Image forming apparatus

Abstract

To provide an image forming apparatus that, even when printing an image in which a specific coloring material color is used in a large amount, can make jitter inconspicuous compared with conventional processing.SOLUTION: An image forming apparatus according to the present disclosure performs image processing on input image data and forms an image on a medium, and comprises: a storage unit that stores the combinations of a plurality of dithers used in dither processing; a color separation processing unit that separates the input image data into coloring material colors and acquires half tone image data of the coloring material colors; a half tone counting unit that calculates, for the half tone image data, the number of pixels to be a half tone of the coloring material colors, and compares the calculated number of pixels between the coloring material colors; and a dither processing unit that, on the basis of a result of comparison of the number of pixels, determines the combination of dithers to be used in the dither processing and performs the dither processing.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to an image forming apparatus, and can be applied to, for example, an electrophotographic printer that uses a plurality of colorant colors.

Background Art

[0002] Conventionally, in an image forming apparatus such as a printer with poor conveyance accuracy, density unevenness (dither) occurring horizontally with respect to the conveyance direction may be pointed out as an image defect.

[0003] The visibility of the above dither varies depending on the angle of the screen (dither) used for printing, and the dither tends to be less visible when the angle of the dither is an angle along the conveyance direction (vertical direction).

[0004] Therefore, in an apparatus where dither is prominent, it is preferable to create the dither so that the angle is in the vertical direction as much as possible and use the above dither for printing. For example, in the technique described in Patent Document 1, a device is devised to suppress deterioration of image quality due to dither by using a dither that is always in the vertical direction when viewed with respect to the conveyance direction regardless of the orientation of the image.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, in color printing using a plurality of colorant colors, it is necessary to prepare dithers for each colorant color (for example, four colors of black (K), yellow (Y), magenta (M), and cyan (C)).

[0007] Images on a recording medium are formed by overlapping dithers of all ink colors. However, moiré (interference fringes) occurs if the angles of the dithers for each color used are not appropriate. Due to such limitations, it is not possible to make all dithers approach an angle perpendicular to the conveyance direction.

[0008] For the above reasons, for ink colors for which the angle of the dither could not be made to approach an angle perpendicular to the conveyance direction, the image quality degradation due to dither is more prominent compared to ink colors for which the angle could be made to approach the perpendicular angle. When printing an image in which a specific ink color is used frequently, there is a problem that the dither becomes more prominent.

[0009] Therefore, there is a demand for an image forming apparatus that can make dither less prominent even when printing an image in which a specific ink color is used frequently.

Means for Solving the Problem

[0010] The present invention is an image forming apparatus that performs image processing on input image data to form an image on a medium, and includes: (1) a storage unit that stores combinations of a plurality of dithers used in dither processing; (2) a color separation processing unit that decomposes the input image data into each ink color and obtains halftone image data for each ink color; (3) a halftone count unit that calculates the number of pixels of pixels that become halftone for each ink color for each of the halftone image data and compares the calculated number of pixels for each ink color; and (4) a dither processing unit that determines the combination of the dithers used in the dither processing based on the result of comparing the number of pixels and performs the dither processing.

Effect of the Invention

[0011] According to the present invention, even when printing an image in which a specific ink color is used frequently, dither can be made less prominent compared to conventional processing.

Brief Description of the Drawings

[0012]

Figure 1

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Figure 14

Mode for Carrying Out the Invention

[0013] (A) First Embodiment Hereinafter, a first embodiment of the image forming apparatus according to the present disclosure will be described in detail with reference to the drawings. Hereinafter, an example in which the image forming apparatus of the present disclosure is applied to a printer will be described.

[0014] (A-1) Configuration of the First Embodiment (A-1-1) Overall Schematic Configuration FIG. 3 is a schematic cross-sectional view showing the overall configuration of the printer according to the first embodiment.

[0015] The printer 1 includes an image forming unit 2 (2K, 2Y, 2M, 2C), a paper feed roller 11, a paper feed sensor 12, a conveyance roller 14, a conveyance roller 15, a writing sensor 16, a conveyance belt driven roller 17, a conveyance belt driving roller 18, a conveyance belt 19, transfer rollers 10 (10K, 10Y, 10M, 10C), a fixing unit 20, a discharge sensor 24, a discharge roller 25, and a discharge roller 26.

