschel mixer (Mitsui Mining Co., Ltd.). The mixture was kneaded at 130 ° C. for 45 minutes, rolled and cooled, and pulverized to 1 mmφ or less with a pulverizer to obtain [Masterbatch 1].

(7) Preparation of oil phase In a container equipped with a stir bar and a thermometer, 378 parts of [low molecular weight polyester 1], 110 parts of synthetic ester wax, 22 parts of CCA (salicylic acid metal complex E-84: Orient Chemical Industry), acetic acid 947 parts of ethyl was charged, heated to 80 ° C. with stirring, maintained at 80 ° C. for 5 hours, and then cooled to 30 ° C. over 1 hour. Next, 500 parts of [Master batch 1] and 500 parts of ethyl acetate were placed in a container and mixed for 1 hour to obtain [Raw material solution 1].
[Material solution 1] 1324 parts were transferred to a container, and using a bead mill (Ultra Visco Mill, manufactured by Imex Co., Ltd.), a liquid feed speed of 1 kg / hr, a disk peripheral speed of 6 m / sec, and 0.5 mm zirconia beads were 80% by volume. Carbon black and WAX were dispersed under conditions of filling and 3 passes. Next, 1324 parts of a 65% ethyl acetate solution of [low molecular weight polyester 1] was added, followed by one pass with a bead mill under the above conditions to obtain [Pigment / WAX Dispersion 1]. The solid content concentration of [Pigment / WAX Dispersion 1] (130 ° C., 30 minutes) was 50%.

(8) Emulsification / desolvation
650 parts of [Pigment / WAX Dispersion 1], 140 parts of [Prepolymer 1] and 6.0 parts of [Ketimine Compound 1] were put in a container and mixed with a TK homomixer (manufactured by Tokushu Kika) at 5000 rpm for 1 minute. Thereafter, 1200 parts of [Aqueous Phase 1] was added to the container, and mixed with a TK homomixer at 13,000 rpm for 20 minutes to obtain [Emulsified Slurry 1].
Next, [Emulsion slurry 1] was put into a container in which a stirrer and a thermometer were set, and after removing the solvent at 30 ° C. for 8 hours, aging was performed at 40 ° C. for 8 hours to obtain [Dispersion slurry 1]. .

(9) Cleaning and drying
[Emulsified slurry 1] After 100 parts of vacuum filtration,
(I): 100 parts of ion-exchanged water was added to the filter cake, mixed with a TK homomixer (rotation speed: 12,000 rpm for 10 minutes), and then filtered.
(Ii): 100 parts of a 10% aqueous sodium hydroxide solution was added to the filter cake of (i) above, mixed with a TK homomixer (30 minutes at 12,000 rpm), and then filtered under reduced pressure.
(Iii): 100 parts of 10% hydrochloric acid was added to the filter cake of (ii) above, mixed with a TK homomixer (10 minutes at 12,000 rpm), and then filtered.
(Iv): Add 300 parts of ion-exchanged water to the filter cake of (iii) above, mix with TK homomixer (rotation speed 12,000 rpm for 10 minutes) and then filter twice to perform [Filter cake 1]. Obtained.

[Filtration cake 1] was dried at 45 ° C. for 48 hours in a circulating drier, sieved with a mesh of 75 μm, and then hydrophobic silica (hexamethyldisilazane surface-treated product, specific surface area) was added to 100 parts of toner particles. and 200m ² /g)0.5 parts, hydrophobized rutile-type titanium oxide (isobutyltrimethoxysilane surface treated product, average primary particle diameter: 0.02 [mu] m) were mixed with 0.5 parts of a Henschel mixer, the toner a Got. This toner had a volume average particle size of 5.43 μm, Tg of 46 ° C., and a THF insoluble content of the resin component of 12%.

  The so-called toner particle size of the toner thus prepared was measured with a particle size measuring device “Coulter Counter TAII” manufactured by Coulter Electronics Co., Ltd., with an aperture diameter of 100 μm. The volume average particle size and the number average particle size are determined by the particle size measuring instrument. The volume average particle diameter of the toner prepared in this example was 5.5 μm. The ratio of pigment to resin in the toner was 6.0%. By using the same manufacturing method, toners of cyan (C), magenta (M), and yellow (Y) were also produced.

