JP2006270449A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technology of easily obtaining an image occurrence of moire and color unevenness in which is excellently suppressed. <P>SOLUTION: An image processing section 500 of an image forming apparatus 1 carries out screen processing and an image forming section 600 sequentially forms toner images in yellow, magenta, cyan, and black. In the case of carrying out the screen processing, the image processing section 500 applies a screen with the same screen angle to the cyan and black toner images. The screen angle of the magenta differs from that of the cyan and the black by 30 degrees or over. Thus, the image forming apparatus can form an image wherein moire is reduced and color unevenness of a shadow part is suppressed. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a technique for forming an image with good image quality in an image forming apparatus.

  A dither method is known as one method for expressing the halftone of an image. For example, Patent Document 1 discloses an electrophotographic image forming apparatus that expresses a halftone using a dither method. In such an image forming apparatus, halftones are expressed by halftone dots, and pseudo gradations are expressed by the size of the halftone dots, but when halftones are expressed using halftone dots, However, there is a problem in that moire occurs due to interference between regular halftone dot patterns, thereby deteriorating image quality.

  In order to suppress moiré, the angle of the screen (screen angle) that determines the direction of the halftone dot pattern of each color may be varied. For example, in the case of an orthogonal screen, if the screen angle is varied by 45 ° for a screen of two colors. Most effectively suppresses moire. Further, when the difference in screen angles is less than 30 °, moire becomes conspicuous. However, in a general full-color image forming apparatus, that is, an image forming apparatus using Y (yellow), M (magenta), C (cyan), and K (black) color materials, screen angles of four colors are respectively set. It cannot be different by 45 °. Therefore, in general, the screen angle of Y, which is the least noticeable color of moire, is set to 0 °, and the screen angles of C, K, and M are changed by 30 °, such as 15 °, 45 °, and 75 °, respectively. , Doing something like that. By doing so, it is possible to suppress moire to some extent, but still moire may occur depending on the image to be formed and various conditions of the image forming apparatus.

Further, as a factor for deteriorating the image quality, color unevenness due to a shift in the printing position of the color material can also be mentioned.
Figure 9 is a diagram for explaining the color unevenness, dot d _M by the color material of the dots d _Y and M in the white of the paper P by the color material of Y indicates a state in which the printing. In the figure, light rays L ₁ is to be reflected by the sheet P passes through the coloring material Y, the reflected light is perceived as "Yellow" complementary color component is reduced human eye Y. Meanwhile, since light L ₂ is reflected by the sheet P passes through the color material of Y and M, the reflected light to the human eye is reduced the complementary color components Y and M "red (so-called gold red)" Perceived as. That is, the color perceived by the human eye is different depending on whether the two color materials are completely overlapped or not. In a region where the printing position of the color material is shifted, a different color is generated, and this color causes color unevenness.

Efforts to suppress color unevenness have also been conventionally made, and examples include the technique described in Patent Document 2. However, it is difficult to suppress color unevenness while suppressing moire well, and it is very difficult to obtain an image with good image quality even if the screen angle and the number of screen lines are variously adjusted.
JP 57-76977 A Japanese Patent Laid-Open No. 11-252367

  The present invention has been made in view of the above-described circumstances, and an object thereof is to provide a technique that makes it possible to easily obtain an image in which moire and color unevenness are satisfactorily suppressed.

In order to achieve the above-mentioned object, the present invention provides an acquisition means for acquiring color image data composed of a plurality of color components and a pseudo gradation corresponding to a screen having a predetermined screen angle in a minute area of the color image data. Storage means for storing a threshold value matrix for defining each color corresponding to the color component, and each color stored in the storage means for each of the minute areas constituting the color image data acquired by the acquisition means A comparison unit that specifies the density indicated by each minute region of the color image data for each color by comparing with the threshold matrix, and each color representing the color image data according to the density specified by the comparison unit Image forming means for sequentially forming images on a recording material, and a screen stored in the storage means is provided in the image forming means. For two-color an image is continuously formed have to provide an image forming apparatus having the same screen angle.
According to such an image forming apparatus, for example, by setting two colors, which are likely to cause color unevenness, to have the same screen angle, it is possible to reduce color shift and reduce color unevenness. In addition, since these two colors have the same screen angle, the difference from the screen angles of the other colors can be made larger, and the occurrence of moire can be suppressed.