[0016] The image forming units 2K, 2Y, 2M, 2C each form a toner image. The image forming units 2K, 2Y, 2M, 2C each form a toner image of the color of the coloring material (toner) of black (K), yellow (Y), magenta (M), and cyan (C). The image forming units 2K, 2Y, 2M, 2C each include LED heads 3K, 3Y, 3M, 3C, and form a toner image on the paper P as a medium fed using the transfer rollers 10K, 10Y, 10M, 10C installed opposite to each other. The details of this image forming unit 2 will be described again when explaining FIG. 1.

[0017] The paper feed roller 11 is a roller that feeds the paper P one by one from a paper cassette (not shown). The paper feed sensor 12 is a sensor for detecting the paper P fed from a paper cassette (not shown).

[0018] The conveyance rollers 14 and 15 are rollers that convey the fed paper P to the conveyance belt 19. The writing sensor 16 is a sensor for determining the writing start timing of the fed paper P.

[0019] The conveying belt 19 is an endless belt for conveying the fed sheet P. The conveying belt driving roller 18 is connected to a motor and gears (not shown) and is a roller for moving the conveying belt 19. The conveying belt driven roller 17 rotates in accordance with the rotation of the conveying belt 19 and is arranged to keep the tension of the conveying belt 19 constant.

[0020] The fixing device 20 incorporates a fixing roller 21 and a fixing backup roller 22. The fixing roller 21 is heated by an internal heater (a heating element such as a halogen lamp) (for example, heated at a temperature of about 170°C) to fix the toner image transferred onto the paper surface. The fixing backup roller 22 is a roller pressed against the fixing roller 21 by pressing means (not shown).

[0021] The discharge sensor 24 is a sensor for detecting the discharge state of the sheet P (such as conveyance errors or jams).

[0022] The discharge rollers 25 and 26 are rollers for discharging the sheet on which the image has been formed to the outside.

[0023] In addition, although not shown, the printer 1 also has as components a motor for rotating each roller, rollers on the conveyance path laid at a distance equal to or less than the minimum media interval, a clutch for turning on / off the power transmission to the rollers on the conveyance path, and the like.

[0024] (A-1-2) Configuration of the control system Next, the configuration of the control system of the printer 1 will be described. FIG. 1 is a block diagram showing the configuration of the control system of the printer according to the first embodiment.

[0025] In FIG. 1, the printer 1 includes a print control unit 100, a data reception unit 110, an image processing unit 120, a storage unit 130, and the above-described image forming unit 2.

[0026] The printing control unit 100 is responsible for the main function of the printer 1 (control of the printing operation). The printing control unit 100 has, for example, a CPU (not shown), and realizes the functions of the printer 1 by, for example, the CPU executing a program stored in the storage unit 130.

[0027] The data reception unit 110 is composed of a scanner, a network interface, etc. The data reception unit 110 reads an image from a document placed on a scanner (not shown), or writes the input image data acquired by receiving image data (printing data) sent from a higher-level device via the network interface into the storage unit 130.

[0028] The storage unit 130 is a storage unit that stores data and programs necessary for the printer 1 to function, and is composed of, for example, a ROM (Read Only Memory), a RAM (Random Access Memory), an HDD (Hard Disc Drive), etc.

[0029] FIG. 2 is a configuration diagram showing data stored in the storage unit according to the first embodiment and used in image processing. In FIG. 2, the storage unit 130 stores the input image data 141 by the above-described data reception unit 110, intermediate image data 142 generated during the processing of the image processing unit 120 described later, output image data 143, a dither 144 used in the dither processing unit 123 in the image processing unit 120, and the number of pixels 145. Details of each data will be described again in the description of the image processing unit 120.

[0030] The image forming unit 2 forms an image on the paper P based on the output image data 143 created by the image processing unit 120 and stored in the storage unit 130. The image forming unit 2 forms a full-color image based on data corresponding to the color of each color material (toner) of the output image data 143 created by the image processing unit 120.

[0031] The image forming unit 2 includes developing units corresponding to each of the colors C, M, Y, and K (a charging device 5 for charging the photosensitive drum 4, an LED head 3 as an exposure device for exposing the charged photosensitive drum 4 with light corresponding to an image to form an electrostatic latent image corresponding to the image on the photosensitive drum 4, and a developing device 6 for developing the electrostatic latent image formed on the photosensitive drum 4). Each color developing unit forms a toner image on the photosensitive drum 4 based on the output image data 143 of each color.