  In this embodiment, the above toner was used. Note that the method for producing the toner of the present invention is not limited to the above method, and besides the above method, a toner produced by a dispersion polymerization method or a pulverization method may be used.

Example 1
The Raw γ characteristics were measured using the pseudo halftone processing in the first embodiment of the present invention (configuration shown in FIGS. 1 to 10). The result is shown in FIG.
The output of the image is manufactured based on an experimental machine (Ricoh's Ipsio Color 5100) capable of outputting an image by changing the pseudo halftone process, and producing a modified experimental machine: resolution 600 dpi, number of gradations 8 bits per pixel) Therefore, an image output experiment is performed. The toner used at this time was a polymerized toner having the above-mentioned volume average particle diameter.

  FIG. 17 shows the result (Raw γ characteristic) of image output and measurement of the density of each color of CMYK in the output image. The output image is output by the above-described experimental machine after the above-described pseudo halftone processing image is applied to the 15 gradations in which the highlight portion is emphasized between the density of 0-bit of 8-bit data, and each of the colors of CMYK is output. The reflection density was measured by using a spectral density colorimeter (X-Rite 938). The horizontal axis in FIG. 17 is the input data value (white: 0 to black: 255), and the vertical axis is the reflection density value of each color of CMYK. As can be seen from FIG. 17, it is possible to realize an image forming apparatus having substantially the same Raw γ characteristics (each CMY curve has almost the same value) for CMY colors. In addition, it has been clarified that the K color exhibits similar Raw γ characteristics such that CMYK density values are multiplied by a constant.

  In addition, 3C gray in which CMYK was mixed at an equal ratio was output, and the area of lightness 90 to 70 was measured in the same manner, and the gray balance was confirmed (when the saturation component of 3C gray was measured). It is clear that the component is about 0.5, and it is possible to realize a good gray balance without performing gradation correction for each CMY component. In addition, because the Raw γ characteristic is similar to that of the C version for the K version, phenomena such as gradation skip and coloring due to the black process occur even in the image subjected to the black process (UCR). It became clear that they would not.

  On the other hand, as Comparative Example 1, a pseudo halftone process was performed using a dither matrix in which the number of lines did not completely match in each of the CMYK plates, and an image was output. The CMYK version dither matrix of Comparative Example 1 is shown in FIGS. Table 1 shows a comparison list of the number of lines in the dither matrix between Example 1 and Comparative Example 1. The image output procedure and output image were exactly the same as those in Example 1 described above.

  FIG. 29 shows the density measurement results (Raw γ characteristics) of CMYK colors. Since the number of lines is the same for the CM color, it has almost the same Raw γ characteristics. However, since the Y color has a different number of lines from the CM color, there is a difference in the Raw γ characteristics. Comparing with the results of Example 1, it can be seen that in Comparative Example 1, CM and Y have slightly different values for the same input value. Further, since the number of K colors is also different from that of both the CM color and the Y color, the tendency of the Raw γ characteristics is also different from the case of the first embodiment.

  Further, as in the case of the first embodiment, 3C gray obtained by mixing CMYK at an equal ratio was also confirmed by the same method as that of the first embodiment. As a result, a gray balance shift was perceived.

(Example 2)
Evaluation was performed by outputting an actual image using the dithermatox shown in FIGS. 13 and 14 shown in the second embodiment of the present invention.
The result is shown in FIG. It was found that the Raw γ characteristics of the respective colors of CMYK can be made substantially the same, and almost the same effect as in Example 1 can be obtained. Thereby, the effect that a gray balance can be maintained favorable can be acquired.

