JP2007259097A - Unit and method for image processing, and program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enable binarization of images represented in color input image data so as to be looked as uniform color, without making the above images vanished, or modified into dashed lines, even when the images are thin lines and characters. <P>SOLUTION: In regard to the thin lines included in the input images, an image processing unit performs binarization using a dither matrix satisfying the conditions of (a) in each pixel, the threshold for each color component constituting the image color is either L representing the lowest tone or lower, or H representing the highest tone or higher, in the above color input image data; and (b) in each pixel, among each color component constituting the above image color, only a threshold of at least one color component is L. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a technique for binarizing multi-tone color input image data.

  2. Description of the Related Art Conventionally, a dither method is known as one of methods for performing tone conversion from multi-tone color input image data to low-tone color output image data. This dither method is a method of determining an output gradation value by comparing the input gradation value in the color input image data with the threshold value in the dither matrix 1: 1.

Techniques for eliminating the disadvantages of the dither method have also been proposed. For example, a technique has been proposed in which a threshold value of a dither matrix is set in accordance with the resolution of color output image data so that the density of the output image is appropriate (see Patent Document 1).
JP 2005-109546 A

  If gradation is converted by dithering when thin lines or characters are drawn in the color input image data, the thin lines or characters disappear in the color output image data, or the thin lines or characters disappear partially and become broken lines. Sometimes it became. This phenomenon will be described with reference to FIG.

  FIG. 15a shows 16-tone color input image data including each color of yellow (Y), magenta (M), cyan (C), and black (K), and each region divided by a grid is one pixel. Represents. In this color input image data, a pixel in which “8” is described is a pixel in which the gradation values of M and Y are 8, and the gradation values of C and K are 1. A pixel in which “1” is described is a pixel having a gradation value of 1 for all color components of C, M, Y, and K. That is, in the color input image data, a thin line image having a width of 2 pixels is drawn in the horizontal direction, and the color of the thin line is a color having a ratio of M to Y of 1: 1.

  FIG. 15b shows a case where the M dither matrix is applied to the color input image data shown in FIG. 15a, and FIG. 15c shows a case where the Y dither matrix is applied to the color input image data shown in FIG. 15a.

  As shown in FIG. 15b, in the color input image data, among the pixels corresponding to the thin lines, the pixels whose M gradation value is equal to or greater than the threshold value of the dither matrix and the color output image data is given the color M , Only pixels whose numbers are outlined. Further, as shown in FIG. 15c, in the color input image data, among the pixels corresponding to the thin lines, the Y gradation value is equal to or greater than the threshold value of the dither matrix, and the color output image data is assigned Y color. A pixel is only a pixel which represented the number by the outline.

  FIG. 15d shows color output image data binarized by applying a dither matrix as shown in FIGS. 15b and 15c. In this color output image data, the pixels represented by solid fill are pixels marked with M and Y, the pixels represented by diagonal lines are pixels marked with Y, and the other pixels are blank. Pixel.

  When the color input image data of FIG. 15a is compared with the color output image data of FIG. 15d, the color input image data image is a continuous thin line, but the color output image data image partially disappears. As a result, it has become a broken line. This is a part of the pixels corresponding to the thin lines in the color input image data, and both the M gradation value and the Y gradation value are lower than the threshold value of the dither matrix, and as a result, This is because the pixel has become blank.

  Further, when the color input image data of FIG. 15a and the color output image data of FIG. 15d are compared, the color of the fine line in the color input image data is that M and Y are 1: 1 colors throughout. As for the color of the broken line in the color output image data, the lower side of the broken line is a color in which M and Y overlap, and the upper side of the broken line is a color of Y alone. For this reason, when printing is performed based on the color output image data, the upper portion of the Y color appears to protrude from the lower portion of the red (M + Y) color.

  The present invention has been made in view of the above points, and even if the image represented in the color input image data is a fine line or a character, the image is not lost or becomes a broken line, and is uniform. An object of the present invention is to provide an image processing apparatus, an image processing method, and a program that can be binarized so as to look like any color.

(1) The invention of claim 1
Each color component in the pixel of interest of the color input image data as means for performing tone conversion from multi-tone color input image data composed of a plurality of color components to binarized color output image data The output tone value of each color component at the target pixel of the color output image data is determined based on the result of comparing the tone value of the color output value with the threshold value for each color component at the target pixel of the dither matrix. An image processing apparatus including a determination unit, wherein the image determination unit determines whether the image represented in the color input image data includes a fine line and the color of the image, and the image determination unit includes: When it is determined that the image does not include a thin line, a normal dither matrix is used as the dither matrix used for gradation conversion of pixels corresponding to the image, and When it is determined to contain a fine line in an image, a selection means for selecting a fine wire dither matrix corresponding to the color of the image, the dither matrix for thin wire corresponding to the color of the image,
(A) In each pixel, a threshold value for each color component constituting the color of the image is selected from among L that is not more than the lowest gradation and H that is not less than the highest gradation in the color input image data. (B) In each pixel, an image satisfying the condition that only one of the color components constituting the color of the image has a threshold value L. The gist of the processing apparatus.

  When the image represented by the color input image data includes a thin line, the image processing apparatus of the present invention uses a thin line dither matrix corresponding to the color of the image in the dither processing of the pixel corresponding to the image.

  In this thin line dither matrix, the threshold value of at least one of the color components included in the corresponding color is L (below the minimum gradation value in the color input image data) in each pixel. It is configured.

  Accordingly, when dither processing is performed using the thin line dither matrix on the pixels in which the image is formed in the color input image data (the gradation of the color component constituting the color of the image is not less than the minimum value), Since at least one gradation value among the color components included in the color of the image is larger than the threshold value L, the corresponding pixel in the color output image data always has that color component.

  That is, the predetermined pixel in the color input image data is originally colored as in the conventional dither processing, but the threshold value of the corresponding pixel in the dither matrix is large. There is no such thing as being blank.