[0032] The toner images formed on the photosensitive drums of each color are sequentially transferred onto the paper P by the operation of the transfer roller 10, and then fixed onto the paper P by the fixing device 20 and output. The developing unit, the transfer roller 10, and the fixing device 20 use the rotation of a motor (not shown) as a driving source, and the rollers rotate through several gears to perform the conveyance, fixing, transfer, and development of the paper P. In a device with poor conveyance accuracy, speed unevenness may occur due to the pitch of the gears or the like, and as a result, the pitch of the colored and colorless portions of the fine pattern of the dither printed due to this changes, which is visually recognized as density unevenness (jitter) occurring horizontally with respect to the conveyance direction.

[0033] FIG. 14 is a simulation diagram showing the state of jitter generation. FIG. 14(A) shows 0-degree and 90-degree dot dither. FIG. 14(B) shows 90-degree line dither, and FIG. 14(C) shows 0-degree line dither.

[0034] In FIG. 14, the jitter generated in the pitch variation regions 200a and 200b changes in the way of jitter depending on the direction (angle) in which the fine pattern (dither) is continuous. The jitter is less noticeable when the angle of the dither is closer to the vertical direction and more noticeable when it is closer to the horizontal direction. That is, in FIG. 14, it can be said that the 90-degree line dither in FIG. 14(B) has less noticeable jitter, and the 0-degree line dither in FIG. 14(C) has a tendency for more noticeable jitter (the dot dither is in the middle between the two).

[0035] The image processing unit 120 is a functional unit that performs image processing to convert the input image data 141 into data that can be output by the image forming unit 2, and includes a color separation processing unit 121, a halftone count unit 122, and a dither processing unit 123.

[0036] <Color separation processing unit> The color separation processing unit 121 is a functional unit that decomposes the image data into the colors of the color materials (toners) held by the image forming unit 2. The color separation processing unit 121 reads out the input image data 141 stored in the storage unit 130, converts it into image data for each color plane of C color, M color, Y color, and K color, which are the colors of the color materials used by each image forming unit 2, and writes it into the storage unit 130 as intermediate image data 142.

[0037] <Halftone count unit> The halftone count unit 122 is a functional unit that counts the number of pixels that are halftones. The halftone count unit 122 reads out the intermediate image data 142 stored in the storage unit 130, counts the number of pixels that are halftones included in the image data of each of the C color, M color, Y color, and K color, and writes it into the storage unit 130 as the number of halftone pixels 145 for each color.

[0038] <Pixel count formula part 1> The halftone count unit 122 calculates the number of halftone pixels for each of the above colors specifically using the following formulas (1) to (4). Number of C color pixels = SUM(pixel count [Cipx]) ∵ C lower limit < Cipx < C upper limit …(1) Number of M color pixels = SUM(pixel count [Mipx]) ∵ M lower limit < Mipx < M upper limit …(2) Number of Y color pixels = SUM(pixel count [Yipx]) ∵ Y lower limit < Yipx < Y upper limit …(3) Number of K color pixels = SUM(pixel count [Kipx]) ∵ K lower limit < Kipx < K upper limit …(4)

[0039] Cipx, Mipx, Yipx, and Kipx in the above formulas (1) to (4) are the pixel values of the pixels included in the image data of each color of C color, M color, Y color, and K color. For example, if it is 8-bit data, it takes values from 0 to 255. However, in this process, only the pixels with intermediate tones are counted, so restrictions are given so that only the pixel values within the range of the lower and upper pixel value limits are used for each color. For example, if the lower limit is "10" and the upper limit is "245", the pixel value range is from "11" to "244".

[0040] Also, the number of pixels [(C, M, Y, K)ipx] in the above formulas (1) to (4) represents the number of pixels for each pixel value of each color included in the image data to be processed, and SUM represents the operation of adding up all the numbers of pixels for each of the above pixel values. The pixels with intermediate tones are the pixels having pixel values with intermediate tones in the dither pattern that appears by the dither processing described later.

[0041] <Dither Processing Unit> The dither processing unit 123 is a functional unit that performs dither processing on the intermediate image data 142 in order to obtain the number of gradations that can be output by the image forming unit 2, and converts it into image data (output image data 143) that represents the color shading (density) with a dither pattern.

[0042] The dither processing unit 123 reads out the intermediate image data 142 stored in the storage unit 130, performs dither processing on the image data of each color of C color, M color, Y color, and K color using the dither 144 prepared in advance for each color, and writes it to the storage unit 130 as the output image data 143.

