JP2019047166A - Image forming apparatus and image forming method - Google Patents

Abstract

To suppress the occurrence of moire while improving the reproducibility and graininess of halftone dots in a low density region.SOLUTION: An image forming apparatus according to the present invention is an image forming apparatus that applies halftone processing to each recording material using a threshold value matrix having a plurality of halftone dot cells to generate a halftone dot image, and in the threshold value matrix, from among the halftone cells, a halftone dot of a predetermined size is sequentially formed from a halftone dot cell in which the degree of dispersion of the halftone dots calculated by superposing the halftone dot images generated for recording materials of at least two colors is the largest to assign a threshold value in predetermined units.SELECTED DRAWING: Figure 8

Description

  The present invention relates to an image forming apparatus that generates a halftone image and an image forming method.

  Conventionally, an electrophotographic method is known as an image recording method used in an image forming apparatus such as a printer or a copying machine. In the electrophotographic system, a latent image is formed on a photosensitive drum using a laser beam, developed with a charged color material (hereinafter referred to as toner), and the developed toner image is transferred onto a sheet and fixed. This is a method for recording images.

  However, the image data input to the image forming apparatus is multi-tone image data including a halftone, and it is difficult to obtain a halftone image by the above-described electrophotographic method. Therefore, it is a common practice to convert multi-gradation image data into halftone image data and record it by a pseudo gradation method using a dither method.

  Here, the pseudo gradation method using the dither method uses a threshold matrix having thresholds of N × M, acquires a threshold from the threshold matrix according to the pixel position of multi-tone image data, and sets a threshold for each pixel. This is a method for converting multi-tone image data into halftone image data by comparison.

  In the pseudo gradation method using the dither method, the threshold values are arranged so that the halftone dots are periodically formed at a predetermined angle in the threshold value matrix. Similarly, in the color image forming apparatus, the threshold value matrix is designed so that the halftone dots have different angles for each color material. However, the halftone dots are periodically overlapped at different angles so that the halftone dots are periodically overlapped. It is known that moiré (rosetta) having a frequency lower than the period of points occurs.

  Thus, for example, Patent Document 1 discloses a method of making it difficult to visually recognize moire having a frequency lower than the halftone dot period by using a specific combination of halftone dot periods of the threshold matrix used for each color material. .

  By the way, particularly in an image forming apparatus using an electrophotographic system, toner is difficult to adhere to the recording paper when the halftone dot is small, and in particular, the reproducibility and graininess of the low density halftone dot are deteriorated. Is a problem. Therefore, for example, in Patent Document 2, a halftone dot is concentrated and grown until it reaches a predetermined size in a low concentration region, and a different halftone dot is grown after the halftone dot reaches a predetermined size. A method for improving the reproducibility and graininess of the low concentration region is disclosed.

Japanese Patent Laid-Open No. 2-134635 JP-A-10-145593

  However, when the halftone dots are grown to a predetermined size or more in order to improve the reproducibility of the halftone dots in the low concentration range according to the method disclosed in Patent Document 2, all the halftone dots become a predetermined size. The frequency of the halftone dot becomes low in the concentration range up to. Thus, when the halftone dot frequency is lowered, the moire frequency is also lowered. In this case, even if the method disclosed in Patent Document 1 is applied, the moire is easily visually recognized in the low density region. There's a problem.

  The present invention has been made in view of the above-described conventional problems, and an object thereof is to suppress the generation of moire while improving the reproducibility and graininess of halftone dots in a low density region.

  In order to achieve the above object, an image forming apparatus of the present invention is an image forming apparatus that generates a halftone image by performing halftone processing using a threshold value matrix having a plurality of halftone cells for each recording material. In the threshold matrix, a halftone dot having the maximum degree of dispersion of halftone dots calculated by superimposing halftone dot images generated for each recording material of at least two colors among the plurality of halftone cells. A threshold value is assigned in a predetermined unit so that a halftone dot having a predetermined size is formed in order from the cell.

  According to the present invention, it is possible to suppress the occurrence of moire while improving the reproducibility and graininess of halftone dots in a low concentration range.

1 is a schematic block diagram of an image forming apparatus. 1 is a cross-sectional view of an image forming apparatus. 2 is a schematic block diagram of an image processing unit of the image forming apparatus. FIG. It is a binary threshold matrix applied to cyan. It is a binary threshold matrix applied to magenta. It is a binary threshold matrix applied to yellow. It is a binary threshold matrix applied to black. It is a flowchart for setting the formation order of the minimum halftone dot unit of a halftone dot cell. It is an example of the image data which formed the halftone dot by the minimum halftone dot unit. This is an example of binarized image data in which halftone dots are formed in units of minimum halftone dots. This is an example of binarized image data in which halftone dots are formed in units of minimum halftone dots. It is a figure which shows the execution result of a halftone process. It is a binary threshold matrix applied to cyan. It is a binary threshold matrix applied to magenta. It is a binary threshold matrix applied to black. It is a flowchart for setting the formation order of the minimum halftone dot unit of a halftone dot cell. It is a figure which shows the execution result of a halftone process. It is a binary threshold matrix applied to cyan. It is a binary threshold matrix applied to magenta. It is a binary threshold matrix applied to yellow. It is a binary threshold matrix applied to black. It is an example of image data in which halftone dots are formed in units of minimum halftone dots and converted to lightness. It is a figure which shows the execution result of a halftone process.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments do not limit the present invention, and all combinations of features described in the present embodiment are not necessarily essential to the solution means of the present invention.

(Embodiment 1)
FIG. 1 is a schematic block diagram of the image forming apparatus 10. The image forming apparatus 10 is shown as a digital multi-function peripheral having functions such as a copy, a printer, and a FAX as an example.

  As shown in FIG. 1, the image forming apparatus 10 includes a scanner unit 101, a controller 102, a printer unit 103, and an operation unit 104 as functions thereof. The scanner unit 101 executes a document reading process. The controller 102 is a control unit, and mainly performs image processing on the image data (input image) read by the scanner unit 101 and stores it in the memory 105 as image data for printing. The printer unit 103 forms an image on the recording paper according to the print setting conditions set by the operation unit 104 by using the printing image data read from the memory 105. The operation unit 104 sets various printing conditions for the image data read by the scanner unit 101.