  For this reason, even if the image displayed in the color input image data is a thin line, the pixel corresponding to the thin line becomes blank and the thin line disappears, or a part of the pixel corresponding to the thin line becomes blank. Thus, the thin line does not become a broken line. In addition, when the image represented in the color output image data is a fine line and a fill having the fine line as a boundary, a part of the fine line is not blank and unevenness is not generated at the boundary.

  Further, the thin line dither matrix is configured such that, in each pixel, only the threshold value of any one of the color components constituting the corresponding color is L. For this reason, when dither processing is performed on a pixel on which an image is formed in the color input image data, the color component whose gradation value exceeds the threshold value L is only 1 for that pixel. Only the color component whose gradation value exceeds the threshold value L is attached to the pixel, and no other color component is attached.

Therefore, according to the image processing apparatus of the present invention, there is no case where a plurality of color components are mixed and the color is not changed in each pixel of the color output image data.
(2) The invention of claim 2
The selection means uses the fine line corresponding to the color of the image as the dither matrix used for gradation conversion of the pixel corresponding to the image only when the color of the image including the thin line is a predetermined specific color. The gist of the image processing apparatus according to claim 1, wherein a dither matrix is selected.

In the present invention, the thin line dither matrix is selected only when the color of the image including the thin line is a specific color set in advance. Therefore, the number of types of the thin line dither matrix can be reduced. Therefore, the capacity of the storage means for storing the thin line dither matrix can be reduced.
(3) The invention of claim 3
When the color of the image including the thin line is composed of two or more color components, in the thin line dither matrix, a pixel whose threshold value of one color component of the two or more color components is L, and other The gist of the image processing apparatus according to claim 1, wherein the pixels whose color component threshold value is L are distributed.

  In the present invention, in the thin line dither matrix, a pixel having a threshold value L of one of two or more color components and a pixel having a threshold value L of another color component are distributed. Has been placed.

  The pixel whose threshold value of the one color component is L corresponds to a pixel to which the color of the first color component is attached in the color output image data after dither processing. In addition, the pixels whose threshold values of the other color components are L correspond to the pixels to which the colors of the color components are added in the color output image data after the dither processing.

In the present invention, as described above, in the thin line dither matrix, a pixel in which the threshold value of one of the two or more color components is L, and a pixel in which the threshold value of the other color components is L Are dispersedly arranged, in the color output image data after dither processing, the pixels with the color of the one color component and the pixels with the other color components are dispersedly arranged. As a result, an image printed based on the color output image data does not collect pixels of a specific color and looks as a uniform color as a whole.
(4) The invention of claim 4
When the color of the image including the thin line is composed of the color component 1 and the color component 2, in the thin line dither matrix, the pixel whose threshold value of the color component 1 is L, and the threshold value of the color component 2 The gist of the image processing apparatus according to claim 1, wherein the pixels of which L is L are alternately arranged in a vertical direction and a horizontal direction in the pixel array.

In the present invention, in the thin line dither matrix, pixels whose color component 1 threshold is L and pixels whose color component 2 threshold is L are alternately arranged.
A pixel having a color component 1 threshold value L corresponds to a pixel to which a color of the color component 1 is attached in the color output image data after dither processing. A pixel having a threshold value of color component 2 corresponding to L corresponds to a pixel to which the color of color component 2 is added in the color output image data after dither processing.

In the present invention, as described above, in the thin line dither matrix, the pixels whose color component 1 threshold is L and the pixels whose color component 2 threshold is L are alternately arranged. In the color output image data after the dither processing, the pixels to which the color component 1 is attached and the pixels to which the color component 2 is attached are alternately arranged. As a result, the image printed based on the color output image data looks like a uniform color as a whole without collecting pixels of a specific color.
(5) The invention of claim 5
The gist of the image processing apparatus according to claim 1, wherein the image including the thin line is an image formed based on a line drawing command.

The present invention illustrates an image including a thin line. According to the present invention, an image formed based on a line drawing command can be subjected to image processing without causing a part of the image to disappear.
(6) The invention of claim 6
In order to perform gradation conversion from multi-tone color input image data composed of a plurality of color components to binarized color output image data, the gradation of each color component in the target pixel of the color input image data An image processing method for determining an output gradation value of each color component in the target pixel of the color output image data based on a result of comparing the value with a threshold value for each color component in the target pixel of the dither matrix If it is determined whether the image represented in the color input image data includes a fine line and the color of the image, and it is determined that the image does not include a thin line, the image corresponds to the image. When a normal dither matrix is used as the dither matrix used for tone conversion of pixels and it is determined that the image includes a thin line, a thin line corresponding to the color of the image Select a dither matrix, a dither matrix for thin wire corresponding to the color of the image,
(A) In each pixel, a threshold value for each color component constituting the color of the image is selected from among L that is not more than the lowest gradation and H that is not less than the highest gradation in the color input image data. (B) In each pixel, an image satisfying the condition that only one of the color components constituting the color of the image has a threshold value L. The processing method is summarized.

  In the image processing method of the present invention, when a thin line is included in the image represented in the color input image data, a thin line dither matrix corresponding to the color of the image is used in the dither processing of the pixel corresponding to the image.

  In this thin line dither matrix, the threshold value of at least one of the color components included in the corresponding color is L (below the minimum gradation value in the color input image data) in each pixel. It is configured.

  Accordingly, when dither processing is performed using the thin line dither matrix on the pixels in which the image is formed in the color input image data (the gradation of the color component constituting the color of the image is not less than the minimum value), Since at least one gradation value of the color components included in the color of the image is larger than the threshold value L, the corresponding pixel in the color output image data always has the color of the color component.

  That is, the predetermined pixel in the color input image data is originally colored as in the conventional dither processing, but the threshold value of the corresponding pixel in the dither matrix is large. There is no such thing as being blank.

