IMAGE FORMING APPARATUS AND DOT PATTERN ADJUSTMENT METHOD

Abstract

An image forming apparatus includes: an image former that includes ink dischargers of two or more colors and that forms an image on a recording medium; and a hardware processor that generates, in each of the ink dischargers, at least a first dot pattern of a first color with uniform density and a second dot pattern in a second color with uniform density. When the first dot pattern is combined with the second dot pattern, the hardware processor adjusts the second dot pattern based on a positional relationship of each dot in the first dot pattern.

Claims

1. An image forming apparatus, comprising: an image former that comprises ink dischargers of two or more colors and that forms an image on a recording medium; and a hardware processor that generates, in each of the ink dischargers, at least a first dot pattern of a first color with uniform density and a second dot pattern in a second color with uniform density, wherein when the first dot pattern is combined with the second dot pattern, the hardware processor adjusts the second dot pattern based on a positional relationship of each dot in the first dot pattern.

2. The image forming apparatus according to claim 1, wherein the hardware processor sets a number of dots of the second dot pattern to be more than a number of dots of the first dot pattern.

3. The image forming apparatus according to claim 2, wherein the hardware processor sets the number of dots of the second dot pattern to be equal to or less than twice the number of dots of the first dot pattern.

4. The image forming apparatus according to claim 1, wherein the hardware processor generates the second dot pattern by generating the dots of the second dot pattern at a center of gravity of a polygon formed by the dots of the first dot pattern.

5. The image forming apparatus according to claim 4, wherein the hardware processor: generates regions that surround each of the dots in the first dot pattern by Voronoi tessellation, and adjusts a position of each of the dots in the first dot pattern by moving the dot to a position of the center of gravity of a polygon forming each of the regions.

6. The image forming apparatus according to claim 1, further comprising: a storage that stores the dots of the first dot pattern in association with the first color, and stores the dots of the second dot pattern in association with the second color.

7. The image forming apparatus according to claim 1, further comprising: an image processor that generates a dither matrix by using the first dot pattern and the second dot pattern.

8. A dot pattern adjustment method executed by an image forming apparatus that comprises an image former, wherein the image former comprises ink dischargers of two or more colors and that forms an image on a recording medium, the method comprising: generating, in each of the ink dischargers, at least a first dot pattern of a first color with uniform density and a second dot pattern in a second color with uniform density; and when the first dot pattern is combined with the second dot pattern, adjusting the second dot pattern based on a positional relationship of each dot in the first dot pattern.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0023] The advantages and features provided by one or more embodiments of the invention will become more fully understood from the detailed description given hereinbelow and the appended drawings which are given by way of illustration only, and thus are not intended as a definition of the limits of the present invention:

[0024] FIG. 1 is a diagram illustrating a schematic configuration of an image forming apparatus according to one or more embodiments of the present invention;

[0025] FIG. 2 is a block diagram illustrating a main functional configuration of the image forming apparatus according to one or more embodiments;

[0026] FIG. 3 is a diagram for explaining dot generation of a second dot pattern according to one or more embodiments;

[0027] FIG. 4 is a diagram for explaining the dot generation of the second dot pattern according to one or more embodiments;

[0028] FIG. 5 is a flowchart illustrating one operation example when executing pattern generation control in the image forming apparatus according to one or more embodiments;

[0029] FIG. 6 is a diagram for explaining dot position adjustment of a first dot pattern according to one or more embodiments; and

[0030] FIG. 7 is a diagram for explaining the dot position adjustment of the first dot pattern according to one or more embodiments.

DETAILED DESCRIPTION OF EMBODIMENTS

[0031] Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the scope of the invention is not limited to the disclosed embodiments.

[0032] Hereinafter, embodiments of the present invention will be described in detail below based on the accompanying drawings. FIG. 1 is a diagram illustrating a schematic configuration of image forming apparatus 1 according to one or more embodiments of the present invention.

[0033] Image forming apparatus 1 is an inkjet image forming apparatus that records an image on recording medium P. Image forming apparatus 1 includes sheet feed section 10, image forming section 20 (i.e., image former), sheet ejection section 30, and control section 40.