  Furthermore, when the dither matrix of Example 2 was used, the reproducibility at extremely high highlights was better than that of the dither matrix of Example 1. In both FIG. 17 and FIG. 30, the vertical axis represents the reflection density of the image, and the larger the value, the higher the density of the image. The second plot point from the left in FIG. 17 (Example 1) and FIG. 30 (Example 2) is an image with an image area ratio of about 3%. Compared to Example 1, Example 2 is slightly smaller. The value is larger. The same can be said when the output image is visually evaluated. The result of Example 2 is that the patch with an image area ratio of 3% can be expressed better than Example 1. It was. Good reproduction of the image up to the very highlight portion is one of the items required for a high-quality image from the viewpoint of smoothly changing the gradation of the output image. In the image forming apparatus according to the second embodiment, it is possible to realize an image forming apparatus capable of gradation expression over the extremely high highlight portion.

( Reference Examples 1-4 )
Next, as Reference Examples 1 to 4 and Comparative Examples 2 to 3, dither matrices were created by changing the screen angle, and image output was performed. The screen angle here is the same as the angle α defined in the section of the first embodiment, and a dither matrix obtained by shaking the angle α in the range of 11.3 to 33.7 degrees has a resolution of 600 dpi and a dither gradation number of 4 bits. Created.
The results are shown in Table 2. Table 2 displays the created dither matrix and a list of image output results. At this time, in the same manner as in the first embodiment, the dither matrix having the angle α described in the above table is a Y-dither matrix, and a dither matrix having an angle of −90 + α, an angle of −α, and an angle of 90−α is created. The K, M, and C dither matrices were used. The dither growth type is a line type.

The detailed presentation of the dither matrix of Reference Examples 1 to 4 and Comparative Examples 2 to 3 is complicated and will be omitted, but in order to more specifically explain the dither matrix, the periodic structure of the dither matrix will be described. The periodic structure of the dither matrix is determined by two vectors, a vector (vector A) indicating the direction of the angle α and a vector (vector B) determining the number of lines (FIG. 31). Incidentally, FIG. 31 shows the Y version of the first embodiment as an example, and the vector A = (2, 1) and the vector B = (1, −3 ). The first component of each vector represents the component in the direction of 0 degrees defined in the first embodiment, and the second component represents the component in the direction of 90 degrees. Table 3 shows the dither matrices of Reference Examples 1 to 4 and Comparative Examples 2 to 3 according to such a display format.

As shown in Table 2, the results of image output cannot perceive color moire in Reference Examples 1 to 4 , while color moire occurs in a dark blue image in Comparative Example 2, and in Comparative Example 3, Color moire occurred in the dark red image of blue. From this, it has been clarified that when the screen angle difference of each color component of CMYK is 25 degrees or more, an adverse effect such as color moiré occurs. Further, it can be seen that also color moire so long as the angle α is 12.5 ° ～32.5 ° de Izamatorikusu does not occur. In Table 3, the angle α is confirmed only up to 31.0 degrees. However, when the point of symmetry (assuming rotation of 45 degrees) is used, an area where the color moire is not a problem has an angle α of 12.5 to It can be considered to be in the range of 32.5 degrees.

Further, from the image output results of Reference Example 1 to Reference Example 4 , the color moire is limited to the range of the above angle α (12.5 degrees to 32.5 degrees), but the edge portion has a backlash. The results show that the above angle is preferably 25 to 32.5 degrees because of the attachment (edge portion of a special right-angled patch).

(Example 3 )
Next, as Example 3 and Comparative Example 4, dither matrices with different dither gradations were created and image output was performed. More specifically, a dither matrix having 2 bits (4 values) and 1 bit (2 values) of dither gradations was created. The dither matrices of Examples 1 and 2 and Reference Examples 1 to 4 were all dither matrices with 4 bits of gradation (16 level values). The screen angle and the number of lines at this time were the same as the dither matrix of Example 1, the screen angle (angle α) was 26.4 degrees, and the number of lines was 191.7 lpi. Table 4 shows image output results when the number of dither gradations is changed to 2 bits and 1 bit when the resolution is 600 dpi.