  For this reason, even if the image displayed in the color input image data is a thin line, the pixel corresponding to the thin line becomes blank and the thin line disappears, or a part of the pixel corresponding to the thin line becomes blank. Thus, the thin line does not become a broken line. In addition, when the image represented in the color output image data is a fine line and a fill having the fine line as a boundary, a part of the fine line is not blank and unevenness is not generated at the boundary.

  Further, the thin line dither matrix is configured such that, in each pixel, only the threshold value of any one of the color components constituting the corresponding color is L. For this reason, when dither processing is performed on a pixel on which an image is formed in the color input image data, the color component whose gradation value exceeds the threshold value L is only 1 for that pixel. Only the color component whose gradation value exceeds the threshold value L is attached to the pixel, and no other color component is attached.

Therefore, according to the image processing apparatus of the present invention, there is no case where a plurality of color components are mixed and the color is not changed in each pixel of the color output image data.
(7) The invention of claim 7
Only when the color of the image is a specific color set in advance, the dither matrix for fine lines corresponding to the color of the image is selected as the dither matrix used for gradation conversion of pixels corresponding to the image. The gist of the image processing method according to claim 6 is as follows.

In the present invention, the thin line dither matrix is selected only when the color of the image including the thin line is a specific color set in advance. Therefore, the number of types of the thin line dither matrix can be reduced. Therefore, the capacity of the storage means for storing the thin line dither matrix can be reduced.
(8) The invention of claim 8
When the color of the image is composed of two or more color components, in the dither matrix for fine lines,
Of the two or more color components, one pixel having a threshold value of L and another pixel having a threshold value of L are dispersedly arranged. The gist of the image processing method according to claim 6.

  In the present invention, in the thin line dither matrix, a pixel having a threshold value L of one of two or more color components and a pixel having a threshold value L of another color component are distributed. Has been placed.

  The pixel whose threshold value of the one color component is L corresponds to a pixel to which the color of the first color component is attached in the color output image data after dither processing. In addition, the pixels whose threshold values of the other color components are L correspond to the pixels to which the colors of the color components are added in the color output image data after the dither processing.

In the present invention, as described above, in the thin line dither matrix, a pixel in which the threshold value of one of the two or more color components is L, and a pixel in which the threshold value of the other color components is L Are dispersedly arranged, in the color output image data after dither processing, the pixels with the color of the one color component and the pixels with the other color components are dispersedly arranged. As a result, an image printed based on the color output image data does not collect pixels of a specific color and looks as a uniform color as a whole.
(9) The invention of claim 9
When the color of the image is composed of the color component 1 and the color component 2, in the thin line dither matrix, the pixel having a threshold value of the color component 1 and the threshold value of the color component 2 is L. The gist of the image processing method according to any one of claims 6 to 8, wherein the certain pixels are alternately arranged in a vertical direction and a horizontal direction in the arrangement of the pixels.

In the present invention, in the thin line dither matrix, pixels whose color component 1 threshold is L and pixels whose color component 2 threshold is L are alternately arranged.
A pixel having a color component 1 threshold value L corresponds to a pixel to which a color of the color component 1 is attached in the color output image data after dither processing. A pixel having a threshold value of color component 2 corresponding to L corresponds to a pixel to which the color of color component 2 is added in the color output image data after dither processing.

In the present invention, as described above, in the thin line dither matrix, the pixels whose color component 1 threshold is L and the pixels whose color component 2 threshold is L are alternately arranged. In the color output image data after the dither processing, the pixels with the color component 1 and the pixels with the color component 2 are alternately arranged. As a result, the image printed based on the color output image data looks like a uniform color as a whole without collecting pixels of a specific color.
(10) The invention of claim 10
The gist of the image processing method according to claim 6, wherein the image including the fine line is an image formed based on a line drawing command.

The present invention illustrates an image including a thin line. According to the present invention, an image formed based on a line drawing command can be subjected to image processing without causing a part of the image to disappear.
(11) The invention of claim 11
The determination means according to claim 1, for performing gradation conversion from multi-tone color input image data composed of a plurality of color components to binarized color output image data. The gist of the present invention is an image processing program that functions as the image determination unit and the selection unit.

  According to the image processing program of the present invention, it is possible to cause a computer to function as the determination unit, the image determination unit, and the selection unit according to claims 1 to 5.

1. Configuration of Color Electrophotographic Printer Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a sectional view showing a schematic configuration of a color electrophotographic printer 1 to which the present invention is applied. In FIG. 1, a color electrophotographic printer 1 is a so-called horizontal tandem type color electrophotographic printer in which four image forming units 20 are arranged in a horizontal direction. A paper feeding unit 9 for feeding recording paper 3 as a recording medium, an image forming unit 4 for forming an image on the fed recording paper 3, and a paper for discharging the recording paper 3 on which an image is formed. And a control unit 90 for controlling the operation of the color electrophotographic printer 1.

  The paper feed unit 9 includes a paper feed tray 12 that is detachably attached to the main body casing 5 from the front side (the right side in FIG. 1) at the bottom of the main body casing 5, and an upper part of one end of the paper feed tray 12 ( A paper feed roller 83 provided on the upper front side and a front side of the paper feed roller 83, a downstream side in the transport direction of the recording paper 3 with respect to the paper feed roller 83 (hereinafter, downstream in the transport direction of the recording paper 3). Conveying rollers 14 a and 14 b provided on the downstream side and the upstream side in the conveying direction of the recording paper 3 may be omitted as upstream.

  The recording paper 3 is stacked in the paper feed tray 12, and the uppermost recording paper 3 is fed toward the transport rollers 14a and 14b one by one by the rotation of the paper feed roller 83. Then, the toner is sequentially fed between the conveying belt 68 and each photosensitive drum 62 (transfer position).

  A guide member 15 disposed in the vertical direction is provided between the transport roller 14a and the transport roller 14b, and the recording paper 3 fed by the paper feed roller 83 is supplied to the transport roller 14a and the guide. The material 15 and the conveyance roller 14b are sequentially fed between the conveyance belt 68 and the photosensitive drum 62 (transfer position).