In this image forming apparatus, it is desirable that the screens have the same number of screen lines of the two colors having the same screen angle.
By doing so, it is possible to further suppress the occurrence of color unevenness and moire.

In the image forming apparatus, it is desirable that the image forming unit continuously forms the two color images having the same screen angle.
In this case, it is more preferable that the image forming unit forms the two color images after the other color images.
By doing in this way, it becomes possible to further suppress the deviation of the printing positions of the two colors.

Further, in this image forming apparatus, the image forming unit forms an image using yellow, magenta, cyan and black color materials, the two colors are cyan and black, and the other colors are yellow and magenta. The structure which is may be sufficient.
In this case, since the difference between the screen angle of cyan and black and the screen angle of magenta can be made larger than 30 °, it becomes possible to further suppress the occurrence of moire and the same screen angle between cyan and black. Therefore, it is possible to suppress the occurrence of color unevenness in the bluish shadow portion.
Further, since the image can be formed in the same order as the image forming order (Y → M → C → K) in the conventional image forming apparatus, the present invention can be easily applied to various image forming apparatuses. Is possible.

  As described above, according to the present invention, it is possible to easily obtain an image in which moire and color unevenness are satisfactorily suppressed.

  FIG. 1 is a block diagram showing a functional configuration of an image forming apparatus 1 according to an embodiment of the present invention. The image forming apparatus 1 includes a control unit 100, a storage unit 200, an input unit 300, an operation unit 400, an image processing unit 500, and an image forming unit 600. The control unit 100 includes an arithmetic device such as a CPU (Central Processing Unit) and various memories such as a ROM (Read Only Memory) and a RAM (Random Access Memory), and controls the operation of each unit of the image forming apparatus 1. . The storage unit 200 includes a storage device capable of reading and writing data such as a hard disk drive (HDD) and a flash memory, and stores various data necessary for image formation. In the present embodiment, the storage unit 200 stores a plurality of threshold matrixes.

  The input unit 300 is an interface device that acquires color image data from an external device such as a computer or a scanner. The color image data is, for example, image data of three colors of R (red), G (green), and B (blue), that is, 8 bits for each color (24 bits in total) expressed in the RGB color system. It is.

  The operation unit 400 includes a touch panel type liquid crystal display, various buttons, and the like, and receives various instructions regarding image formation from the user. The image processing unit 500 includes an ASIC (Application Specific Integrated Circuit) and an LSI (Large Scale Integration), and executes predetermined image processing to be described later on the input color image data to generate image data of each color of YMCK. The image forming unit 600 creates toner images of each color of YMCK based on the halftone dot image data of each color of YMCK generated by the image processing unit 500, and transfers this to a sheet to form an image. The detailed configuration of the image forming unit 600 will be described in detail later.

  FIG. 2 is a block diagram showing an outline of image processing executed by the image processing unit 500 in the present embodiment. The main image processing executed by the image processing unit 500 is color conversion processing, under color removal processing, and screen processing. Note that the processing actually performed by the image processing unit 500 includes enlargement / reduction of images, various correction processes, and the like, but the description thereof is omitted here for convenience of explanation.

  The color conversion process is, for example, a process of converting a color system of color image data expressed by three colors of RGB and converting this to color image data expressed by three colors of CMY. The under color removal process is a process of adding a K color component corresponding to the gray density to a portion where the three color components of CMY overlap and become gray. That is, by this under color removal processing, the color image data expressed in the three colors CMYK is converted into the color image data expressed in the four colors CMYK.

  Screen processing refers to extracting a specific color component from color image data expressed in four colors of CMYK, and representing the image with this color component (hereinafter referred to as “image data”) as a threshold value for that color. By comparing with the matrix, a process for specifying the density of each of the minute regions constituting the image data is performed. By performing the screen processing, the image data of each color of CMYK becomes halftone dot image data of each color of CMYK described by the density of each minute area. At this time, the threshold matrix is arranged at a predetermined angle and interval is called “screen”, the angle at which the threshold matrix is arranged is “screen angle”, and the interval (pitch) between adjacent threshold matrices is called “screen”. This is called "screen line number".