  The image forming apparatus 10 is connected via a network 106 to a server 107 that manages image data, a PC (Personal Computer) 108 that instructs the image forming apparatus to execute printing, and the like.

  FIG. 2 is a cross-sectional view of the image forming apparatus 10, and a more detailed configuration of the image forming apparatus 10 will be supplemented with reference to FIG. As shown in FIG. 2, the image forming apparatus 10 mainly includes a scanner 201, a document feeder 202, and a print recording printer 213 including a four-color drum.

  In the configuration shown in FIG. 2, first, a supplementary explanation will be given for the reading operation in the scanner 201. When reading a document as electronic data, the user sets the document on the document table 207 and closes the document feeder 202. When the open / close sensor 224 detects that the document table 207 is closed, the light reflection type document size detection sensors 226 to 230 in the housing of the scanner 201 detect the size of the set document.

  When the size of the set original is detected, the light source 210 irradiates the original, and a CCD (charge-coupled device) 231 receives reflected light from the original via the reflector 211 and the lens 212. By reading the image. The image data read by the CCD 231 is converted into a digital signal by the controller 102 of the image forming apparatus 10, and when image processing for the scanner is performed, the image data is stored in the memory 105 in the controller 102 as image data for printing. . In this case, the image data for printing is composed of signals of three colors of red (R), green (G), and blue (B).

  Next, a supplementary explanation will be given for the reading operation in the document feeder 202. When the user sets a document on the document feeder 202 and performs a reading operation, the user places the document face up on the document setting tray 203 of the document feeder 202. When the original is placed and the original presence / absence sensor 204 detects that the original is set, the original is conveyed by the rotation of the paper supply roller 205 and the conveying belt 206, and a predetermined position on the original platen 207 is detected. Set to

  In the subsequent operation, as in the reading operation in the scanner 201, the document is read as image data and finally stored in the memory 105 in the controller 102 as image data for printing. When the reading is completed, the conveyance belt 206 is rotated again, and the document is fed rightward in FIG. 2 and discharged to the document discharge tray 209 via the discharge roller 208 on the discharge side. When there are a plurality of documents, when the scanned document is discharged from the document table 207 in the right direction in FIG. 2, the next document passes through the paper feed roller 205 at the same time. In FIG. 2, the paper is fed from the left side and the next original is continuously read.

  Finally, a printing operation in the printer 213 will be described. The image data for printing temporarily stored in the memory 105 in the controller 102 is transferred to the printer 213 after image processing for printing described later is performed again in the controller 102. In the printer 213, the laser recording unit converts the recording laser light into four color toners (recording materials) of yellow (Y), magenta (M), cyan (C), and black (K). The

  Then, the converted recording laser light is irradiated to the photosensitive member 214 of each color, and an electrostatic latent image is formed on each photosensitive member. When the electrostatic latent image is formed on each photoconductor, the printer 213 develops the toner on each photoconductor with the toner supplied from the toner cartridge 215, and the toner image visualized on each photoconductor is transferred to the intermediate transfer belt 219. Primary transcription. The intermediate transfer belt 219 rotates in the clockwise direction in FIG. 2, and when the recording paper fed from the paper cassette 216 through the paper feed conveyance path 217 reaches the secondary transfer position 218, the intermediate transfer belt 219 A toner image is transferred to the recording paper.

  The recording paper onto which the image has been transferred is fixed with toner by pressurization and heat in the fixing device 220, and is conveyed in the paper discharge conveyance path, and then is transferred to the face-down center tray 221 or the face-up side tray 222. The paper is ejected. The flapper 223 is for switching the paper discharge conveyance path in order to switch these paper discharge ports. For example, in the case of double-sided printing, after the recording paper passes through the fixing device 220, the flapper 223 switches the conveyance path, and then the recording paper is switched back and sent downward, passes through the double-sided printing paper conveyance path 225, and again. The paper is fed to the secondary transfer position 218 and double-sided printing is performed.

  Next, image processing for printing will be described in detail with reference to FIG. FIG. 3 is a block diagram illustrating image processing for printing (that is, a hardware configuration diagram for image processing). The image processing unit 301 illustrated in FIG. 3 includes a color conversion processing unit 302, a gamma correction unit 303, a halftone processing unit 304, a ROM 305, a CPU 306, and a RAM 307, and performs image processing for printing based on instructions from the controller 102. Run.

  Here, the image data for printing temporarily stored in the memory 105 in the controller 102 is multi-gradation image data composed of 8-bit pixels, and a pixel value within a range of 0 to 255 per pixel. The number of colors is 256. First, in the color conversion processing unit 302, printing image data input from the memory 105 is converted from three RGB colors into four CMYK colors corresponding to toner color materials for each pixel. Next, the gamma correction unit 303 performs gamma correction for each color. Finally, halftone processing is performed for each color in the halftone processing unit 304, converted from 8 bits to 1-bit halftone image data that can be printed by the printer 213, and sent to the printer unit 103. .

  The ROM 305 stores a control program, and the CPU 306 controls the overall operation of the image processing unit 301 based on the control program stored in the ROM 305. The RAM 307 is used as a work area for the CPU 306. As will be described later, a threshold matrix is recorded in the RAM 307 for each color material (for each recording material) corresponding to CMYK toner. In addition, in the present embodiment, as described above, it has been described that the halftone processing unit 304 outputs 1-bit halftone image data to the printer unit 103. However, the printer unit 103 uses the PWM control or the like to perform multi-value printing. However, the present invention is not limited to this. In this case, for example, the halftone processing unit 304 uses a multi-value threshold matrix.

  Next, the processing in the halftone processing unit 304 of the image processing unit 301 and the threshold matrix used in the halftone processing will be described in detail with reference to FIGS. 4, 5, 6, and 7. Note that the threshold matrixes shown in FIGS. 4, 5, 6, and 7 are shown as 600 dpi threshold matrices as an example.

  A threshold matrix 400 shown in FIG. 4 is a binary threshold matrix applied to cyan, and the binary threshold matrix is composed of a single matrix. As shown in FIG. 4, the threshold value matrix 400 is a repetitive arrangement of halftone cells composed of a plurality of threshold values, and is repeatedly used so as to cover the pixel positions of the image data.