  The image forming unit 4 transfers the images formed by the four image forming units 20Y, 20M, 20C, and 20K that form images and the image forming units 20 to the recording paper 3 in the intermediate portion in the main body casing 5. The image forming apparatus includes a transfer unit 17 and a fixing unit 8 that heats and presses an image transferred onto the recording paper 3 and fixes the image on the recording paper 3. The subscripts Y, M, C, and K represent yellow (Y), magenta (M), cyan (C), and black (K), respectively. These need to be individually distinguished. If there is no, the above suffix is omitted.

  Each image forming unit 20 forms an electrostatic latent image on the photosensitive drum 62 and the photosensitive drum 62 that charges the photosensitive drum 62 around the photosensitive drum 62 as the image carrier. Developing means for forming a toner image by attaching toner as a developer to the photosensitive drum 62 by a developing bias applied between the exposure unit 41 that is an electrostatic latent image forming means and the photosensitive drum 62. The developing unit 51 is arranged.

  The charger 31 is, for example, a scorotron charger for positive charging that generates corona discharge from a charging wire made of tungsten or the like and uniformly charges the surface of the photosensitive drum 62 to positive polarity. The exposure unit 41 includes an LED array that generates light for forming an electrostatic latent image on the surface of the photosensitive drum 62.

  In this exposure unit 41, light emitted from the LED array is irradiated onto the photosensitive drum 62, and an electrostatic latent image is formed on the surface of the photosensitive drum 62. Note that the exposure unit 41 is not necessarily an LED array, and may be, for example, an exposure scanning unit (laser scanner) that exposes the photosensitive drum 62 by scanning a laser beam. It is.

  The developing unit 51 includes a hopper 56, a supply roller 32, and a developing roller 52 in a developing casing 55. The hopper 56 is formed as an internal space of the developing casing 55. In each hopper 56, yellow (Y), magenta (M), cyan (C), and black (K) toners (for example, positively charged nonmagnetic one-component toner) are provided for each image forming unit 20. Polymerized toner) is contained.

  That is, the four image forming units 20 described above include an image forming unit 20Y in which yellow (Y) toner is stored in the hopper 56, an image forming unit 20M in which magenta (M) toner is stored in the hopper 56, and The image forming unit 20C contains cyan (C) toner in the hopper 56 and the image forming unit 20K contains black (K) toner in the hopper 56. The only difference is the color of the toner. , Having the same configuration (in FIG. 1, some symbols are omitted).

  The supply roller 32 is disposed below the hopper 56, and a metal roller shaft is covered with a roller portion made of a conductive sponge member. The supply roller 32 is rotatably supported so as to rotate in a direction opposite to the developing roller 52 at a nip portion facing and contacting the developing roller 52.

  The developing roller 52 is rotatably disposed at a position opposite to and in contact with the supply roller 32 on the side of the supply roller 32. The developing roller 52 is formed by covering a metal roller shaft with a roller portion made of an elastic member such as a conductive rubber material. As will be described later, a predetermined developing bias voltage is applied from a power source 110 (see FIG. 2). It is configured to be.

  The transfer unit 17 is provided in the main body casing 5 so as to face the photosensitive drum 62. The transfer unit 17 includes a transport belt driving roller 63, a transport belt driven roller 64, a transport belt 68 that is an endless belt, and a transfer roller 61.

  The conveyance belt driven roller 64 is upstream (front side) from the photosensitive drum 62 of the yellow image forming unit 20 </ b> Y on the most upstream side with respect to the conveyance direction of the recording paper 3, and above the sheet feed roller 83. It is arranged. Further, the conveyance belt drive roller 63 is downstream (rear side) of the photosensitive drum 62 of the black image forming unit 20K on the most downstream side with respect to the conveyance direction of the recording paper 3, and is more than the fixing unit 8. Arranged on the upstream side (front side).

  The conveyor belt 68 is wound between the conveyor belt driving roller 63 and the conveyor belt driven roller 64. The conveyor belt 68 is disposed such that the outer surface around which it is wound is in opposed contact with all the photosensitive drums 62 of each image forming unit 20.

  The conveyance belt driven roller 64 is driven by the driving of the conveyance belt driving roller 63, and the conveyance belt 68 rotates in a counterclockwise direction between the conveyance belt driving roller 63 and the conveyance belt driven roller 64. That is, the transport belt 68 moves in the same direction as the photosensitive drum 62 on the contact surface that faces and contacts each photosensitive drum 62 of each image forming unit 20.

  Further, the transfer roller 61 is disposed inside the wound conveyance belt 68 so as to face the photosensitive drum 62 of each image forming unit 20 with the conveyance belt 68 interposed therebetween. The transfer roller 61 is formed by covering a metal roller shaft with a roller portion made of an elastic member such as a conductive rubber material.

  The transfer roller 61 is rotatably provided in the counterclockwise direction so that the transfer roller 61 rotates in the same direction as the circumferential movement direction of the conveyor belt 68 on the contact surface facing the conveyor belt 68. A predetermined voltage is applied between the transfer roller 61 and the photosensitive drum 62 from a power source (not shown) in a direction in which the toner image carried on the photosensitive drum 62 is transferred (transferred) to the recording paper 3 during transfer. And an appropriate transfer bias is applied by constant current control.

  The fixing unit 8 is disposed on the downstream side (rear side) of the image forming unit 20 and the transfer unit 17. The fixing unit 8 includes a heating roller 81 and a pressing roller 82. The heating roller 81 is made of a metal base tube having a release layer formed on the surface thereof, and a halogen lamp is provided along the axial direction thereof. Then, the surface of the heating roller 81 is heated to the fixing temperature by the halogen lamp. The pressing roller 82 is disposed so as to press the heating roller 81.