The screen processing in this embodiment will be described in more detail.
FIG. 3 is a block diagram illustrating a hardware configuration and a data flow for realizing the screen processing according to the present embodiment. The screen processing is realized by an address generator 51, a matrix memory 52, and a comparator 53. The address generator 51 acquires image position information indicating the size and coordinates of the image data and matrix information for selecting a threshold matrix corresponding to the color and output gradation of the image data, and performs comparison processing described later. Address information for specifying a minute region is generated. The matrix memory 52 acquires the address information generated by the address generator 51, and acquires a threshold matrix corresponding to the color and output gradation of the image data from the storage unit 200 based on the address information. The matrix memory 52 supplies threshold information corresponding to the threshold matrix to the comparator 53. The threshold information is information indicating the position of each dot represented by the threshold matrix and the threshold value at that dot. The comparator 53 compares the threshold information with the image data, and outputs halftone image data as the comparison result.

Here, a specific operation performed by the comparator 53 of the present embodiment will be described while showing threshold information.
FIG. 4 is a diagram showing a threshold matrix (in the case of resolution 1200 dpi) used in the present embodiment. Threshold value matrix M _Y shown in the figure is a threshold matrix is applied to yellow in the present embodiment, 0 °, it is to form a 240 line screen. Since the threshold value matrix _MY is a matrix of dots in 5 rows and 5 columns, it indicates 26 gradations.

For example where the threshold matrix M _Y compared to the image data will be described with reference to FIG. For example, the comparator 53, among the image data of yellow, concentrations of 0%, i.e. when comparing a small region with a threshold value matrix M _Y is "0/26", the threshold of any dot be so does not become "0" The comparator 53 outputs the data of FIG. 5A indicating that the entire area is “0” as a comparison result. Also, among the image data of yellow, concentration 50%, i.e., "13/26" When compared minute domains and the threshold matrix M _Y is, the comparator 53 threshold is "13" or less and made dots "1" The data of FIG. 5 (b) indicating this is output as a comparison result. Also, among the image data of yellow, concentration 65%, i.e., "17/26" When compared minute domains and the threshold matrix M _Y is, the comparator 53 threshold is "17" or less and made dots "1" The data of FIG. 5 (c) indicating this is output as a comparison result. In the same manner, when the density is 100%, all dots are “1” as shown in FIG.

  The comparator 53 sequentially performs such processing for each of the minute areas constituting the image data, and outputs data giving density information for each minute area as shown in FIG. 5, that is, halftone dot image data. The comparator 53 performs the above processing on the image data of each color of CMYK and outputs halftone dot image data of a total of four colors. The gradation of the color image data is expressed in a pseudo manner by the size of the halftone dot indicated by the halftone dot image data.

The threshold matrix used in this embodiment is the same for cyan and black, and is characterized in that the screen angle of the magenta screen is 30 ° or more away from the screen angle of the cyan and black screen described above. is doing.
FIG. 6 is a diagram illustrating a threshold matrix M _M that forms a magenta screen. This threshold matrix M _M forms a 26.5 °, 268 line screen. 7 is a diagram showing a threshold matrix M _C to form a screen for cyan and black. The threshold value matrix _{M C} is, 64.4 °, to form a 268 line screen.

In this way, the screen angle of the cyan and black screens formed by the threshold matrix M _C is 37.9 ° away from the screen angle of the magenta screen formed by the threshold matrix M _M. It is possible to suppress the occurrence of moiré compared to a simple image forming apparatus. The screen angle of the yellow screen is about 25 ° away from the screen angles of the other color screens. In this case, the difference in screen angle does not become 30 ° or more, but the screen angle can be separated from that of a conventional general image forming apparatus, so that the effect of suppressing moire is recognized even in yellow.
Further, by making the screen angles of the cyan and black screens the same, it is possible to suppress color unevenness that occurs, for example, in a bluish shadow portion.