[0034] Under control by control section 40, image forming apparatus 1 conveys recording medium P stored in sheet feed section 10 to image forming section 20, discharges ink onto recording medium P in image forming section 20 to record an image, and conveys recording medium P on which the image is recorded to sheet ejection section 30.

[0035] For details, image forming apparatus 1 records a color image on recording medium P by overlaying and outputting each of four colors of yellow (Y), magenta (M), cyan (C), and black (K) on recording medium P with a predetermined number of recording gradations for each of the colors.

[0036] As recording medium P, it is possible to use paper, such as normal paper and coated paper, as well as various media, such as a fabric and a sheet-like resin, that can fix the ink impacted on the surface.

[0037] Sheet feed section 10 includes sheet feed tray 11 that stores recording medium P, and medium supply section 12 that conveys and supplies recording medium P from sheet feed tray 11 to image forming section 20. Medium supply section 12 includes a ring-shaped belt whose inner side is supported by two rollers, and conveys recording medium P from sheet feed tray 11 to image forming section 20 by rotating the rollers in a state in which recording medium P is placed on the belt.

[0038] Image forming section 20 includes conveyance section 21, transfer unit 22, heating section 23, head unit 24, fixing section 25, delivery section 27, and the like.

[0039] Conveyance section 21 holds recording medium P placed on a conveyance surface of conveyance drum 211 that has a cylindrical shape, and conveyance drum 211 rotates and moves around a rotation axis (cylindrical axis) extending in an X direction perpendicular to FIG. 1 so that recording medium P on conveyance drum 211 is conveyed in a conveying direction along the conveyance surface.

[0040] Conveyance drum 211 includes a claw section (not illustrated) and an intake section (not illustrated) for holding recording medium P on the conveyance surface of conveyance drum 211. An edge of recording medium P is pressed by the claw section, and recording medium P is pulled toward the conveyance surface by the intake section so that recording medium P is held on the conveyance surface.

[0041] Transfer unit 22 is provided at a position between medium supply section 12 of sheet feed section 10 and conveyance section 21, and holds and takes up one end of recording medium P, which is conveyed from medium supply section 12, with swing arm section 221, and transfers recording medium P to conveyance section 21 via transfer drum 222.

[0042] Heating section 23 is provided between a position where transfer drum 222 is arranged and a position where head unit 24 is arranged, and heats recording medium P such that recording medium P conveyed by conveyance section 21 is at a temperature within a predetermined temperature range. Heating section 23 includes, for example, an infrared heater or the like, and energizes the infrared heater based on a control signal supplied from control section 40 to cause the infrared heater to generate heat.

[0043] Head unit 24 records the image by discharging the ink onto recording medium P from a nozzle opening section provided in an ink discharge surface facing the conveyance surface of conveyance drum 211 at an appropriate timing in accordance with the rotation of conveyance drum 211 on which recording medium P is held.

[0044] Head unit 24 is arranged such that the ink discharge surface and the conveyance surface are apart from each other by a predetermined distance. In image forming apparatus 1 of one or more embodiments, four head units 24 corresponding to inks of four colors of Y, M, C, and K, respectively, are arranged so as to be aligned at predetermined intervals in an order of the Y, M, C, and K colors from an upstream side of the recording medium P-conveying direction. Head unit 24 corresponds to the ink discharge section (i.e., ink dischargers) of one or more embodiments of the present invention.

[0045] Head unit 24 is used at a fixed position when the image is recorded, and records the image with a single pass method by successively discharging the ink at a predetermined interval (conveying direction interval) at different positions in the conveying direction in accordance with the conveyance of recording medium P.

[0046] Note that, the configuration of head unit 24 is not limited to the above configuration as long as a plurality of recording elements are provided at positions different from each other in the X direction.

[0047] Fixing section 25 includes an energy ray irradiation section arranged over a width of the X direction of conveyance section 21, and irradiates recording medium P placed on conveyance section 21 with an energy ray such as an ultraviolet ray from the energy ray irradiation section to cure and fix the ink discharged onto recording medium P. The energy ray irradiation section of fixing section 25 is arranged facing the conveyance surface between a position where head unit 24 is arranged and a position where transfer drum 271 of delivery section 27 is arranged in the conveying direction.