  The paper discharge unit 6 is disposed on the downstream side of the fixing unit 8 in the upper part of the main body casing 5. The paper discharge unit 6 is provided with a pair of paper discharge rollers 11 for discharging the recording paper 3 on which image fixing has been completed to the paper discharge tray 10, and a downstream side of the paper discharge rollers 11. A paper discharge tray 10 for accumulating all the recording sheets 3 that have been completed is provided.

  Further, a density sensor 80 for reading a batch or the like formed on the conveyor belt 68 is provided opposite to the outer surface of the conveyor belt 68 at the lower rear side of the conveyor belt drive roller 63. A toner recovery unit 107 for recovering toner (such as the patch) attached on the conveyor belt 68 is obliquely forward and downward, and the toner recovery roller 105 of the toner recovery unit 107 contacts the outer surface of the conveyor belt 68. It is arranged like this.

  Next, while explaining the electrical configuration of the color electrophotographic printer 1 with reference to FIG. 2, the process until the color electrophotographic printer 1 forms a color image on the recording paper 3 by the above-described cooperative operation of each unit in the apparatus. The process will be described. FIG. 2 is a block diagram schematically showing the electrical configuration of the color electrophotographic printer 1.

  As shown in FIG. 2, the color electrophotographic printer 1 includes a control unit 90 (including a CPU 210, a ROM 230, a RAM 220, a driver 250, an I / O, etc.) that controls each unit of the apparatus. The image forming operation is performed. The control unit 90 is connected to an external personal computer (PC) (not shown) and can receive a print command from the PC.

  In the image forming operation, the control unit 90 of the color electrophotographic printer 1 performs initial setting of each unit to be controlled by the main control processing unit (program) and then charges the surface of the photosensitive drum 62. The unit 31 is uniformly charged, and light is irradiated from the exposure unit 41 according to the image information to form an electrostatic latent image on the surface of the photosensitive drum 62. Next, toner is attached to the surface of the photosensitive drum 62 by the developing unit 51 to develop the electrostatic latent image on the surface of the photosensitive drum 62. As the photosensitive drum 62 rotates, the developed toner image is moved to the transfer position.

  Further, the controller 90 feeds the recording paper 3 onto the transport belt 68 by operating the paper feed roller 83 and the transport rollers 14 a and 14 b. Then, the conveyance belt driving roller 63 is driven to move the conveyance belt 68 around, and the recording paper 3 is supplied to the transfer position. At the transfer position, a transfer bias is applied between the transfer roller 61 and the photosensitive drum 62 to transfer the toner image onto the recording paper 3.

  Next, the control unit 90 moves the conveyance belt 68 around and conveys the recording paper 3 to the fixing unit 8. In the fixing unit 8, the recording paper 3 is nipped and conveyed by the heating roller 81 and the pressing roller 82, and the toner image on the recording paper 3 is heated and pressurized to be fixed on the recording paper 3. Then, the paper discharge roller 11 is operated to discharge the recording paper 3 to the paper discharge tray 10 above the main body casing 5, and the image forming operation is finished.

Next, the control unit 90 will be described in more detail with reference to FIG. FIG. 3 is a block diagram showing a part of the configuration of the control unit 90.
As shown in FIG. 3, the control unit 90 includes a CPU 210, a RAM 220, and a ROM 230, and these are communicably connected via a bus 250.

  The RAM 220 has an input image storage unit 221 for storing color input image data and an output image storage unit 222 for storing color output image data obtained by binarizing the color input image data. .

  Here, the color input image data is four types of color input image data of black (K), cyan (C), magenta (M), and yellow (Y) corresponding to each print color for printing a color image. That is, the color electrophotographic printer 1 generates color output image data of each print color corresponding to the color input image data by processing to be described later. A color image is expressed by printing the print color images based on the color output image data so as to overlap each other on the paper.

  The ROM 230 has a processing program (corresponding to an image processing program) 231 for causing the CPU 210 to perform processing described later using a print command received from an external personal computer, and a program for executing normal processing in a color electrophotographic printer. It is remembered. The reason why the color input image data is set to 16 gradations is for the sake of convenience in order to make the explanation easy to understand, and is applied to color input image data of other gradations, for example, 256 gradations. Is possible.

  The ROM 230 records a normal dither matrix 234 and a thin line dither matrix 235 as information used in halftone processing described later. Here, as shown in FIG. 4, the normal dither matrix 234 stores the threshold value T of 1 to 16 in a total of 16 pixels, 4 in each of the vertical and horizontal directions. This is conventionally used as a dither matrix for binarizing data by the dither method. The normal dither matrix 234 is prepared for each of C, M, Y, and K (for convenience, only M and Y are shown in FIG. 4), and the threshold arrangement pattern is different for each color.

  On the other hand, as shown in FIG. 5, the dither matrix 235 for thin lines is one in which threshold values T of 1 or 16 are stored in a total of four pixels, two in the vertical direction and two in the horizontal direction. It is used as a dither matrix for binarizing input image data by the dither method. Thin line dither matrices 235 are prepared corresponding to specific colors (color numbers 1, 2, 3, 4...) Shown in the dither setting color table of Table 1. That is, a thin line dither matrix 235-1 corresponding to color number 1, a thin line dither matrix 235-2 corresponding to color number 2, a thin line dither matrix 235-3 corresponding to color number 3, and a color number 4 Dither matrixes 235-4... For thin lines corresponding to are prepared.

  The configuration of the thin line dither matrix 235 will be described by taking the thin line dither matrix 235-4 corresponding to the color number 4 as an example. The color number 4 is a color in which M and Y are 1: 1. Therefore, the thin line dither matrix 235-4 corresponding to the color number 4 includes the M dither matrix 235-4M and the Y dither matrix 235-4Y shown in FIG.