Next, a specific configuration of the image forming unit 600 will be described.
FIG. 8 is a diagram showing a device configuration of the image forming unit 600. The image forming unit 600 includes image forming units 10Y, 10M, 10C, and 10K, an intermediate transfer belt 12, a drive roll 13, a secondary transfer roll 14, a backup roll 15, a fixing device 16, and a belt cleaner 17. , A plurality of transport rolls 18, a resist roll 19, and paper trays 20 and 21. The alphabets attached to the end of the reference numerals of the image forming units 10Y, 10M, 10C, and 10K are for indicating the color of the toner, and have the same configuration. Therefore, in the following description of the configuration, these alphabets are omitted and collectively referred to as “image forming unit 10”. Similarly, each part of the image forming unit 10 will be described by omitting the alphabet as appropriate.

  The image forming unit 10 includes a photosensitive drum 101, a charger 102, an exposure device 103, a developing device 104, a primary transfer roll 105, a drum cleaner 106, and a charge removal device 107. The photosensitive drum 101 is an image carrier having a photoconductive layer formed on the surface thereof, and is rotated in the direction of arrow A in the figure. The charger 102 uniformly charges the surface of the photosensitive drum 101 by generating a predetermined potential difference. The exposure device 103 includes a beam emission source such as a laser diode, and the charged light on the surface of the photosensitive drum 101 is irradiated with the beam light so that the charge of the irradiated portion disappears, and the halftone image data of each color is input. An electrostatic latent image is formed. The developing device 104 includes toners of Y, M, C, and K colors, and forms a toner image by applying toner to the electrostatic latent image on the surface of the photosensitive drum 101. The primary transfer roll 105 transfers the toner image to the intermediate transfer belt 12 at a position facing the photosensitive drum 101 via the intermediate transfer belt 12. The transferred toner image is conveyed in the B direction in the figure.

  The drum cleaner 106 collects untransferred toner remaining on the surface of the photosensitive drum 101 after the toner image is transferred. The neutralization device 107 neutralizes the surface of the photosensitive drum 101. That is, the drum cleaner 106 and the charge eliminating device 107 remove unnecessary toner and electric charges from the photosensitive drum 101, and prepare the photosensitive drum 101 for the next image forming process.

  The intermediate transfer belt 12 is an endless belt member. The intermediate transfer belt 12 is rotated in the direction of arrow B in the figure by the driving roll 13 and conveys the toner image formed in the image forming unit 10 to the nip region formed by the secondary transfer roll 14. In the nip region, the secondary transfer roll 14 facing the backup roll 15 transfers the toner image on the surface of the intermediate transfer belt 12 onto the paper P. The paper P is supplied from the paper tray 20 or 21 and is transported by a plurality of transport rolls 18. When the toner image is transferred to the paper P, the registration roll 19 performs timing control for adjusting the transfer position of the toner image.

  The fixing device 16 melts and fixes the toner image by heating and pressurizing the paper P on which the toner image is transferred by the secondary transfer roll 14. The paper P on which the toner image is fixed is discharged to the outside. When these series of processes are completed, the belt cleaner 17 collects untransferred toner remaining on the intermediate transfer belt.

  The apparatus configuration of the image forming apparatus 1 is as described above. With such a configuration, the image forming apparatus 1 generates YMCK halftone dot image data corresponding to the input color image data (color image data). Then, the image forming apparatus 1 forms a toner image corresponding to the halftone dot image data of each color, and transfers and fixes the toner image on the paper P to form an image.

  Note that the image forming apparatus 1 according to the present embodiment forms images in the order of yellow, magenta, cyan, and black as in a general image forming apparatus. However, as in the present embodiment, a cyan and black screen is formed. When the angles are the same, it is most preferable to form images in this order. This is because the shift in the printing position between the two image forming units 10 becomes more prominent as the distance between the units increases. This is because the greater the distance between the units, the stronger the influence of the skew of the intermediate transfer belt 12.

  As described above, according to the image forming apparatus 1 of the present embodiment, by setting cyan and black, which are likely to cause color unevenness, to have the same screen angle, it is possible to reduce the printing position shift and suppress color unevenness. Is possible. In addition, by setting these two colors to the same screen angle, it is possible to further increase the difference from the screen angles of the other colors, and to suppress the occurrence of moire.