[0043] The dither processing in the first embodiment is a conversion process called a commonly used dither method. As represented by the following formula (5), the target image (ipx[x, y] in the following formula (5). x and y are the coordinate positions of the pixels) is covered in a tile shape with a prepared dither (a two-dimensionally arranged threshold value), and the pixel value of the pixel at the same position is compared with the threshold value (th[x, y] in the following formula), and it is determined whether the output pixel (opix[x, y] in the following formula) is 0 or 1. In the dither processing, this is performed for all pixels to obtain the output image data 143. opix[x, y] = (ipx[x, y] > th[x, y])? 1 : 0 …(5)

[0044] <Dither to be held in the memory unit> In the first embodiment, the dither 144 held (prepared) in the memory unit 130 prepares a combination of two dithers. One combination of dithers has dithers for all colors of C, M, Y, and K. A base combination (dither combination A) and a combination different from the base combination (dither combination B) are prepared respectively.

[0045] FIG. 4 shows, for dither combination A among the combinations of dithers 144 used in the dither processing according to the first embodiment, a dither pattern after conversion corresponding to the pixel values of the original image.

[0046] In FIG. 4, examples of dither patterns after dither processing for each color of C, M, Y, and K are shown in the case where the pixel values in the image data are 0, 64, 128, 192, 255 (Duty0%, Duty25%, Duty50%, Duty75%, DuDuty100%).

[0047] As shown in FIG. 4, for the highlight part (= near pixel value 0) and the shard part (= near pixel value 255), the dithers for all colors have a similar pattern, but for the other parts (midtones), the patterns are different for each color. Generally, for the angles of each color of the dither used in color printing, it is necessary to separate the angles of each color as much as possible so that moire (interference fringes) does not occur when overlaid.

[0048] In the example of the dither combination A in FIG. 4, for color C, a dither with an angle of 117 degrees is used; for color M, a dither with an angle of 27 degrees is used; for color Y, a dither with an angle of 153 degrees is used; for color K, a dither with an angle of 63 degrees is used. A combination of dithers with the angles of the dithers for each color sufficiently separated is used, such that the angles of the dithers for each color are as different as possible. The arrow at the top of FIG. 4 indicates the conveyance direction. For the two dithers of colors C and K, they are close to the angle along the conveyance direction (vertical direction), while for the two dithers of colors M and Y, they are dithers close to the angle perpendicular to the conveyance direction (horizontal direction).

[0049] FIG. 5 shows, for the dither combination B among the combinations of dithers 144 used in the dither processing of the first embodiment, the dither pattern after conversion corresponding to the pixel values of the original image.

[0050] Also in FIG. 5, similar to FIG. 4 described above, examples of the dither patterns after dither processing for each of the colors C, M, Y, and K when the pixel values in the image data are 0, 64, 128, 192, and 255 (Duty0%, Duty25%, Duty50%, Duty75%, Duty100%) are shown.

[0051] However, in the example of the dither combination B in FIG. 5, for color C, a dither with an angle of 27 degrees is used; for color M, a dither with an angle of 117 degrees is used; for color Y, a dither with an angle of 153 degrees is used; for color K, a dither with an angle of 63 degrees is used. A combination of dithers with the angles of the dithers for each color sufficiently separated is used.

[0052] That is, the dither combination B in FIG. 5 uses a combination of dithers in which the dither for color C and the dither for color M are swapped from the dither combination A shown in FIG. 4. Therefore, for the two dithers of colors M and K, they are close to the angle along the conveyance direction (vertical direction), while for the two dithers of colors C and Y, they are dithers close to the angle perpendicular to the conveyance direction (horizontal direction).

[0053] (A-2) Operation of the First Embodiment Next, the operation of the printer 1 according to the first embodiment having the above configuration will be described.

[0054] (A-2-1) Image Processing FIG. 6 is a flowchart showing the characteristic operation (processing of the image processing unit) of the printer according to the first embodiment. When the printer 1 acquires image data from a host device or a scanner (not shown), after storing the acquired input image data 141 in the storage unit 130, the following processing is performed.

[0055] <Step S101> The image processing unit 120 acquires an image to be processed from the input image data 141 in units of objects or pages.

[0056] For example, in the example of the input image data 141 in FIG. 7, the objects are divided in units of the image O1 where the C color is frequently used and the image O2 where the M color is frequently used, and the subsequent processing will be performed in units of the above objects.

[0057] <Step S102> The color separation processing unit converts the input image data 141 into image data of each color plane of C, M, Y, and K colors, and writes it as intermediate image data 142 into the storage unit 130.

[0058] <Step S103> The halftone count unit 122 reads the intermediate image data 142, counts the number of halftone pixels included in the image data of each color of C, M, Y, and K colors according to the above formulas (1) to (4), and writes it as the number of halftone pixels 145 of each color into the storage unit 130.