  The halftone processing unit 304 converts each pixel of the image data into two-gradation image data having a value of 0 or 1. That is, binarization is performed. Specifically, the halftone processing unit 304 first reads out a threshold value corresponding to each pixel from a predetermined position of the matrix for each pixel. Next, the halftone processing unit 304 compares the pixel value with the read threshold value, and outputs 1 when the pixel value is equal to or greater than the read threshold value, and outputs 0 otherwise. To do.

  In the threshold matrix of this embodiment, a technique called a sub-matrix is used, and the threshold matrix is configured by combining a plurality of halftone cells having different thresholds in order to obtain a sufficient number of gradations. Specifically, the threshold matrix 400 is an array of a total of 1600 vertical 40 and horizontal 40, and as described above, 16 types of halftone cells 401 to 416 each having a different combination of threshold values are regularly arranged. It is configured by arranging them in a line. In addition, each of the halftone cells 401 to 416 includes 20 threshold values.

  In the threshold value matrix 400, the period in which halftone cells are arranged is expressed by two vectors, and the vertical and horizontal two-component vectors of the threshold value array are repeated at periods of (4, 2) and (2, -4). Further, in the present embodiment, in order to improve the reproducibility and graininess of the low density region, a predetermined number of threshold values are arranged in each halftone cell, and the threshold value is continuously arranged. In the other portions, the threshold value is set so as to increase in each halftone cell (in ascending order).

  In this way, by arranging the threshold value in the halftone cell, the halftone dot can be concentrated and grown until it reaches a predetermined size in the low density region. It becomes possible to form. For example, the halftone cell 401 includes four continuous threshold values (0, 1, 2, 2) from 0 to 2, and the halftone cell 411 includes continuous threshold values (3, 4, 5, 5). Each halftone cell 401-416 has four consecutive thresholds, such as including 6). In the present embodiment, the size of halftone dots formed with the continuous threshold is referred to as a minimum halftone dot unit. With respect to this minimum halftone dot unit, the dot cells 401, 411, 402, 407, 412, 403, 416, 413, 410, 408, 404, 409, 414, 405, 415, 406 are determined in this order from the low density range ( Set) to form halftone dots.

  A threshold matrix 500 shown in FIG. 5 is a binary threshold matrix applied to magenta. As shown in FIG. 5, the threshold value matrix 500 is a repetitive arrangement of halftone cells composed of a plurality of threshold values, and is repeatedly used so as to cover the pixel positions of the image data.

  The threshold matrix 500 has a total of 1600 vertical 40 and horizontal 40 arrays, and as described above, 16 types of halftone dot cells 501 to 516 each having a different combination of threshold values are arranged in a regular and continuous manner. It is composed by being done. Each of the halftone cells 501 to 516 includes 20 threshold values.

  In the threshold value matrix 500, the period in which the halftone cells are arranged is expressed by two vectors, and the vertical and horizontal two-component vectors of the threshold value array are repeated at periods of (2, 4) and (4, -2). The halftone cell 501 includes four continuous threshold values (0, 1, 2, 2) from 0 to 2, and the halftone cell 509 includes three continuous threshold values (3, 4, 5). , 6) and the like, each halftone cell 501 to 516 has four consecutive threshold values. Thus, the minimum halftone dot units are in the order of the halftone cells 501, 509, 516, 508, 512, 513, 514, 505, 515, 503, 506, 507, 510, 511, 504, 502 from the low density region. It is set (determined) and a halftone dot is formed.

  A threshold matrix 600 shown in FIG. 6 is a binary threshold matrix applied to yellow. As shown in FIG. 6, the threshold value matrix 600 is a repetitive arrangement of halftone cells composed of a plurality of threshold values, and is repeatedly used so as to cover the pixel positions of the image data.

  The threshold matrix 600 is a total of 256 arrays of 16 in the vertical direction and 16 in the horizontal direction. As described above, the 16 types of halftone cells 601 to 616 each having a different combination of threshold values are arranged regularly and continuously. It is composed by being done. Each of the halftone cells 601 to 616 includes 16 threshold values.

  In the threshold value matrix 600, the period in which halftone cells are arranged is expressed by two vectors, and the vertical and horizontal two-component vectors of the threshold value array are repeated at periods of (4, 0) and (0, 4). The halftone cell 601 includes four continuous threshold values (0, 1, 2, 3) from 0 to 3, and the halftone cell 616 includes four continuous threshold values (4, 5, 6). , 7) and so on, each halftone cell 601-616 has four consecutive threshold values. Thus, the minimum halftone dot units are in the order of the halftone cells 601, 616, 611, 606, 609, 607, 610, 608, 603, 614, 615, 612, 602, 604, 613, 605 from the low density region. It is set (determined) and a halftone dot is formed.

  A threshold matrix 700 shown in FIG. 7 is a binary threshold matrix applied to black. As shown in FIG. 7, the threshold value matrix 700 is a repetitive arrangement of halftone cells composed of a plurality of threshold values, and is repeatedly used so as to cover the pixel positions of the image data.

  The threshold value matrix 700 is an arrangement of a total of 576, 24 in the vertical direction and 24 in the horizontal direction. As described above, the 16 types of halftone dot cells 701 to 716 each having a different combination of threshold values are arranged regularly and continuously. It is composed by being done. Each of the halftone cells 701 to 716 includes 18 threshold values.

  In the threshold value matrix 700, the period in which halftone cells are arranged is expressed by two vectors, and the vertical and horizontal two-component vectors of the threshold value array are repeated at periods of (3, 3) and (3, -3). The halftone cell 708 includes four continuous threshold values (0, 1, 2, 3) from 0 to 3, and the halftone cell 711 includes continuous threshold values (4, 4, 5). , 6) and the like, each halftone cell 701 to 716 has four consecutive threshold values. In this way, halftone dots are formed in the order of the halftone cells 708, 711, 705, 710, 702, 716, 714, 703, 701, 706, 709, 712, 704, 715, 713, and 707 from the low density region. The

  Note that the order of threshold values included in the halftone cells of the threshold value matrices 400, 500, 600, and 700, that is, the formation order of the minimum halftone dot unit is set based on the degree of dispersion according to the procedure described later. Further, in the present embodiment, for ease of explanation, an example in which halftone processing is performed by repeatedly using the pixel positions of image data to perform halftone processing has been described, but the present invention is not necessarily limited thereto. is not.