  When attention is paid to the upper left pixel in each of the dither matrix 235-4M and the dither matrix 235-4Y, the former threshold value T is 1 and the latter threshold value T is 16. When attention is paid to the upper right pixel, the former threshold value T is 16 and the latter threshold value T is 1. When attention is paid to the lower left pixel, the former threshold value is 16 and the latter threshold value T is 1. When attention is paid to the lower right pixel, the former threshold value T is 1 and the latter threshold value T is 16. That is, when attention is paid to the corresponding pixels of the dither matrix 235-4M and the dither matrix 235-4Y, only one of the threshold values T is 1. Further, in each of the dither matrix 235-4M and the dither matrix 235-4Y, the pixels having the threshold value T of 1 and the pixels having the threshold value T of 16 are alternately arranged in the vertical direction and the horizontal direction (distributed arrangement). Have been).

  The thin line dither matrices 235-1 to 235-3 corresponding to the color numbers 1, 2, 3, 5... Are basically the same as the case of the color number 4. However, in the case of the color number 4, the combination of the M dither matrix 235-4M and the Y dither matrix 235-4Y is used, but in the case of other color numbers, the combination corresponds to each color. .

  The dither matrix 235 for fine lines is not limited to a total of 4 pixels, 2 in the vertical direction and 2 in the horizontal direction. You may have. However, in order to reduce the capacity of the ROM 230 that stores the thin line dither matrix 235, it is preferable that the number of pixels is small.

Further, the ROM 230 stores a correspondence table of color numbers 1, 2, 3, 4,... And a thin line dither matrix corresponding to each color number.
2. Processing Performed by Control Unit 90 First, an outline of control performed by the control unit 90 will be described with reference to FIG. FIG. 6 is a flowchart showing print processing executed by the control unit 90 when a print command is received from an external personal computer.

In step 100, a page description language is input from a printer driver such as an external personal computer. This page description language includes a print command.
In step 110, RGB input image data expressed in RGB is created from the print command by drawing processing. This drawing process will be described in detail later.

In step 120, color conversion is performed. That is, the RGB input image data is converted into four types of color input image data K, C, M, and Y.
In step 130, binarized color output image data is created by halftone processing. This halftone process will be described in detail later.

In step 140, printing is performed based on the color output image data.
Next, the drawing process (step 110 in FIG. 6) will be described with reference to FIGS. FIG. 7 is a flowchart showing a drawing process, FIG. 8 is an explanatory diagram for a print command, and FIG. 9 is an explanatory diagram for registration of a thin line portion.

  In step 200, the type of print command input from a printer driver such as an external personal computer is checked. As types of print commands, there are a line drawing command, a graphic drawing command, and a character drawing command as shown in FIG.

  In step 210, it is determined whether or not the print command checked in step 200 is a line drawing command. If yes, then continue with step 220, otherwise continue with step 250.

  In step 220, in the color input image data, a pixel on which an image based on the line drawing command is formed is registered as a thin line portion. For example, as shown in FIG. 9, a line is applied to a pixel A having x coordinate = 1 and y coordinate = 2, a pixel B having x coordinate = 1 and y coordinate = 3, and a pixel C having x coordinate = 1 and y coordinate = 4. When an image based on the drawing command is formed, the pixels A, B, and C are registered as thin line portions.

  In step 230, color input image data is created based on the print command. As described above, this color input image data is four types of color input image data K, C, M, and Y corresponding to each print color for printing a color image. Gradation values (any one of 1 to 16) are recorded. If the print command is a line drawing command, the fine line location registration made in step 220 is present in the pixel on which the image (line) based on the line drawing command is formed.

  In step 240, it is determined whether all print commands input from the printer driver of the external personal computer have been checked. If YES, the process is terminated, and if NO, the process returns to Step 200.

  On the other hand, if NO in step 210, the process proceeds to step 250. In step 250, the print command checked in step 200 is a print command (graphic drawing command or character drawing command) other than the line drawing command. Determine whether or not. If yes, then continue with step 230, otherwise continue with step 240.

  Next, halftone processing (step 130 in FIG. 6) will be described with reference to FIGS. FIG. 10 is a flowchart showing halftone processing, and FIG. 11 is an explanatory diagram showing halftone processing in pixels registered as thin line portions.

  In step 300, a predetermined pixel in the color input image data is selected, and it is checked whether or not the thin line portion is registered for the pixel (see step 220 in FIG. 7). Note that the pixel to be selected in step 300 is determined by, for example, assigning a number to each pixel of the color input image data, and when executing step 300 for the first time, when executing the first pixel for the second time. Can be selected as a second pixel.

  In step 310, it is determined whether or not a fine line portion is registered in the pixel checked in step 300. If yes, then continue with step 320, otherwise continue with step 360.

In step 320, the color attached to the pixel is checked.
In step 330, it is determined whether or not the color detected in step 330 is one of color numbers 1, 2, 3, 4,... (Preset dither setting color). to decide. If yes, then continue with step 340, otherwise continue with step 360.

  In step 340, the selected pixel is binarized using the thin line dither matrix 235. This process will be described based on the case shown in FIG. FIG. 11 is an explanatory diagram showing the principle of the binarization process executed in step 340.

  FIG. 11a represents color input image data before binarization, and each area partitioned by a grid represents one pixel. In this color input image data, a pixel in which “8” is described is a pixel in which the gradation values of M and Y are 8, and the gradation values of C and K are 1. A pixel in which “1” is described is a pixel having a gradation value of 1 for all color components of C, M, Y, and K. That is, in the color input image data, a thin line image having a width of 2 pixels is drawn in the horizontal direction. This fine line is formed based on a line drawing command, and the pixel on which the fine line is drawn has a registration of the fine line part. The color of the thin line is a color having a ratio of M to Y of 1: 1, and this color is color number 4 (see Table 1 above) which is one of the dither setting colors.

  In this example, since the color of the fine line is color number 4 as described above, the control unit 90 reads the dither matrix 235-4 for fine line corresponding to the color from the ROM 230 and performs dither processing for the selected pixel. .