[0059] <Step S104> The halftone count section 122 determines which of the C color and the M color is used more by using the number of pixels 145 of each color calculated in the above-described step S103. When the number of C-color pixels is larger than the number of M-color pixels, the halftone count section 122 transfers the process to step S105. On the other hand, when the number of C-color pixels is smaller than the number of M-color pixels, the halftone count section 122 controls to transfer the process to step S106.

[0060] In addition, when the number of M-color pixels and the number of C-color pixels among the pixels 145 are the same, the process may transition to either step S105 or step S106 (it may be appropriately set which transition to make when they are the same according to the usage environment, etc.).

[0061] <Step S105> When it is determined in the above-described step S104 that the number of C-color pixels is larger than the number of M-color pixels, the dither processing section 123 performs dither processing on the target (object or page) using the dither 144 of the dither combination A, and writes it to the storage section 130 as the output image data 143.

[0062] <Step S106> On the other hand, when it is determined in the above-described step S104 that the number of C-color pixels is smaller than the number of M-color pixels, the dither processing section 123 performs dither processing on the target (object or page) using the dither 144 of the dither combination B, and writes it to the storage section 130 as the output image data 143.

[0063] For example, in the example of the input image data 141 in FIG. 7, since the image O1 is an image in which the C color is used more and the image O2 is an image in which the M color is used more, for the image O1, it is determined that the number of C-color pixels is large and dither processing using the dither combination A (step S105) is performed, and for the image O2, it is determined that the number of M-color pixels is large and dither processing using the dither combination B (step S106) is performed.

[0064] As described above, by executing the processing of the image processing unit 120 on the object and the object in page units, the output image data 143 is generated. After that, the generated output image data 143 is to be formed on the paper P by the operation of the image forming unit 2 described above.

[0065] (A-2-2) Result of Image Processing FIG. 8 is an explanatory diagram showing the result of processing the input image data by the image processing unit according to the first embodiment. Also, for comparison, the result in the configuration when the first embodiment is not applied is shown to explain the difference.

[0066] FIG. 8(a) shows the result (output result before application) when processed with a conventional configuration to which the first embodiment is not applied. On the other hand, FIG. 8(b) shows the result (output result of this example) when the first embodiment is applied.

[0067] In the output result before application in FIG. 8(a), for both the image O1 where the C color is used a lot and the image O2 where the M color is used a lot, the dither 144 uses the dither combination A. Regarding the image O1, the dither is not noticeable, but regarding the image O2, the angle of the M-color dither is close to horizontal, and the M-color dither that is frequently used in the image is noticeable.

[0068] On the other hand, in the output result according to the first embodiment in FIG. 8(b), the dither of the dither combination A that is the same as before the application of the example is used for the image O1 where the C color is used a lot, and the dither of the dither combination B is used for the image O2 where the M color is used a lot. As a result, similar to before application, the dither is not noticeable for the image O1, and for the image O2, the angle of the M-color dither is close to vertical, and the dither for the M color that is frequently used in the image is also not noticeable.

[0069] (A-3) Effects of the First Embodiment According to the first embodiment, even when printing an image in which a specific colorant color is frequently used, it is possible to make the dither less noticeable compared to the conventional processing.

[0070] (B) Second Embodiment Hereinafter, a second embodiment of the image forming apparatus according to the present disclosure will be described in detail with reference to the drawings. In the second embodiment, an example in which the image forming apparatus of the present disclosure is applied to a printer will be described.

[0071] (B-1) Configuration of the Second Embodiment The overall schematic and the configuration of the control system of the printer 1 in the second embodiment are the same as those shown in FIGS. 1 to 3 of the first embodiment. However, since the pixel number calculation formula used in the halftone count unit 122 and the combination of the dither 144 held in the storage unit 130 are different from those in the first embodiment, the following description will focus on these differences.

[0072] <Halftone Count Unit of the Second Embodiment> The halftone count unit 122 in the second embodiment counts the number of pixels using the following pixel number calculation formula 2 (formulas (6) to (9)) or pixel number calculation formula 3 (formulas (10) to (13)).