  Therefore, for example, with respect to the threshold value matrix 400, as described above, it is sufficient to use 16 types of halftone dot cells including 20 threshold values. Therefore, it is necessary to repeat the halftone process while shifting the 320 threshold value arrays in accordance with the pixel positions. Processing can also be performed. The same applies to the threshold matrixes 500 and 700, and the threshold matrix 500 can be subjected to halftone processing repeatedly while shifting 320 threshold arrays, and the threshold matrix 700 is shifted to 288 threshold arrays according to the pixel positions. In the present embodiment, each halftone cell has been described as including four continuous threshold values. However, the present invention is not necessarily limited to this, and it is sufficient that at least two or more threshold values are continuous.

  In addition, it has been described that each halftone cell of the threshold matrixes 400, 500, 600, and 700 corresponding to cyan, magenta, yellow, and black colors includes four continuous threshold values in all halftone cells. It is not necessarily limited to this. Therefore, for example, the threshold value matrix may be generated so that a different number of threshold values are consecutive for each color. In addition, regarding the threshold matrixes 400, 500, 600, and 700, threshold matrices having different vectors can be used. In this case, the shape of the halftone cell, the number of included thresholds, and the size of the threshold matrix are set according to the vector.

  Next, a threshold arrangement procedure in the threshold matrix will be described with reference to FIGS. 8, 9, and 10. FIG. In the present embodiment, the threshold value matrix is stored in the RAM 307 in accordance with a later-described procedure (rule) executed by a CPU (not shown) of the PC 108.

  FIG. 8 is a flowchart for setting (determining) the formation order of the minimum halftone dot unit of the halftone cell in the present embodiment. Here, for example, the program for executing the processing shown in FIG. 8 is stored in the HDD (not shown) of the PC 108, read into the RAM (not shown) of the PC 108, and executed by the CPU (not shown) of the PC 108. Is done. Note that the program for executing the processing shown in FIG. 8 is not limited to being stored on the PC 108 but may be stored in the ROM 305 of the image forming apparatus 10 and executed by the CPU 306.

  FIG. 9 is an example of image data in which halftone dots are formed in units of minimum halftone dots in the present embodiment. FIG. 10 is a diagram in which halftone dots are formed in units of minimum halftone dots in the present embodiment and then binarized. It is an example of the processed image data.

  Hereinafter, a description will be given according to the flowchart shown in FIG. First, in step S801, halftone dots are formed in units of minimum halftone dots on image data for calculating the degree of dispersion described later for halftone cells in which the formation order has already been set for all colors of CMYK.

  Note that the size of the image data for calculating (measuring) the degree of dispersion is preferably larger than the least common multiple of the matrix sizes of all CMYK in width and height. Specifically, both the width and the height are preferably 240 pixels or more, and in this embodiment, the size of the image data for calculating the degree of dispersion is set to 500 × 500 pixels. In addition, since there is no halftone cell whose formation order is determined at the start of the flowchart shown in FIG. 8, no halftone dot is formed in any pixel, and the process proceeds to step S802.

  In step S802, among the halftone cells 401 to 416 included in the threshold matrix 400 of the first color, one of undecided halftone cells whose formation order has not been determined is selected, and the minimum is displayed on the image data. Halftone dots are formed in halftone dots.

  In this embodiment, the first color is cyan. FIG. 9A shows a state in which one of the cyan halftone dot cells 401 is selected and halftone dots 901 are formed on the image data in units of minimum halftone dots.

  The CPU of the PC calculates the degree of dispersion using the image data in which the halftone dots are formed in step S802 (S803). FIG. 10A shows the image data binarized from FIG. 9A. In this embodiment, the degree of dispersion is calculated from the binarized image data. Details of the degree of dispersion will be described later.

  Next, it is determined whether or not the calculation of the degree of dispersion has been completed for all the halftone cells for which the formation order has not been determined (S804). If the calculation of the degree of dispersion has been completed (Yes in S804). ), The process proceeds to step S805. If the calculation of the degree of dispersion has not been completed (No in S804), the process returns to Step S801, and a halftone dot is formed at a position corresponding to an undecided halftone cell that has not yet been selected in Step S802.

  When the process proceeds to step S805, the CPU of the PC 108 assigns a threshold value to the halftone cell having the largest degree of dispersion among the degrees of dispersion calculated in step S803, and adds the threshold value to the already determined halftone cell (that is, the already-determined network). Set as a point cell). Note that the threshold value to be assigned is set so that the threshold value increases for each halftone cell whose order is determined.

  Next, in step S806, halftone dots are formed in units of minimum halftone dots on the image data for calculating the degree of dispersion for the halftone cells in which the formation order has already been determined for all colors of CMYK as in step S801. To do.

  In step S807, one of undecided halftone cells whose order of formation has not been determined is selected from among the halftone cells 501 to 516 included in the second color threshold matrix 500, and the minimum is displayed on the image data. Halftone dots are formed in halftone dots.

  In this embodiment, the second color is magenta. FIG. 9B shows a state in which a halftone dot 902 is formed in units of minimum halftone dots on image data by selecting one of magenta halftone dot cells 501.

  Similarly to step S803, the CPU of the PC 108 calculates the degree of dispersion using the image data in which the halftone dots are formed in step S807 (S808). FIG. 10B shows the image data binarized from FIG. 9B. In this embodiment, the degree of dispersion is calculated from the binarized image data.

  Next, it is determined whether or not the calculation of the degree of dispersion has been completed for all the halftone cells for which the formation order has not been determined (S809), and if the calculation of the degree of dispersion has been completed (Yes in S809). ), The process proceeds to step S810. If the calculation of the degree of dispersion has not been completed (No in S809), the process returns to step S806, and a halftone dot is formed at a position corresponding to an undecided halftone cell that has not yet been selected in step S807.

  When the process proceeds to step S810, the CPU of the PC 108 assigns a threshold value to the halftone cell having the largest degree of dispersion among the degrees of dispersion calculated in step S808, and adds the threshold value to the determined halftone cell (ie, the already-determined network). Set as a point cell). Note that the threshold value to be assigned is set so that the threshold value increases for each halftone cell whose order is determined.

  Next, in step S811, as in step S801, halftone dots are formed in units of minimum halftone dots on the image data for calculating the degree of dispersion for the dot cells whose formation order has already been determined for all colors of CMYK. .

  In step S812, among the halftone cells 601 to 616 included in the threshold matrix 600 for the third color, one of undecided halftone cells whose formation order has not been determined is selected, and the minimum is displayed on the image data. Halftone dots are formed in halftone dots.