  Here, FIG. 11b is obtained by fitting the dither matrix 235-4M of the thin line dither matrix 235-4 to the color input image data of FIG. 11a, and FIG. 11c is the color of FIG. 11a. Of the dither matrix 235-4 for thin lines, the Y dither matrix 235-4Y is applied to the input image data.

  In FIG. 11b, the pixels whose numbers are outlined are pixels whose M gradation value is larger than the threshold value T of the dither matrix 235-4M. In addition, in FIG. 11c, pixels whose numbers are outlined are pixels whose Y gradation value is larger than the threshold value T of the dither matrix 235-4Y.

  Therefore, if the selected pixel is a pixel whose number is white in FIG. 11b, the pixel is given a color M, and if it is a pixel whose number is white in FIG. 11c, Only the Y color is assigned, and if it is a pixel other than those, it is blank.

  FIG. 11d shows the color output image data after dither processing. In this color output image data, pixels represented by solid fill are pixels marked with only M, and pixels represented by diagonal lines are pixels marked with only Y.

In step 350, it is determined whether or not processing has been completed for all pixels. If YES, the process is terminated, and if NO, the process returns to step 300.
On the other hand, if NO is determined in step 310 or step 330, the process proceeds to step 360. In step 360, normal binarization processing is executed using the normal dither matrix 234 (see FIG. 4).
3. Effects produced by the color electrophotographic printer 1 (a) As described above, in the color electrophotographic printer 1, the image represented in the color input image data is a fine line derived from a line drawing command, and the color of the fine line is If the dither setting color is preset, the dither matrix 235 for thin lines corresponding to the color of the thin line (see FIG. 5) is used in the dither processing of the pixel corresponding to the thin line (step 310 in FIG. 10). ~ 340).

  In the thin line dither matrix 235, the threshold value T of at least one of the color components (M and Y in the example shown in FIG. 5) included in the corresponding color is 1 in each pixel. Therefore, when the thin line is dithered using the thin line dither matrix 235, a certain color component is always attached to the pixel corresponding to the thin line in the color output image data (see FIG. 11). For this reason, even if the image displayed in the color input image data is a thin line, the pixel corresponding to the thin line becomes blank and the thin line disappears, or a part of the pixel corresponding to the thin line becomes blank. Thus, the thin line does not become a broken line.

  Further, the thin line dither matrix 235 is configured so that only the threshold value T of any one of the color components constituting the corresponding color is 1 in each pixel (see FIG. 5). Therefore, when dither processing is performed on a pixel in which fine lines are formed in the color input image data, the color component whose gradation value exceeds the threshold value T is only 1 in the pixel. A pixel has only a single color component whose gradation value exceeds the threshold value T, and no other color component (see FIG. 11d). Therefore, in each pixel of the color output image data, a plurality of color components are not mixed and the color does not change.

  (B) Since the color electrophotographic printer 1 selects the thin line dither matrix 235 only when the color of the image including the thin line is a specific dither color set in advance, the number of types of the thin line dither matrix 235 is small. I'll do it. Therefore, the storage capacity of the ROM 230 that stores the thin line dither matrix 235 is small.

(C) In the thin-line dither matrix 235 shown in FIG. 5, pixels with an M threshold value of 1 and pixels with a Y threshold value of 1 are alternately arranged.
By the way, a pixel having an M threshold value of 1 corresponds to a pixel having an M color in the color output image data after dither processing. A pixel having a Y threshold value of 1 corresponds to a pixel with a Y color in the color output image data after dither processing.

  As described above, in the thin line dither matrix 235, the pixels whose M threshold is 1 and the pixels whose Y threshold is 1 are alternately arranged. In the color output image data, pixels with M and pixels with Y are alternately arranged (see FIG. 11d). As a result, an image printed based on the color output image data does not collect pixels of a specific color (M or Y), and looks as a uniform color as a whole.

  As mentioned above, although one Embodiment of this invention was described, it cannot be overemphasized that this invention can take a various form. For example, the present invention can be applied not only to a printer but also to a personal computer or the like. That is, the processing shown in FIG. 12 can be executed by software installed in a personal computer. In step 400, image information is input from the application in step 400, drawing processing is performed in step 410, color conversion is performed in step 420, and halftone processing is performed in step 430. The processing in steps 410 to 430 is the same as the processing in steps 110 to 130 in FIG. Next, in step 440, the color output image data is output to the printer. Furthermore, the present invention can also be applied to a copy machine and a multi-function machine.

  As another embodiment, in the drawing process (step 110 in FIG. 6), only the pixels in which the image is a thin line portion and the image color is a color set with dither may be registered. This drawing process will be described based on the flowchart of FIG.

In step 500, the type of print command input from a printer driver such as an external personal computer is checked.
In step 510, it is determined whether or not the print command checked in step 500 is a line drawing command. If yes, then continue with step 520, otherwise continue with step 560.

  In step 520, it is determined whether the color of the line drawn by the line drawing command is a preset dither setting color. If yes, then continue with step 530, otherwise continue with step 540.

  In step 530, in the color input image data, a pixel on which an image based on the line drawing command is formed is registered as a pixel “which is a color with a thin line portion and dither set”.

  In step 540, color input image data is created based on the print command. When registration is performed in step 530, the registration is performed on the corresponding pixel in the color input image data.

  In step 550, it is determined whether all print commands input from a printer driver such as an external personal computer have been checked. If YES, the process is terminated, and if NO, the process returns to step 500.

  On the other hand, if NO is determined in step 510, the process proceeds to step 560. In step 560, the print command checked in step 500 is a print command (graphic drawing command or character drawing command) other than the line drawing command. Determine whether or not. If yes, then continue with step 540, otherwise continue with step 550.

When the drawing process is performed as described above, the halftone process (step 130 in FIG. 6)
Whether to perform binarization using the normal dither matrix or binarization using the dither matrix for thin lines only with or without the registration of “the color is a thin line location and dither set” Can be determined. Such halftone processing will be described with reference to the flowchart of FIG.