[0048] Delivery section 27 includes belt loop 272 that includes a ring-shaped belt whose inner side is supported by two rollers, and transfer drum 271 that has a cylindrical shape and transfers recording medium P from conveyance section 21 to belt loop 272. Delivery section 27 uses belt loop 272 to convey recording medium P transferred from conveyance section 21 onto belt loop 272 by transfer drum 271 so that recording medium P is sent to sheet ejection section 30.

[0049] Sheet ejection section 30 includes sheet tray 31 which has a plate shape and on which recording medium P sent from image forming section 20 by delivery section 27 is placed.

[0050] FIG. 2 is a block diagram illustrating a main functional configuration of image forming apparatus 1. Image forming apparatus 1 includes control section 40, head unit drive section 50, conveyance drive section 60, image processing section 70, input/output interface 80, and pattern generating section 100 (i.e., hardware processor).

[0051] Control section 40 includes CPU 41 (Central Processing Unit), RAM 42 (Random Access Memory), ROM 43 (Read Only Memory), and storage section 44 (i.e., storage), and integrally controls overall operation of image forming apparatus 1.

[0052] CPU 41 reads out programs for various types of control and setting data stored in ROM 43 and stores the programs and setting data in RAM 42, and executes the programs to perform various types of arithmetic processing.

[0053] RAM 42 provides CPU 41 with a memory space for work, and stores temporary data. RAM 42 may include a non-volatile memory.

[0054] ROM 43 stores the programs for various types of control, the setting data, and the like to be executed by CPU 41. Note that, a rewritable non-volatile memory such as an EEPROM (Electrically Erasable Programmable Read Only Memory) and a flash memory may be used instead of ROM 43.

[0055] Storage section 44 stores a print job that is input from an external apparatus (not illustrated) via input/output interface 80, image data of an image to be recorded by the print job, or the like. As storage section 44, an HDD (Hard Disk Drive) may be used, for example, and a DRAM (Dynamic Random Access Memory) or the like may be used in combination.

[0056] Head unit drive section 50 supplies a driving signal in accordance with the image data to the recording element of head unit 24 based on the control by control section 40 at an appropriate timing so that the ink in an amount in accordance with a pixel value of image data is discharged from a nozzle of head unit 24.

[0057] Conveyance drive section 60 supplies a driving signal to a conveyance drum motor provided in conveyance drum 211 based on a control signal supplied from control section 40 to rotate conveyance drum 211 at a predetermined speed and timing. Further, conveyance drive section 60 supplies a driving signal to a motor for operating medium supply section 12, transfer unit 22, and delivery section 27 based on a control signal supplied from control section 40 to cause recording medium P to be supplied to conveyance section 21 and to cause recording medium P to be ejected from conveyance section 21.

[0058] Image processing section 70 performs predetermined image processing to the image data stored in storage section 44, and stores the resulting image data in storage section 44. The image processing encompasses, in addition to correction processing that corrects the image data by applying a correction table (not illustrated) or the like to the image data, color conversion processing, tone correction processing, pseudo continuous tone processing, and the like.

[0059] Input/output interface 80 is connected to an input/output interface of the external apparatus (for example, a personal computer), and mediates transmission and reception of data between control section 40 and the external apparatus. Input/output interface 80 is configured with, for example, either various serial interfaces or various parallel interfaces, or a combination thereof.

[0060] Further, in one or more embodiments, after the image processing such as the tone correction processing (gamma correction or the like) is performed to the image data as needed, the pseudo continuous tone processing in which image data with 8 bits in each pixel are converted into pseudo continuous tone image data with 1 bit in each pixel (2 gradations) is performed. As a method of the pseudo continuous tone processing, dither processing in which each pixel value is binarized in accordance with each threshold value arranged in a matrix is used.

[0061] Image processing section 70 outputs processed image data obtained by performing the dither processing to the image data subjected to the image processing. A dither matrix configured with a plurality of cell elements is set in image processing section 70. In the dither matrix, threshold value TH corresponds to each cell element (corresponding to one pixel).

[0062] It is assumed here that the number of cell elements in a main scanning direction is M and the number of cell elements in a sub scanning direction is N. For example, in a case where M is assumed as 256 and N is assumed as 256, the number of the elements is 256256=65536, so there are 65536 threshold values.