  Here, in this embodiment, the third color is yellow. FIG. 9C shows a state in which a halftone dot 903 is formed in units of the minimum halftone dot on the image data by selecting 601 which is one of yellow halftone dot cells.

  Similarly to step S803, the CPU of the PC 108 calculates the degree of dispersion using the image data in which the halftone dots are formed in step S812 (S813). FIG. 10C shows image data binarized from FIG. 9C. In this embodiment, the degree of dispersion is calculated from the binarized image data.

  Next, it is determined whether or not the calculation of the degree of dispersion has been completed for all the halftone cells for which the formation order has not been determined (S814). If the calculation of the degree of dispersion has been completed (Yes in S814). ), The process proceeds to step S815. If the calculation of the degree of dispersion has not been completed (No in S814), the process returns to Step S811, and a halftone dot is formed at a position corresponding to an undecided halftone cell that has not yet been selected in Step S812.

  When the process proceeds to step S815, the CPU of the PC 108 assigns a threshold value to the halftone cell having the largest degree of dispersion among the degrees of dispersion calculated in step S813, and adds the threshold value to the already-determined halftone cell (ie, the already-determined network). Set as a point cell). Note that the threshold value to be assigned is set so that the threshold value increases for each halftone cell whose order is determined.

  Next, in step S816, as in step S801, halftone dots are formed in units of minimum halftone dots on the image data for calculating the degree of dispersion for the halftone cells in which the formation order has already been determined for all colors of CMYK. To do.

  In step S817, among the halftone cells 701 to 716 included in the fourth color threshold value matrix 700, one of undecided halftone cells whose formation order has not been determined is selected, and the minimum is displayed on the image data. Halftone dots are formed in halftone dots.

  Here, in the present embodiment, the fourth color is black. FIG. 9D shows a state in which a halftone dot 904 is formed in units of the minimum halftone dot on the image data by selecting 708 which is one of black halftone dot cells.

  Similar to step S803, the CPU of the PC 108 calculates the degree of dispersion using the image data in which the halftone dots are formed in step S817 (S818). FIG. 10D is image data binarized from FIG. 9D. In this embodiment, the degree of dispersion is calculated from the binarized image data.

  Next, it is determined whether or not the calculation of the degree of dispersion has been completed for all the halftone cells for which the formation order has not been determined (S819), and if the calculation of the degree of dispersion has been completed (Yes in S819). ), The process proceeds to step S820. If the calculation of the degree of dispersion has not been completed (No in S819), the process returns to step S816, and a halftone dot is formed at a position corresponding to an undecided halftone cell that has not yet been selected in step S817.

  When the process proceeds to step S820, the CPU of the PC 108 assigns a threshold value to the halftone cell having the largest degree of dispersion among the degrees of dispersion calculated in step S818, and adds the threshold value to the already-determined halftone cell (ie, the already-determined network). Set as a point cell). Note that the threshold value to be assigned is set so that the threshold value increases for each halftone cell whose order is determined.

  Next, the CPU of the PC 108 determines whether or not the dot formation order has been determined in all the dot cells of all the colors (S821), and if determined (S821 Yes), FIG. The process shown in FIG. If not determined (No in S821), the process returns to step S801, and the formation order is determined for the remaining halftone cells from cyan.

  Next, a method for calculating the degree of dispersion in the present embodiment will be described in detail with reference to FIGS. 10 and 11 described above. FIG. 11 shows binarized image data in which halftone dots are formed in units of minimum halftone dots in the image forming apparatus according to the present embodiment.

  Further, in the present embodiment, as the degree of dispersion, the binarized image data is divided by a unit of a predetermined size, and a local average value obtained by calculating the average of the pixel values by the unit of the divided area is obtained and divided. A value (Equation 3) obtained by taking the reciprocal from the variance of the local average value of the entire region is used.

  Here, for example, in FIG. 10 described above, the image data is divided in units of regions 1000. In the present embodiment, the size of the region 1000 is 16 pixels wide and 16 pixels high, and the image data is divided into 31 × 31 regions.

  The region 1000 including the halftone dots 1001 and 1002 in FIG. 10B includes four pixels of cyan and magenta, and 248 pixels of white having a value of 0, out of all 256 pixels. The local average value is obtained by (Equation 1), the width of the region 1000 is m = 16, the height is n = 16, and the local average value f (0,0) of the first divided region 1000 is 0.03125. Become. I in (Expression 1) is a pixel value of binary image data.

  In FIG. 10B, the local average values f (i, j) of other blocks are similarly obtained, and the average value μ of the local average values shown in (Expression 2) is obtained. As described above, the number of divisions of the image data is o = 31 in the width direction and p = 31 in the height direction. Using (Equation 3), the reciprocal of the variance value of the local average value of all divided regions is It is calculated as 13009.84.

  Note that the size of the region 1000 is desirably determined according to the vector components of the halftone cells of each threshold value matrix and the arrangement of the sub-matrices. For example, the threshold value matrix 400 is 16 pixels wide and 16 pixels high because the large value of the vector of halftone cells is 4 and the period of the sub-matrix is 4 × 4.

  Next, the size of the divided region for obtaining the local average value when the number of halftone dots increases (for example, when the formation order of three halftone dots is determined) will be described. In the present embodiment, when the size of the divided area for obtaining the local average value is fixed, the difference in the degree of dispersion calculated when the density of halftone dots increases, so the size of the divided area for obtaining the local average value is It is dynamically changed according to the number of already determined halftone cells.

  In this regard, the description will be supplemented with reference to FIG. FIG. 11 shows image data obtained by forming halftone dots in units of minimum halftone dots and binarizing as described above. Specifically, after determining the formation order of three halftone dots for each color of CMYK, FIG. This is image data obtained by binarizing halftone dots when determining the formation order of the fourth halftone cell of each color of CMYK. In FIG. 11, the density of the halftone dots is higher than that in FIG. 10, and the divided area for obtaining the local average value is half of that from the area 1000 (16 pixels wide and 16 pixels high) shown in FIG. The width is changed to 8 pixels and the height is 8 pixels (divided area 1100).