  In step 600, a predetermined pixel in the color input image data is selected, and it is checked whether or not registration (step 530 in FIG. 13) that “the color is a thin line portion and a dither is set” is made to the pixel. To do.

  In step 310, it is determined whether or not the pixel checked in step 600 is registered. If yes, then continue with step 620, otherwise continue with step 640.

In step 620, binarization is performed using the dither matrix 235 for fine lines. This process is the same as the process in step 340 of FIG.
In step 630, it is determined whether or not processing has been completed for all pixels. If YES, the process is terminated, and if NO, the process returns to Step 600.

  On the other hand, if NO is determined in step 610, the process proceeds to step 640. In step 640, normal binarization processing is executed using the normal dither matrix 234 (see FIG. 4).

1 is an explanatory diagram showing a schematic configuration of a color electrophotographic printer 1 to which the present invention is applied. FIG. 2 is a block diagram schematically illustrating an electrical configuration of the color electrophotographic printer 1. FIG. 3 is a block diagram illustrating a configuration of a main part in a control unit 90. FIG. 5 is an explanatory diagram showing a normal dither matrix 234. FIG. It is explanatory drawing showing the dither matrix 235 for thin lines. 6 is a flowchart illustrating a printing process executed by a control unit 90. It is a flowchart showing the drawing process which the control part 90 performs. It is explanatory drawing showing a print command. It is explanatory drawing showing registration of a thin line location. It is a flowchart showing the halftone process which the control part 90 performs. It is explanatory drawing showing the halftone process in the pixel registered as a thin line location. It is a flowchart showing the process at the time of applying this invention to a personal computer. In another embodiment, it is a flowchart showing the drawing process which the control part 90 performs. In another embodiment, it is a flowchart showing the halftone process which the control part 90 performs. It is explanatory drawing showing the conventional dither process.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Color electrophotographic printer 90 ... Control part 210 ... CPU
220 ... RAM
230 ... ROM
234: Normal dither matrix 235: Dither matrix for fine lines

Claims (11)

Each color component in the pixel of interest of the color input image data as means for performing tone conversion from multi-tone color input image data composed of a plurality of color components to binarized color output image data The output tone value of each color component at the target pixel of the color output image data is determined based on the result of comparing the tone value of the color output value with the threshold value for each color component at the target pixel of the dither matrix. An image processing apparatus provided with a determining means,
Image determining means for determining whether an image represented in the color input image data includes a thin line and a color of the image;
When the image determining unit determines that the image does not include a thin line, the image determining unit uses a normal dither matrix as the dither matrix used for gradation conversion of pixels corresponding to the image, and the image includes a thin line. A selection means for selecting a dither matrix for fine lines corresponding to the color of the image,
The dither matrix for fine lines corresponding to the color of the image is
(A) In each pixel, a threshold value for each color component constituting the color of the image is selected from among L that is not more than the lowest gradation and H that is not less than the highest gradation in the color input image data. (B) In each pixel, an image satisfying the condition that only one of the color components constituting the color of the image has a threshold value L. Processing equipment.   The selection means uses the fine line corresponding to the color of the image as the dither matrix used for gradation conversion of the pixel corresponding to the image only when the color of the image including the thin line is a predetermined specific color. The image processing apparatus according to claim 1, wherein a dither matrix is selected.   In the case where the color of the image including the fine line is composed of two or more color components, in the fine line dither matrix, a pixel whose threshold value of one color component of the two or more color components is L, and other The image processing apparatus according to claim 1, wherein the pixels whose color component threshold value is L are distributed.   When the color of the image including the thin line is composed of the color component 1 and the color component 2, in the thin line dither matrix, the pixel having the threshold value of the color component 1 being L, and the threshold value of the color component 2 The image processing apparatus according to claim 1, wherein the pixels whose L is L are alternately arranged in a vertical direction and a horizontal direction in the pixel array.   The image processing apparatus according to claim 1, wherein the image including the thin line is an image formed based on a line drawing command. In order to perform gradation conversion from multi-tone color input image data composed of a plurality of color components to binarized color output image data, the gradation of each color component in the target pixel of the color input image data An image processing method for determining an output gradation value of each color component in the target pixel of the color output image data based on a result of comparing the value with a threshold value for each color component in the target pixel of the dither matrix Because
Determining whether the image represented in the color input image data includes a fine line and the color of the image;
When it is determined that the image does not include a thin line, a normal dither matrix is used as the dither matrix used for gradation conversion of the pixel corresponding to the image, and when it is determined that the image includes a thin line. , Select a dither matrix for fine lines corresponding to the color of the image,
The dither matrix for fine lines corresponding to the color of the image is
(A) In each pixel, a threshold value for each color component constituting the color of the image is selected from among L that is not more than the lowest gradation and H that is not less than the highest gradation in the color input image data. (B) In each pixel, an image satisfying the condition that only one of the color components constituting the color of the image has a threshold value L. Processing method.   Only when the color of the image is a specific color set in advance, the dither matrix for fine lines corresponding to the color of the image is selected as the dither matrix used for gradation conversion of pixels corresponding to the image. The image processing method according to claim 6. When the color of the image is composed of two or more color components, in the dither matrix for fine lines,
Of the two or more color components, one pixel having a threshold value of L and another pixel having a threshold value of L are dispersedly arranged. The image processing method according to claim 6 or 7.   When the color of the image is composed of the color component 1 and the color component 2, in the thin line dither matrix, the pixel having a threshold value of the color component 1 and the threshold value of the color component 2 is L. The image processing method according to claim 6, wherein the certain pixels are alternately arranged in a vertical direction and a horizontal direction in the arrangement of the pixels.   The image processing method according to claim 6, wherein the image including the thin line is an image formed based on a line drawing command.   The determination means according to claim 1, for performing gradation conversion from multi-tone color input image data composed of a plurality of color components to binarized color output image data. An image processing program that functions as the image determination unit and the selection unit.

Images