Next, in order to explain the dither matrix of the third embodiment, a C version dither matrix is presented. FIG. 32 is a dither matrix of the C plate according to the seventh embodiment. The actually used matrix size is 28 × 28, but this is for uniform dither growth. If the number of gradations to be reproduced is 256, the matrix size is 14 ×. Since there is no problem if it is 14, here the dither matrix size is presented as 14 × 14 for simplification of illustration. A dither matrix was also created for the MYK plate other than the C plate by assigning screen angles to the examples. As can be seen from FIG. 32, the dither growth mode is a line type.

When an image was output using the dither matrix of Example 3 , it was possible to realize pseudo halftone processing in which color moire was completely eliminated as in Example 1. Further, the Raw γ characteristics are the same for each color component of CMYK as in Example 1, and a good gray balance could be realized.

On the other hand, as Comparative Example 4, a dither matrix with 1-bit dither gradation was created and an image was output. FIG. 33 is a C-dither matrix of Comparative Example 4. For the MYK version, a dither matrix with the screen angle set was created in the same manner as in Example 1, and an image was output. As a result, by the low-frequency structure of the image, including a similar abnormal image to the color moire, an image is output, the effect (the realization of high-quality images eliminate color moire) as in Example 1 and Example 3 are viewed I couldn't.

(Examples 4 and 5 )
Next, as Examples 4 and 5 , image output was performed by changing the number of gradations of the dither matrix at a resolution of 1200 dpi. The change in resolution was realized by modifying the optical unit and the image data processing portion of the experimental machine (Ricoh's Ipsio Color 8150) described in the section of Example 1. A dither matrix was also created for a resolution of 1200 dpi. As for the screen angle and the number of lines of the dithers in Examples 4 and 5 , the angle α is 26. The number of lines was 61.7 and 191.7 lpi. As for the number of dither gradations, the fourth embodiment is 2 bits and the fifth embodiment is 1 bit. As for the dither growth type, Example 4 has the same form as Example 3 and Example 5 has the same form as Comparative Example 4. Since it seems that the dither matrix can be specified in the above description, the presentation of the dither matrix is omitted.

  As a result, if the resolution is 1200 dpi, even if the number of gradations is 2 bits or 1 bit, the problem as in Comparative Example 4 does not occur, and color moiré is eliminated to realize a high-quality image. Was possible.

(Example 6 )
As Example 6 , the number of lines was changed from 149.1 to 223.6 lpi, and image output was performed under the same conditions as in Example 1 except for that. Further, as Comparative Example 5, image output with the number of lines being 122.0 and 134.2 lpi was performed. The results are shown in Table 5.
Further, according to the experiments conducted by the inventors, if the number of lines is 141 lpi or more, the object of the present invention can be achieved without generating color moire in two colors and tertiary colors. Became clear. On the other hand, when the number of lines in the pseudo halftone process is lower than 141 lpi, the color moire is not perceived, but the structure of the pseudo halftone process in K color or M color with strong brightness contrast. Has been perceived.