[0073] <Pixel Number Calculation Formula 2> Number of C-color pixels = SUM(pixel number [Cipx]) × C-color density coefficient ∵ C lower limit < Cipx < C upper limit... (6) Number of M-color pixels = SUM(pixel number [Mipx]) × M-color density coefficient ∵ M lower limit < Mipx < M upper limit... (7) Number of Y-color pixels = SUM(pixel number [Yipx]) × Y-color density coefficient ∵ Y lower limit < Yipx < Y upper limit... (8) Number of K-color pixels = SUM(pixel number [Kipx]) × K-color density coefficient ∵ K lower limit < Kipx < K upper limit... (9)

[0074] Cipx, Mipx, Yipx, Kipx, and [(C, M, Y, K)ipx], and SUM in the above formulas (6) to (9) are the same as those described in the <Pixel Number Calculation Formula 1> of the first embodiment, so the description is omitted.

[0075] In the above formulas (6) to (9), the value obtained by multiplying the sum of the number of pixels for each pixel value by a predetermined coefficient (color density coefficient for (C, M, Y, K) colors) for each color is calculated as the number of pixels for each color.

[0076] The above coefficients are weighting coefficients, taking into account that the visibility of dither varies depending on the density of each colorant color (dither occurs due to the fluctuation of the space between the colored and colorless parts of the fine pattern, and the higher the density of the colorant color, the more prominent it tends to be). For example, for the K color, since it tends to be more prominent than the C color and M color, the density coefficient of the K color is set to "2.0", the density coefficients of the C color and M color are "1.0", and since the Y color is not prominent at all, it is set to "0.0" to adjust the number of pixels in the midtones of each color.

[0077] <Formula for calculating the number of pixels, version 3> Number of pixels of C color = SUM(pixel number [Cipx] × C dither difference [Cipx]) × C color density coefficient …(10) Number of pixels of M color = SUM(pixel number [Mipx] × M dither difference [Mipx]) × M color density coefficient …(11) Number of pixels of Y color = SUM(pixel number [Yipx] × Y dither difference [Yipx]) × Y color density coefficient …(12) Number of pixels of K color = SUM(pixel number [Kipx] × K dither difference [Kipx]) × K color density coefficient …(13)

[0078] The above formulas (10) to (13) are expanded versions of the above formulas (6) to (9). That is, in formulas (10) to (13), further, for the number of pixels for each pixel value, a coefficient for each pixel value (dither difference [(C, M, Y, K)ipx]) is multiplied, then all are added together, and multiplied by the density coefficient of each color to calculate.

[0079] The coefficients for each pixel value of the intermediate tone are considered in view of the fact that the visibility of dither varies within the intermediate tone. For example, as shown in FIG. 9, the visibility of dither for each pixel value is evaluated using dither with a horizontal angle and dither with a vertical angle respectively, and the difference therebetween is determined in advance as a coefficient (dither difference). That is, by finely reflecting the degree of improvement in dither when changing from horizontal dither to vertical dither for each color and each pixel value, it becomes possible to accurately select the dither used in dither processing.

[0080] <Dither Prepared in the Storage Unit of the Second Embodiment> In the second embodiment, in addition to the combinations of the two dithers shown in the first embodiment (dither combination A in FIG. 4 and dither combination B in FIG. 5), a dither combination (dither combination C) shown in FIG. 10 is further used.

[0081] FIG. 10 shows, for dither combination C among the combinations of dithers 144 used in the dither processing of the second embodiment, the dither patterns after conversion corresponding to the pixel values of the original image.

[0082] In FIG. 10, similar to FIGS. 4 and 5 described above, examples of the dither patterns after dither processing for each of the C, M, Y, and K colors when the pixel values in the image data are 0, 64, 128, 192, and 255 (Duty0%, Duty25%, Duty50%, Duty75%, DuDuty100%) are shown.

[0083] In the example of dither combination C in FIG. 10, a combination of dithers is used in which the C color uses a dither with an angle of 117 degrees, the M color uses a dither with an angle of 63 degrees, the Y color uses a dither with an angle of 153 degrees, the K color uses a dither with an angle of 27 degrees, and the angles of the dithers for each color are sufficiently separated.

[0084] The dither combination C in FIG. 10 uses a combination of dithers in which the M-color dither and the K-color dither in the dither combination A shown in FIG. 4 are swapped. Therefore, for the two dithers of C color and M color, they are closer to the angle (vertical direction) along the conveyance direction, while the two dithers of Y color and K color are dithers that are closer to the angle (horizontal direction) perpendicular to the conveyance direction.

[0085] (B-2) Operation of the Second Embodiment Next, the operation of the printer 1 according to the second embodiment having the above configuration will be described.

[0086] (B-2-1) Image Processing FIG. 11 is a flowchart showing the characteristic operation (processing of the image processing unit) of the printer according to the second embodiment. In FIG. 11, the same or corresponding processes as those in FIG. 6 according to the first embodiment are denoted by the same reference numerals. The detailed description of the same or corresponding processes in FIG. 6 according to the first embodiment is omitted because of duplication.