[0063] Image processing section 70 inputs an image signal for each pixel, determines sai and saj indicative of the position of the pixel in the dither matrix by the below-described equations 1 and 2, and specifies a numerical value at the position (sai, saj) as cell element e corresponding to the pixel by the below-described equation 3.

sai=i % M(1)

saj=j % N(2)

e=sai+sajM(3)

[0064] In the equations, i and j denote position coordinates of the inputted pixel in the entire image, and % denotes a modulus operator.

[0065] Subsequently, image processing section 70 determines two threshold values TH that correspond to the cell element e corresponding to the pixel. Threshold values TH above can be taken out by inputting the value of the cell element e in look-up table TBL [MN] stored in storage section 44 or the like (see equation 4).

TH=TBL[e](4)

[0066] Look-up table TBL [MN] holds threshold values TH of every cell element e. Since the dither matrix includes 65536 elements, 65536 numbers are arranged in TBL [MN].

[0067] Image processing section 70 then calculates SC that is image data after the dither processing by the following equation 5.

SC=(ISTH)(5)

[0068] Note that, IS denotes image data before performing the dither processing. In equation 5, SC=0 (non-dotted) when SC<0, and SC=1 (dotted) when SC>0.

[0069] In image processing section 70, the above-described processing is repeatedly performed for each pixel.

[0070] Based on the image data of the Y, M, C, and K colors to which the pseudo continuous tone processing is performed by image processing section 70, control section 40 controls head unit drive section 50 such that the recording elements of four head units 24 discharge the inks onto recording medium P. Thus, the image is formed on recording medium P.

[0071] Pattern generating section 100 generates a dot pattern in each of head units 24 of two or more colors. The dot patterns generated by pattern generating section 100 are used in the dither processing described above.

[0072] Specifically, in a case where a first dot pattern of a first color with a uniform density and a second dot pattern of a second color with a uniform density are combined, pattern generating section 100 adjusts each dot position in the second dot pattern based on a positional relationship of each dot in the first dot pattern.

[0073] The first color is, for example, the C color. The second color is, for example, the M color. The first dot pattern is a dot pattern in a random dot arrangement by the FM screening.

[0074] The density of each of the dot patterns of the C and M colors is not so high by itself, but the density of an overlapping portion of the respective dot patterns of the C and M colors becomes high, and the overlapping portion becomes easily visually recognizable by an observer.

[0075] A skin color portion of a face photograph can be expressed mainly by the respective dot patterns of the M and Y colors when the density is low, and a dot pattern(s) of the C color or/and the K color is/are added thereto as the density becomes higher. As a result of the addition of the dot pattern of the C color as mentioned above, there are more cases where the dot patterns of the C and M colors overlap.

[0076] In the case of the FM screening, when a ratio of overlapping of the respective dot patterns of the C and M colors increases, the feeling of roughness peculiar to random dots increases. Accordingly, there has been a need for a dot arrangement in which the overlapping of the respective dot patterns of the C and M colors is reduced.

[0077] Given the above, in one or more embodiments, pattern generating section 100 generates the first dot pattern in a random dot arrangement. Pattern generating section 100 then adjusts each dot position of the second dot pattern based on the positional relationship of each dot in the first dot pattern.

[0078] In the case of the skin color, the M color (the second color) has a larger ratio of dots than the C color (the first color). Accordingly, pattern generating section 100 adjusts the number of dots of the second dot pattern to be more than the number of dots of the first dot pattern and to be equal to or less than twice the number of dots of the first dot pattern.

[0079] More specifically, as illustrated in FIGS. 3 and 4, pattern generating section 100 generates the second dot pattern by generating dot D2 at a position of a center of gravity of a Delaunay triangle (see the broken line) formed by three dots D1 among each dot D1 of the first dot pattern.

[0080] The Delaunay triangle is a triangle that includes, as sides, line segments that connect adjacent generating points. Accordingly, the respective three dots mentioned above are located at positions such that there is no other dot on a line connecting two dots, and such that the dots have a positional relationship in which the dots are adjacent to each other.

[0081] A plurality of Delaunay triangles as described above are generated by the first dot pattern. Further, the second dot pattern is generated by generating dot D2 at the position of the center of gravity of each Delaunay triangle.