In the above, an example of implementation of the present invention has been described using the image forming apparatus 1, but the present invention can of course be implemented in other forms. An example is shown below.
In the above-described embodiment, an image forming apparatus that forms toner images of four colors C, M, Y, and K has been described. However, the present invention can also be applied to image forming apparatuses using other colors. . Furthermore, the image forming apparatus is not limited to a four-color image forming apparatus, and may be a multicolor image forming apparatus having six colors, for example. In such a multicolor image forming apparatus, there may be two or more sets of two colors having the same screen angle.

In any of the above cases, the two colors having the same screen angle are not limited, and various colors may be selected according to various conditions of the image forming apparatus and input color image data. . Further, the screen angles of colors other than the two colors having the same screen angle are not particularly limited, and may be arbitrary screen angles.
In addition, in two-color screens having the same screen angle, it is desirable that the number of screen lines be the same. However, the present invention can achieve certain effects even if the number of screen lines is not the same.

  In the above-described embodiment, the image forming apparatus using the intermediate transfer belt has been described. However, the present invention is not limited to such an aspect. For example, the present invention can be applied to an image forming apparatus that includes a sheet conveying belt instead of the intermediate transfer belt and directly transfers a toner image from a photosensitive drum onto a sheet.

  A plurality of threshold matrixes having different screen angles and screen line numbers may be stored for each color. At this time, it is more preferable that an image forming mode corresponding to the type of color image data is provided and an optimum threshold value matrix is selected according to the image forming mode.

1 is a block diagram illustrating a functional configuration of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a block diagram illustrating an outline of image processing executed by an image processing unit in the embodiment. It is the block diagram which showed the hardware constitutions and data flow which implement | achieve the screen process of the embodiment. It is the figure which illustrated the threshold value matrix which forms a yellow screen in the embodiment. It is the figure which illustrated the comparison process of a threshold value matrix and image data. It is the figure which illustrated the threshold value matrix which forms a magenta screen in the embodiment. It is the figure which illustrated the threshold value matrix which forms the screen of cyan and black in the embodiment. FIG. 2 is a diagram illustrating a device configuration of an image forming unit according to the embodiment. It is a figure for demonstrating a color nonuniformity.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 100 ... Control part, 200 ... Memory | storage part (memory | storage means), 300 ... Input part (acquisition means), 400 ... Operation part, 500 ... Image processing part, 51 ... Address generator, 52 ... Matrix memory 53: Comparator (comparing means) 600: Image forming unit (image forming means) 10, 10Y, 10M, 10C, 10K ... Image forming unit, 12 ... Intermediate transfer belt, 13 ... Drive roll, 14 ... Secondary transfer Roll, 15 ... Backup roll, 16 ... Fixing device, 17 ... Belt cleaner, 18 ... Transport roll, 19 ... Registration roll, 20, 21 ... Paper tray, 101 ... Photosensitive drum, 102 ... Charger, 103 ... Exposure device, 104 ... developer, 105 ... primary transfer roll, 106 ... drum cleaner, 107 ... static elimination device

Claims (5)

Storage means for storing, for each color corresponding to the color component, a threshold matrix for giving a pseudo gradation in units of a minute area corresponding to a screen having a predetermined screen angle for color image data composed of a plurality of color components When,
A comparison unit for specifying the density indicated by each minute region of the color image data for each color by comparing the color image data with a threshold matrix of each color stored in the storage unit in units of the minute region;
Image forming means for sequentially forming an image of each color representing the color image data on a recording material according to the density specified by the comparison means,
The image forming apparatus, wherein the screen stored in the storage unit has the same screen angle for two colors on which images are continuously formed in the image forming unit. The image forming apparatus according to claim 1, wherein the screens have the same number of screen lines of the two colors having the same screen angle. The image forming apparatus according to claim 1, wherein the image forming unit continuously forms the two color images having the same screen angle. The image forming apparatus according to claim 3, wherein the image forming unit forms the two color images after the other color images. The image forming unit includes:
Create an image using yellow, magenta, cyan and black colorants,
The image forming apparatus according to claim 4, wherein the two colors are cyan and black, and the other colors are yellow and magenta.

Images