[0087] <Step S201> After the above step S102, the halftone count unit 122 of the second embodiment reads the intermediate image data 142, and counts the number of pixels that are halftone in the image data of each color of C, M, Y, and K according to the above equations (6) to (9) (or equations (10) to (13)), and writes them as the number of halftone pixels 145 of each color to the storage unit 130.

[0088] Hereinafter, in steps S202 to S204, the halftone count unit 122 compares the calculated number of pixels 145 of each color, and determines which color among the three colors of C, M, and K is used the least.

[0089] <Step S202> The halftone count unit 122 determines which of the C color and the M color is used more by using the number of pixels 145 of each color calculated in the above-described step S201. When the number of C-color pixels is greater than the number of M-color pixels, the halftone count unit 122 transfers the process to step S203. On the other hand, when the number of C-color pixels is less than the number of M-color pixels, the halftone count unit 122 controls to transfer the process to step S204.

[0090] <Step S203> When it is determined in step S202 described above that the number of C-color pixels is greater than the number of M-color pixels, the halftone count unit 122 further determines which of the K color and the M color is used more. When the number of K-color pixels is greater than the number of M-color pixels, the halftone count unit 122 transfers the process to step S105 described above. On the other hand, when the number of K-color pixels is less than the number of M-color pixels, the halftone count unit 122 controls to transfer the process to step S205.

[0091] <Step S204> On the other hand, when it is determined in step S202 described above that the number of C-color pixels is less than the number of M-color pixels, the halftone count unit 122 further determines which of the K color and the C color is used more. When the number of K-color pixels is greater than the number of C-color pixels, the halftone count unit 122 transfers the process to step S106 described above. On the other hand, when the number of K-color pixels is less than the number of C-color pixels, the halftone count unit 122 controls to transfer the process to step S205.

[0092] <Step S205> When it is determined in step S203 described above that the number of K-color pixels is less than the number of M-color pixels (or when it is determined in step S204 described above that the number of K-color pixels is less than the number of C-color pixels), the dither processing unit 123 performs dither processing on the target (object or page) using the dither 144 of the dither combination C, and writes it to the storage unit 130 as the output image data 143.

[0093] For example, in the example of the input image data 141 in FIG. 12, image O1 is an image with a large amount of color C and a small amount of color M, image O2 is an image with a large amount of color M and a small amount of color C, and image O3 is an image with a large amount of two colors, C and M, and a small amount of color K.

[0094] In this case, for image O1, it is determined that the number of pixels of color M is the smallest, and dithering processing (step S105 described above) using dither combination A is performed. For image O2, it is determined that the number of pixels of color C is the smallest, and dithering processing (step S106 described above) using dither combination B is performed. For image O3, it is determined that the number of pixels of color K is the smallest, and dithering processing (step S205 described above) using dither combination C is performed.

[0095] (A-2-3) Result of Image Processing FIG. 13 is an explanatory diagram showing the result of processing input image data by the image processing unit according to the first embodiment. For comparison, the result in the configuration of the first embodiment is also shown to explain the difference.

[0096] FIG. 13(a) shows the result (output result of the first embodiment) when processed in the configuration of the first embodiment. On the other hand, FIG. 13(b) shows the result (result of the second embodiment) when processed in the configuration of the second embodiment.

[0097] Regarding the results of image O1 and image O2, they are the same as the results of the previous first embodiment, and the dither is not prominent in either configuration.

[0098] However, for image O3, in the configuration of the first embodiment, since both C and M colors are used frequently, and dither combination A with a dither angle of color M being close to horizontal is selected, the dither of color M becomes prominent.

[0099] On the contrary, in the configuration of the second embodiment, for image O3, dither combination C with a dither angle close to vertical for both C and M colors is selected, resulting in a less prominent dither.

[0100] (B-3) Effects of the Second Embodiment According to the second embodiment, even when printing an image in which a specific coloring material color is frequently used, it is possible to make the dither less noticeable compared to the conventional process, similar to the first embodiment.

[0101] Furthermore, in the second embodiment, it is possible to select a dither (dither combination C) by carefully considering the visibility of dither due to the coloring material color and the visibility of dither in the halftone, with respect to the first embodiment, and it is possible to improve the dither in more cases.

[0102] (C) Other Embodiments Although various modified embodiments have been mentioned in each of the above-described embodiments, the present disclosure is also applicable to the following modified embodiments.