  As shown in FIG. 11, even when the density of halftone dots is increased by changing the divided area from the area 1000 to the area 1100 in half, the halftone dots when the colors are superimposed are arranged more evenly. Can be made (distributed). As described above, in this embodiment, the size of the divided area for obtaining the local average value is changed according to the number of halftone cells that have been determined. Specifically, if there is no halftone dot cell, the width and height of the divided area are changed to 16 pixels, 1 for 12 pixels, 3 for 8 pixels, and 7 for 4 pixels. To do.

  In this embodiment, the size of the divided area for obtaining the local average value is common to all colors of CMYK. However, the size may be changed for each color according to the arrangement of the halftone dot vector components and sub-matrices. Good. In addition, the degree of dispersion has been described as a value obtained by taking the reciprocal from the dispersion of the local average value of all the divided areas, but it is not necessarily limited to this if it can be evaluated that the arrangement of halftone dots is uniform when colors are superimposed. Not. Therefore, for example, binarized image data (binarized image) can be subjected to frequency analysis by Fourier transform or the like, and the ratio of high frequency components can be set as the degree of dispersion.

  FIG. 12 is a diagram showing the execution result (halftone image data) of halftone processing in the image forming apparatus according to the present embodiment. In FIG. 12, the execution result of the halftone process in the present embodiment is shown as FIG. 12C, and in order to more effectively show the effect of suppressing the occurrence of moire in the present embodiment (ie, comparison). For reference), FIGS. 12A and 12B are shown as a reference.

  FIG. 12A shows a dot cell cycle similar to that of the present embodiment. The minimum halftone dot unit is one pixel, that is, a threshold matrix that does not concentrate halftone dots in a low density region is used, and four colors of CMYK are displayed. The halftone image data of the gradation of the low density area which overlapped is shown. FIG. 12B shows a threshold matrix in which the halftone dot unit is 4 pixels in the same halftone cell cycle as in the present embodiment, and the arrangement of halftone dots when the colors are overlapped is locally concentrated. FIG. 5 shows low-density gradation halftone dot image data in which four colors of CMYK are superimposed. However, the threshold value matrix applied in FIG. 12B is one in which threshold values are set (arranged) without considering the degree of dispersion in the generation. Further, FIG. 12C shows a halftone dot image data of a low density region in which four colors of CMYK are overlapped using the threshold value matrixes 400, 500, 600, and 700 described in the present embodiment as described above. Is shown.

  In FIG. 12A, moire (rosetta) generated when colors are superimposed can be visually recognized at the same period, and in FIG. 12B, moire having a lower frequency than that in FIG. 12A is visually recognized. be able to. That is, the frequency is gradually lowered toward a lower concentration range (left side of FIG. 12B). On the other hand, in FIG. 12C, the low-frequency moire visually recognized in the low density region of FIG. 12B is improved, and the arrangement of halftone dots is more uniform when the colors are superimposed. Recognize.

  As described above, in the image forming apparatus according to the present embodiment, it is possible to suppress the occurrence of moire while improving the reproducibility and granularity of halftone dots in the low density region.

(Embodiment 2)
In the above-described embodiment, the degree of dispersion is calculated for binary image data obtained by superimposing the four colors of CMYK, and the halftone dots when the four colors of CMYK are superimposed are evenly dispersed, thereby overlapping the CMYK four colors. Improved the generation of low-frequency moire.

  However, among the four colors of CMYK, yellow (Y) is difficult for humans to visually recognize. Therefore, in the present embodiment, a method for improving the generation of low-frequency moire in the overlapping of three CMK colors excluding yellow (Y) that is difficult for humans to visually recognize will be described.

  Note that the difference from the above-described first embodiment is only the threshold matrix to be applied and the threshold array rule of the threshold matrix. Therefore, the same parts as those in the above-described embodiment are denoted by the same reference numerals and the description thereof is omitted. To do.

  First, the threshold value matrix used in the processing in the halftone processing unit 304 of the image processing unit 301 and the halftone processing will be described in detail with reference to FIGS. The threshold matrixes shown in FIGS. 13, 14, and 15 are shown as 600 dpi threshold matrices as an example.

  A threshold matrix 1300 shown in FIG. 13 is a binary threshold matrix applied to cyan. The threshold matrix 1300 is an arrangement of a total of 1600 vertical 40 and horizontal 40, and is configured by regularly arranging 16 types of halftone cells 1301 to 1316 having different threshold combinations. Is done. Each of the halftone cells 1301 to 1316 includes 20 threshold values.

  Further, the period in which the halftone cells are arranged is expressed by two vectors similarly to the threshold matrix 400, and is repeated at a period of (4, 2) and (2, -4) in the vertical and horizontal two-component vector of the threshold array. It is.

  In addition, in the second embodiment, as in the first embodiment, in each halftone cell, threshold values are arranged so that a predetermined number of threshold values are continuous, and portions other than the portion in which the threshold values are continuously arranged Are set in order so that the threshold value is increased in each halftone cell. As a result, in the example shown in FIG. A halftone dot is formed.

  A threshold matrix 1400 illustrated in FIG. 14 is a binary threshold matrix applied to magenta. The threshold matrix 1400 is a total of 1600 vertical and horizontal 40 arrays, and is configured by regularly arranging 16 types of halftone dot cells 1401 to 1416 having different combinations of threshold values. Is done. Each of the halftone cells 1401 to 1416 includes 20 threshold values.

  The period in which the halftone cells are arranged is expressed by two vectors, similar to the threshold matrix 500, and is repeated at periods of (2, 4) and (4, -2) in the vertical and horizontal two-component vectors of the threshold array. It is.

  In addition, as described above, also in the second embodiment, in each halftone cell, threshold values are arranged so that a predetermined number of threshold values are continuous, and in portions other than the portion where the threshold values are continuously arranged, The threshold values are set in order so as to increase the threshold value for each halftone cell. Thus, in the example shown in FIG. A halftone dot is formed.

  A threshold matrix 1500 shown in FIG. 15 is a binary threshold matrix applied to black. The threshold matrix 1500 has a total of 576 arrays of 24 in the vertical direction and 24 in the horizontal direction, and is arranged so that 16 types of halftone cells 1501 to 1516 each having a combination of different threshold values are regularly arranged. Each of the halftone cells 1501 to 1516 includes 18 threshold values.

  Further, the period in which the halftone cells are arranged is expressed by two vectors similarly to the threshold matrix 700, and is repeated at a period of (3, 3) and (3, -3) in the vertical and horizontal two-component vector of the threshold array. It is.