1 is a schematic view of an image forming apparatus of a tandem type / intermediate transfer system according to the present invention. It is a cyan (C) plane dimatrix (1) in the first embodiment of the present invention. It is a cyan (C) plane dimatrix (2) in the first embodiment of the present invention. It is a magenta (M) plate dematrix (1) in the first embodiment of the present invention. It is a magenta (M) plate dematrix (2) in the first embodiment of the present invention. It is a yellow (Y) plane dimatrix (1) in the first embodiment of the present invention. It is a yellow (Y) plane dimatrix (2) in the first embodiment of the present invention. It is a black (K) plane dimatrix (1) in the first embodiment of the present invention. It is a black (K) plane dimatrix (2) in the first embodiment of the present invention. It is a figure which shows the relationship of the screen angle of the dither matrix of each color of CMYK in the 1st Embodiment of this invention. 1 is a schematic view of an image forming apparatus of a tandem type / direct transfer system according to the present invention. 1 is a schematic view of a revolver type image forming apparatus according to the present invention. It is a cyan (C) plane dimatrix (1) in the second embodiment of the present invention. It is a cyan (C) plane dimatrix (2) in the second embodiment of the present invention. It is a block diagram explaining the structure of the pseudo halftone process part (error diffusion process part) in the 3rd Embodiment of this invention. It is an example of an error diffusion matrix. It is a figure which shows the result of the Raw (gamma) characteristic of Example 1. FIG. 3 is a cyan (C) plane dematrix (1) of Comparative Example 1. FIG. 4 is a cyan (C) plane dematrix (2) of Comparative Example 1. FIG. 4 is a cyan (C) plane dematrix (3) of Comparative Example 1. FIG. 2 is a magenta (M) plate dematrix (1) of Comparative Example 1; 3 is a magenta (M) plate dematrix (2) of Comparative Example 1; 3 is a magenta (M) version dematrix (3) of Comparative Example 1; 3 is a yellow (Y) plate dematrix (1) of Comparative Example 1. FIG. 3 is a yellow (Y) plate dematrix (2) of Comparative Example 1. FIG. 4 is a black (K) plate dematrix (1) of Comparative Example 1; 4 is a black (K) plate dematrix (2) of Comparative Example 1; 4 is a black (K) plate dematrix (3) of Comparative Example 1; It is a figure which shows the result of the Raw-gamma characteristic of the comparative example 1. It is a figure which shows the result of the Raw-gamma characteristic of Example 2. FIG. It is explanatory drawing of two vectors which determine the periodic structure of a dither (in the case of Y version of Example 1). 4 is a cyan (C) plane dematrix according to the third embodiment. 10 is a cyan (C) plane dematrix of Comparative Example 4. FIG. It is the schematic of the image forming apparatus using the conventional electrophotographic process. It is a block diagram of an image processing unit in a conventional image forming apparatus.

Explanation of symbols

1 Photosensitive drum 2 Charging means (charging device)
DESCRIPTION OF SYMBOLS 3 Exposure means 4 Developing means 5 Transfer means 6 Recording sheet 7 Cleaning means 8 Fixing apparatus 10 Image forming apparatus 11, 91 Image input section 12, 92 MTF filter processing section 13, 93 Color correction / tone correction (γ conversion) processing section 14, 94 Pseudo halftone processing unit 15 Image output unit 16, 95 Video signal processing unit 21, 22 Optical unit 31, 32 Image forming unit (x4)
34 Error diffusion processing section 341 Addition section 342 Quantization section 343 Correction value calculation section 344 Error storage section 345 Error calculation section 41 Intermediate transfer member (belt)
42, 52 Registration roller 43 Secondary transfer portion 44, 54 Fixing device 96 CPU
97 ROM
98 RAM
99 Operation part

Claims (14)