[0082] Thus, it is possible to generate dot patterns of the M color (the second color) and the C color (the first color) at positions where the dot patterns do not overlap with each other. As a result, it is possible to reduce the overlapping of the respective dot patterns of the two colors.

[0083] Further, by generating the second dot pattern as described above, a ratio of the number of dots of the first dot pattern to the number of dots of the second dot pattern can be substantially 1 to 2. Note that, the substantially 1 to 2 discussed here encompasses, in addition to a ratio of the number of dots of the first dot pattern to the number of dots of the second dot pattern of 1 to 2, ratios of the number of dots of the first dot pattern to the number of dots of the second dot pattern within an error range of approximately 1% as well.

[0084] Further, pattern generating section 100 causes the generated first dot pattern and the generated second dot pattern to be stored in storage section 44. Storage section 44 stores each dot of the first dot pattern in association with the first color, and stores each dot of the second dot pattern in association with the second color.

[0085] Thus, when performing the image processing such as the dither matrix processing by using the first dot pattern and the second dot pattern, it is possible to accurately distinguish between the first color and the second color.

[0086] Further, image processing section 70 generates the dither matrix by using the first dot pattern and the second dot pattern that are generated by pattern generating section 100 as described above. Methods for generating the dither matrix include, for example, a void-and-cluster method.

[0087] This method provides an initial dot pattern (the first dot pattern and the second dot pattern), and specifies the sparsest position (void) and the densest position (cluster) in the dot pattern by low-pass filtering or the like. The method then adds dots one by one to a sparse position or deletes dots one by one from a dense position to generate a dot pattern for all gradation values, and ultimately generates a dither matrix in which an order for dots to grow is represented by numerical values.

[0088] Image processing section 70 generates the dither matrix by the void-and-cluster method. Image processing section 70 then performs the foregoing pseudo continuous tone processing, based on the generated dither matrix, to the image data.

[0089] Thus, in a case where an image with the skin color is generated, for example, it is possible to reduce the feeling of roughness due to the overlapping of the C and M colors.

[0090] Further, dot patterns may be generated as described above for the K and Y colors as well. With respect to the K and Y colors, however, the dot patterns of the K and Y colors may also be generated at random since the K and Y colors hardly affect the visual density even when dots of the K and Y colors overlap with dots of another color.

[0091] Next, an operation example when executing pattern generation control in image forming apparatus 1 will be described. FIG. 5 is a flowchart illustrating one operation example when executing the pattern generation control in image forming apparatus 1. The processing in FIG. 5 is appropriately executed when control section 40 receives an execution command of a print job relating to the pseudo continuous tone processing.

[0092] As illustrated in FIG. 5, pattern generating section 100 generates the first dot pattern (step S101). Pattern generating section 100 generates Delaunay triangles with each dot based on the first dot pattern (step S102).

[0093] Pattern generating section 100 generates the second dot pattern based on the generated Delaunay triangles (step S103). Next, pattern generating section 100 generates the dither matrix by using the generated first dot pattern and the generated second dot pattern (step S104).

[0094] Pattern generating section 100 then executes the pseudo continuous tone processing (step S105). Thereafter, the control ends.

[0095] Note that, in step S104, the dither matrix is generated by sequentially generating dot patterns with a lower density or a higher density by removing or adding dots one by one in the initial dot pattern generated in step S103 by the void-and-cluster method. When generating a dot pattern with a higher density than the initial dot pattern generated in step S103 described above, the dither matrix may be generated by, in addition to the void-and-cluster method, adding dots one by one with AM growth to generate the pattern. The AM growth discussed here means growing a dot size by continuing to add dots adjacent to a dot that is already present. When using the void-and-cluster method on a high density side, the distance between dots may become locally short in the vicinity of an added dot. Accordingly, in such a case, growing the dot size by the AM growth makes it possible to create a pattern with a high density without breaking a positional relationship between the dots determined in the initial pattern since the number of dots does not change. As a result, it is possible to restrain the distance between the dots from becoming locally short, thus preventing the granularity from deteriorating.

[0096] According to one or more embodiments, it is possible to reduce overlapping of respective dot patterns of two colors.

[0097] Further, since a dot pattern is generated for each one kind of color, the processing can be simple in comparison with a configuration in which two kinds of colors are processed simultaneously.