  In addition, as described above, also in the second embodiment, in each halftone cell, threshold values are arranged so that a predetermined number of threshold values are continuous, and in portions other than the portion where the threshold values are continuously arranged, The threshold values are set in order so as to increase the threshold value for each halftone cell. Thus, in the example shown in FIG. A halftone dot is formed.

  Note that the binary threshold matrix applied to yellow is not shown, and the threshold of the binary threshold matrix applied to yellow is the formation order of the minimum halftone dot unit of the halftone dot cell described below with reference to FIG. It is set regardless of the procedure for setting.

  FIG. 16 is a flowchart for setting (determining) the formation order of the minimum halftone dot unit of the halftone dot cell in the present embodiment (that is, the second embodiment). Also in the present embodiment, as in the first embodiment, the threshold matrix is stored in the RAM 307 in which the dot formation order is set according to the later-described procedure executed by the CPU of the PC 108.

  16 is stored in, for example, the HDD of the PC 108, read out to the RAM of the PC 108, and executed by the CPU of the PC 108, as in the first embodiment. Note that a program for executing the processing shown in FIG. 16 is not limited to being stored on the PC 108 but may be stored in the ROM 305 of the image forming apparatus 10 and executed by the CPU 306.

  In FIG. 8, the dot formation order in the low density region is set so that the dispersion degree is maximized when the CMYK four colors overlap. However, in FIG. 16, the dispersion degree is maximized when the CMK three colors overlap. Next, the dot formation order in the low density region is set.

  Therefore, the procedure of the process shown in FIG. 16 is almost the same as the procedure of the process shown in FIG. 8, and the processes of steps S1601 to S1605 related to the first color (cyan) are the processes of steps S801 to S805 of FIG. Corresponding to That is, the cyan threshold value matrix 1300 is set (determined) in the processes of steps S1601 to S1605.

  Further, the processing of steps S1606 to S1610 related to the second color (magenta) is the processing of steps S806 to S810 in FIG. 8, and the processing of steps S1611 to S1615 related to the third color (black) is shown in FIG. This corresponds to the processing in steps S816 to S820. That is, a magenta threshold matrix 1400 and a black threshold matrix 1500 are set. In addition, the processing in step S1616 is the same as the processing in step S821 in FIG. 8, and it is determined whether or not the dot formation order has been determined in all halftone cells of all colors of CMK.

  FIG. 17 is a diagram illustrating an execution result (halftone image data) of halftone processing in the image forming apparatus according to the present embodiment (second embodiment). In FIG. 17, the execution result of the halftone processing in the present embodiment is shown as FIG. 17C, and in order to more clearly show the effect of suppressing the occurrence of moire in the present embodiment, FIG. 17 (a) and (b) are listed.

  FIG. 17A shows the same period of halftone cells as in the present embodiment, and the minimum halftone dot unit is one pixel, that is, using a threshold matrix that does not concentrate halftone dots in the low density region, the three colors of CMK The halftone image data of the gradation of the low density area which overlapped is shown. FIG. 17B shows a threshold matrix in which the halftone dot unit is 4 pixels in the same halftone cell cycle as in the present embodiment, and the arrangement of the halftone dots when the colors are superimposed is locally concentrated. FIG. 2 shows low-density gradation halftone dot image data in which three colors of CMK are superimposed. However, the threshold value matrix applied in FIG. 17B is one in which threshold values are set (arranged) without considering the degree of dispersion in the generation thereof. Further, FIG. 17C shows gradation halftone dot image data in a low density region in which three colors of CMK are overlaid using the threshold matrixes 1300, 1400, 1500 described in the present embodiment as described above. It is a thing.

  In FIG. 17A, moire (rosetta) generated when colors are superimposed can be visually recognized in the same cycle, and in FIG. 17B, moire having a lower frequency than that in FIG. 17A is visually recognized. be able to. That is, the frequency gradually becomes lower toward a lower concentration range (left side of FIG. 17B). On the other hand, in FIG. 17C, the low-frequency moire visually recognized in the low density region of FIG. 17B is improved, and the arrangement of halftone dots is more even when the three CMK colors are overlaid. I understand.

  In the present embodiment, the method of using the threshold value matrix in which the degree of dispersion is calculated by superimposing halftone dots of the CMK three colors and the order in which the halftone dots are formed is described. However, the present invention is not necessarily limited thereto. Therefore, for example, in the superposition of two or more different colors such as three colors of CMY and two colors of CM, the degree of dispersion can be obtained and the dot formation order can be set (determined).

  As described above, according to the present embodiment, in the three colors of CMK that are easy to be visually recognized by humans, it occurs when the colors are overlapped while improving the reproducibility and graininess of the low-density halftone dots. Generation of moire with a lower frequency can be suppressed.

(Embodiment 3)
In the first embodiment, in order to improve the generation of low frequency moire in the overlapping of four colors of CMYK, the degree of dispersion is calculated for binary image data in which the four colors of CMYK are superimposed. The degree of dispersion is calculated for the image data in which the halftone dots are converted into the color material brightness. Since the difference from the first embodiment is only the threshold matrix to be applied, the same parts as those in the first embodiment are denoted by the same reference numerals and the description thereof is omitted.

  Next, the processing in the halftone processing unit 304 of the image processing unit 301 and the threshold matrix used in the halftone processing will be described in detail with reference to FIGS. 18, 19, 20, and 21. Note that the threshold matrixes shown in FIGS. 18, 19, 20, and 21 are shown as 600 dpi threshold matrices as an example.

  A threshold matrix 1800 illustrated in FIG. 18 is a binary threshold matrix applied to cyan. The threshold matrix 1800 has an arrangement of a total of 1600 vertical 40 and horizontal 40, and is configured by regularly arranging 16 types of halftone cells 1801 to 1816 having different combinations of threshold values. Is done. Each of the halftone cells 1801 to 1816 includes 20 threshold values.

  Further, the period in which the halftone cells are arranged is expressed by two vectors similarly to the threshold matrix 400, and is repeated at a period of (4, 2) and (2, -4) in the vertical and horizontal two-component vector of the threshold array. It is.