In an electrophotographic image forming apparatus that forms a full color image by superimposing toner images corresponding to four color components of cyan (C), magenta (M), yellow (Y), and black (K),
Pseudo halftone processing using line dithering is performed from input image data in the CMYK color space, and the angle of 26.6 ° (main scanning direction is 0 ° angle and counterclockwise direction is defined as positive) A dither matrix having a periodic structure in which the vector A and the vector B are not orthogonal and have different lengths. based on, the screen angle of the first color of the CMYK color angle 26.6 °, the screen angle of the second color is angle -63.4 °, the screen angle of the third color is angle -26.6 °, the pseudo-halftoning apparatus fourth color screen angles of an angle 63.4 ° der is, and the number of lines of the CMYK colors to generate image data which is 191.7 lpi,
An optical writing device that performs optical writing based on image data output from the pseudo halftone processing device;
An image forming apparatus comprising: a developing device that develops the toner image to form an image corresponding to the optical writing. In an electrophotographic image forming apparatus that forms a full color image by superimposing toner images corresponding to four color components of cyan (C), magenta (M), yellow (Y), and black (K),
Pseudo halftone processing is performed from the input image data in the CMYK color space by an error diffusion method having a line dither type threshold matrix, and an angle of 26.6 ° (the main scanning direction is an angle of 0 ° and A vector A indicating a direction of a clockwise direction) and a vector B for determining the number of lines, and the vector A and the vector B are not orthogonal and have different lengths Of the CMYK colors, the screen angle of the first color is 26.6 °, the screen angle of the second color is −63.4 °, and the third color screen angle is an angle -26.6 °, the screen angle of the fourth color is angle 63.4 ° der is, and pseudo halftone processing unit lPI CMYK colors to generate image data which is 191.7 lpi When,
An optical writing device that performs optical writing based on image data output from the pseudo halftone processing device;
An image forming apparatus comprising: a developing device that develops the toner image to form an image corresponding to the optical writing. The M color and the C color are assigned to two colors having a screen angle difference of 90 ° among the first to fourth colors of the image data generated by the pseudo halftone processing device. Item 3. The image forming apparatus according to Item 1 or 2 . The halftone processing apparatus, an image forming apparatus according to any one of claims 1 to 3 in which the number of gradations per pixel located at a resolution 600dpi is characterized in that for halftoning at least 2bit . The halftone processing apparatus, an image forming apparatus according to any one of claims 1-3, characterized in that for halftoning the resolution is not less than 1200 dpi. 2. A gradation correction processing device that detects a toner image formed based on a preset pattern and performs gradation correction processing on input image data based on the detection result. The image forming apparatus according to any one of 5 to 5 . The image forming apparatus according to claim 6 , wherein the gradation correction processing apparatus performs gradation correction processing by creating a conversion lookup table by approximating the detection result of the toner image by polynomial approximation. In an electrophotographic image forming method for forming a full color image by superimposing toner images corresponding to four color components of cyan (C), magenta (M), yellow (Y), and black (K),
From the input image data in the CMYK color space, a vector A and a line indicating the direction of an angle of 26.6 ° (the angle in which the main scanning direction is an angle of 0 ° and the counterclockwise direction is defined as positive) as a series of pixels A first color among the CMYK colors based on a dither matrix composed of a periodic structure in which the vector A and the vector B are not orthogonal and have different lengths. Screen angle is 26.6 °, second color screen angle is −63.4 °, third color screen angle is −26.6 °, and fourth color screen angle is angle 63. .4 ° der is, and the number of lines of the CMYK colors is generated by the pseudo-halftone processing by the ten thousand linear dither method the image data is 191.7 lpi,
Perform optical writing based on the image data generated by the pseudo halftone processing,
An image forming method comprising developing the toner image to form an image corresponding to the optical writing. In an electrophotographic image forming method for forming a full color image by superimposing toner images corresponding to four color components of cyan (C), magenta (M), yellow (Y), and black (K),
From the input image data in the CMYK color space, a vector A and a line indicating the direction of an angle of 26.6 ° (the angle in which the main scanning direction is an angle of 0 ° and the counterclockwise direction is defined as positive) as a series of pixels A first color among the CMYK colors based on a dither matrix composed of a periodic structure in which the vector A and the vector B are not orthogonal and have different lengths. Screen angle is 26.6 °, second color screen angle is −63.4 °, third color screen angle is −26.6 °, and fourth color screen angle is angle 63. .4 ° der is, and the number of lines of the CMYK colors is generated by the pseudo-halftone processing by the error diffusion method with the threshold matrix of parallel line dithered image data is 191.7 lpi,
Perform optical writing based on the image data generated by the pseudo halftone processing,
An image forming method comprising developing the toner image to form an image corresponding to the optical writing. The M color and the C color are assigned to two colors having a screen angle difference of 90 ° among the first to fourth colors of the image data generated by the pseudo halftone process. The image forming method according to 8 or 9 . The image forming method according to any one of claims 8 to 10 , wherein the pseudo halftone processing is performed at a resolution of 600 dpi and a gradation number per pixel of 2 bits or more. The image forming method according to claim 8 , wherein the pseudo halftone process is performed at a resolution of 1200 dpi or more. Detects the toner image formed on the basis of the preset pattern, in any one of claims 8 to 12, characterized in that the gradation correction process on the input image data based on the detection result The image forming method described. 14. The image forming method according to claim 13 , wherein the tone correction processing is processing for tone correction using a conversion lookup table created by approximating the detection result of the toner image by polynomial approximation.

Images