[0098] Further, since the second dot pattern is adjusted based on the positional relationship of each dot in the first dot pattern, the positional relationship between each dot of the first dot pattern and each dot of the second dot pattern can become uniform easily.

[0099] For example, when arranging each dot of the second dot pattern randomly, such a case may occur where a dot of the second dot pattern is located extremely close to each dot of the first dot pattern so that both are likely to overlap at the time of the formation on recording medium P.

[0100] In one or more embodiments, however, the positional relationship between each dot of the first dot pattern and each dot of the second dot pattern becomes uniform so that overlapping of both can be reduced at the time of the formation on recording medium P.

[0101] Further, since the second dot pattern is generated based on triangles configured with three dots among the dots of the first dot pattern, the ratio of the number of dots of the first dot pattern to the number of dots of the second dot pattern can be substantially 1 to 2. That is, when generating a dot pattern of a color (for example, the skin color) in which the number of dots of the second dot pattern is at higher density than the number of dots of the first dot pattern, it is possible to realize a dot arrangement with uniformity in each color and with reduced overlapping.

[0102] Note that, in one or more embodiments, the first dot pattern is configured in a random dot arrangement, but the present invention is not limited thereto. For example, pattern generating section 100 may perform dot position adjustment of the first dot pattern by Voronoi tessellation.

[0103] Specifically, as illustrated in FIG. 6, pattern generating section 100 first generates the first dot pattern with random dot positions. Pattern generating section 100 generates a plurality of regions (regions surrounded by the solid lines) surrounding each dot D1 by performing the Voronoi tessellation by using each dot D1 as generating points.

[0104] Then, as illustrated in FIG. 7, pattern generating section 100 moves each dot D1 (the broken line) to a position of a center of gravity of a polygon (Voronoi polygon) forming each of the plurality of regions so that each dot D1 is positioned at the position of dot D1 (the black circle).

[0105] Pattern generating section 100 performs such position adjustment for each dot D1, then performs the Voronoi tessellation again to adjust the position of dot D1. Pattern generating section 100 repeatedly performs such position adjustment until dot D1 matches the position of the center of gravity of a newly generated Voronoi polygon.

[0106] Thus, it is possible to generate the first dot pattern with good dispersibility. Further, since the second dot pattern is generated by the first dot pattern for which the position adjustment is performed, the dispersibility of the second dot pattern can also be good.

[0107] Further, although the ratio of the number of dots of the first dot pattern to the number of dots of the second dot pattern is substantially 1 to 2 in one or more embodiments, the present invention is not limited thereto. For example, dot D2 of the second dot pattern may be generated at a position of a center of gravity of, in addition to a Delaunay triangle, a quadrangle configured with four dots D1 among each dot of the first dot pattern. That is, in a generation pattern of dot D2 of the second dot pattern, a pattern in which dot D2 is generated at the position of the center of gravity of the triangle configured with three dots D1 may be mixed with a pattern in which dot D2 is generated at the position of the center of gravity of the quadrangle configured with four dots D1.

[0108] Thus, it is possible to configure the number of dots of the second dot pattern to be less than twice the number of dots of the first dot pattern so that a ratio of a number of dots of each color can be easily adjusted. Further, the ratio of the number of dots of each color may also be adjusted by causing no dot D2 to be generated in some of each triangle configured with dots D1.

[0109] Further, although the C color is exemplified as the first color and the M color is exemplified as the second color in one or more embodiments, the first color and the second color can be appropriately selected in accordance with colors forming an image (colors in which every color is combined).

[0110] Further, although pattern generating section 100, image processing section 70 and the like are separated in one or more embodiments, the present invention is not limited thereto. For example, the image processing section may have a function of the pattern generating section.

[0111] In addition, the embodiments described above merely illustrates one example of one or more embodiments for carrying out the present invention, and the technical scope of the present invention shall not be construed in a limited manner thereby. That is, one or more embodiments of the present invention can be carried out in various forms without deviating from the gist or essential characteristics of one or more embodiments of the present invention.

[0112] Although the disclosure has been described with respect to only a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that various other embodiments may be devised without departing from the scope of the present invention. Accordingly, the scope of the invention should be limited only by the attached claims.