[0103] (C-1) In the above-described embodiment, an electrophotographic printer has been exemplified as the image forming apparatus, but the present disclosure is applicable to any apparatus that prints on a medium, such as an MFP (Multifunction Peripheral), a copying machine, a facsimile machine, etc.

[0104] (C-2) In the above-described embodiment, the dither process has been described on the premise of quantization to binary values, but the present disclosure is also applicable to a dither process that performs quantization of three or more values.

[0105] (C-3) In the above-described second embodiment, for the Y color, since dither is generally less noticeable, it was not considered at all when selecting the dither. As a modification, in an apparatus where the dither of the Y color is noticeable, after making the density coefficient greater than 0, when calculating the number of Y color pixels and the number of Y color pixels is larger compared to other colors, dither processing may be performed using a dither combination in which the dither angle of the Y color is set vertically.

[0106] (C-4) The printer 1 may be used by combining all or part of the configurations and / or functions of the above-described embodiments and the modified examples described in (C-1) to (C-3). Further, the printer 1 may omit at least part of the configurations and / or functions of the above-described embodiments and the modified examples described in (C-1) to (C-3).

Explanation of Reference Numerals

[0107] 1…Printer, 2(2C, 2K, 2M, 2Y)…Image forming unit, 3(3C, 3K, 3M, 3Y)…LED head, 4…Photoconductor drum, 5…Charging device, 6…Developing device, 10(10C, 10K, 10M, 10Y)…Transfer roller, 11…Paper feed roller, 12…Paper feed sensor, 14…Conveyor roller, 15…Conveyor roller, 16…Writing sensor, 17…Conveyor belt driven roller, 18…Conveyor belt driving roller, 19…Conveyor belt, 20…Fuser, 21…Fuser roller, 22…Fuser backup roller, 24…Discharge sensor, 25…Discharge roller, 26…Discharge roller, 100…Print control unit, 110…Data receiving unit, 120…Image processing unit, 121…Color separation processing unit, 122…Halftone count unit, 123…Dither processing unit, 130…Storage unit, 141…Input image data, 142…Intermediate image data, 143…Output image data, 144…Dither, 145…Number of pixels, 200a, 200b…Pitch variation region, P…Paper.

Claims

1. An image forming apparatus that performs image processing on input image data to form an image on a medium, comprising: a storage unit that stores combinations of a plurality of dithers used in dither processing; a color separation processing unit that decomposes the input image data into each colorant color and obtains intermediate tone image data for each colorant color; an intermediate tone count unit that calculates the number of pixels of pixels that become intermediate tones for each colorant color in the intermediate tone image data and compares the calculated number of pixels for each colorant color; a dither processing unit that determines the combination of dithers used in the dither processing based on the result of comparing the number of pixels and performs the dither processing. The image forming apparatus is characterized by comprising the above.

2. Each of the colorant colors is four colors of cyan, magenta, black, and yellow, the storage unit stores a first dither combination that is a combination of dithers in which only cyan and black have dither angles set to an angle close to the medium conveyance direction, and a second dither combination that is a combination of dithers in which only magenta and black have dither angles set to an angle close to the medium conveyance direction, when the number of cyan pixels calculated by the intermediate tone count unit is larger than the number of magenta pixels, the dither processing unit uses the first dither combination, and when the number of cyan pixels is smaller than the number of magenta pixels, the dither processing unit uses the second dither combination to perform the dither processing. The image forming apparatus according to claim 1, characterized by the above.

3. The storage unit further stores a third dither combination that is a combination of dithers in which only cyan and magenta have dither angles set to an angle close to the medium conveyance direction, when, as a result of comparing the number of pixels of cyan, magenta, and black calculated by the intermediate tone count unit, the number of magenta pixels is the smallest, the dither processing unit uses the first dither combination, when the number of cyan pixels is the smallest, the dither processing unit uses the second dither combination, and when the number of black pixels is the smallest, the dither processing unit uses the third dither combination to perform the dither processing. The image forming apparatus according to claim 2, characterized by the above.

4. When calculating the number of pixels of pixels that become intermediate tones for each colorant color, the intermediate tone count unit weights each colorant color and then calculates the number of pixels for each colorant color. The image forming apparatus according to claim 1 is characterized by the above.

5. The intermediate tone count unit according to claim 4, wherein when calculating the number of pixels that are in the intermediate tone of each colorant color, the number of pixels for each colorant color is calculated in consideration of the degree of improvement in density unevenness of each colorant color.

Images