  In addition, in the third embodiment, as in the first embodiment, in each halftone cell, threshold values are arranged so that a predetermined number of threshold values are continuous, and portions other than the portion in which the threshold values are continuously arranged Are set in order so that the threshold value is increased in each halftone cell. Accordingly, in the example shown in FIG. 18, the halftone dot cells 1801, 1811, 1807, 1813, 1804, 1812, 1805, 1803, 1810, 1808, 1815, 1806, 1816, 1814, 1802 are arranged in this order from the low concentration range. A point is formed.

  A threshold matrix 1900 shown in FIG. 19 is a binary threshold matrix applied to magenta. The threshold matrix 1900 is an arrangement of a total of 1600 vertical 40 and horizontal 40, and is configured by regularly arranging 16 types of halftone cells 1901 to 1916 having different threshold combinations. Is done. Each of the halftone cells 1901 to 1916 includes 20 threshold values.

  The period in which the halftone cells are arranged is expressed by two vectors, similar to the threshold matrix 500, and is repeated at periods of (2, 4) and (4, -2) in the vertical and horizontal two-component vectors of the threshold array. It is.

  In addition, as described above, also in the third embodiment, in each halftone cell, threshold values are arranged so that a predetermined number of threshold values are continuous, and portions other than the portion where the threshold values are continuously arranged are provided in the portions. The threshold values are set in order so as to increase the threshold value for each halftone cell. Accordingly, in the example shown in FIG. 19, the dot cells 1901, 1909, 1911, 1903, 1902, 1915, 1906, 1907, 1912, 1908, 1914, 1904, 1905, 1916, 1913, 1910 are arranged in this order from the low density region. A halftone dot is formed.

  A threshold matrix 2000 shown in FIG. 20 is a binary threshold matrix applied to yellow. The threshold matrix 2000 has a total of 256 arrangements of 16 in the vertical direction and 16 in the horizontal direction, and is configured by regularly arranging 16 types of halftone cells 2001 to 2016 each having a different threshold combination. Is done. Each of the halftone cells 2001 to 2016 includes 16 threshold values.

  Further, the period in which the halftone cells are arranged is expressed by two vectors similarly to the threshold matrix 600, and is repeated in the cycle of (4, 0) and (0, 4) in the vertical and horizontal two-component vector of the threshold array. .

  In addition, as described above, also in the third embodiment, in each halftone cell, threshold values are arranged so that a predetermined number of threshold values are continuous, and portions other than the portion where the threshold values are continuously arranged are provided in the portions. The threshold values are set in order so as to increase the threshold value for each halftone cell. Accordingly, in the example shown in FIG. 20, the dot cells 2001, 2016, 2011, 2006, 2009, 2007, 2010, 2008, 2003, 2014, 2015, 2012, 2002, 2004, 2013, 2005 from the low concentration range. A halftone dot is formed.

  A threshold matrix 2100 illustrated in FIG. 21 is a binary threshold matrix applied to black. The threshold matrix 2100 has a total of 576 arrangements of 24 in the vertical direction and 24 in the horizontal direction, and is configured by regularly arranging 16 types of halftone cells 2101 to 2116 having different combinations of threshold values. Is done. Each of the halftone cells 2101 to 2116 includes 18 threshold values.

  In addition, as described above, also in the third embodiment, in each halftone cell, threshold values are arranged so that a predetermined number of threshold values are continuous, and portions other than the portion where the threshold values are continuously arranged are provided in the portions. The threshold values are set in order so as to increase the threshold value for each halftone cell. Accordingly, in the example shown in FIG. 21, the dot cells 2109, 2116, 2110, 2113, 2107, 2102, 2108, 2115, 2112, 2111, 2104, 2105, 2101, 2106, 2114, and 2103 are ordered in this order from the low density region. A halftone dot is formed.

  Next, a method for calculating the degree of dispersion in the present embodiment will be described in detail with reference to FIG. FIG. 22 is an example of image data in which halftone dots are formed in units of minimum halftone dots and converted to lightness in this embodiment.

  Further, in the present embodiment, as the degree of dispersion, the image data converted into lightness is divided in units of a predetermined size, and a local average value obtained by calculating the average of pixel values in the unit of the divided area is obtained and divided. A value obtained by taking the reciprocal from the variance of the local average value of the entire region is used.

  The dispersity is calculated using the above-described (formula 1) to (formula 3) as in the first embodiment. In the present embodiment (third embodiment), I in (formula 1) is This is the pixel value of the image data converted to lightness.

  FIG. 22 shows an example of image data in which halftone dots are formed in units of minimum halftone dots and converted to lightness as described above. Specifically, the order of forming the first halftone dot cells for each color of CMYK is determined. It is image data converted into the brightness at the time of performing. In FIG. 22, the image data is divided in units of areas 2205 as illustrated. Note that the size of the region 2205 is the same as the size of the region 1000 of the first embodiment.

  In addition, the procedure for setting the formation order of the minimum halftone dot unit of the halftone dot cell is roughly the same as that of the first embodiment except that the degree of dispersion is calculated for the image data obtained by converting the halftone dots into the brightness of the color material. That is, it is the same as the process shown in FIG.

  FIG. 22A is an example of lightness image data in which the first halftone dot 2201 determined in the process corresponding to step S805 in FIG. 8 is formed. In FIG. 22A, paper white 2200 is the brightness (L * = 93) of the recording paper used in the image forming apparatus 10 of this embodiment, and cyan halftone dot 2201 is the color used in the image forming apparatus 10 of this embodiment. Converted to the lightness of the material (L * = 58).

  FIG. 22B is an example of lightness image data in which the first determined magenta halftone dot 2202 is formed in the processing corresponding to step S810 in FIG. In FIG. 22B, the magenta halftone dot 2202 is converted into the lightness (L * = 49) of the color material used in the image forming apparatus 10 of this embodiment.

  FIG. 22C is an example of lightness image data in which the first halftone dot 2203 determined in the process corresponding to step S815 in FIG. 8 is formed. In FIG. 22C, the yellow halftone dot 2203 is converted into the lightness (L * = 90) of the color material used in the image forming apparatus 10 of this embodiment.

  FIG. 22D is an example of lightness image data in which the first black dot 2204 determined in the process corresponding to step S820 in FIG. 8 is formed. In FIG. 22D, the black halftone dot 2204 is converted into the lightness (L * = 10) of the color material used in the image forming apparatus 10 of the present embodiment.

  Here, for example, in the area 2205 in FIG. 22B, among the 256 pixels, there are 4 cyan pixels with lightness (L * = 58) and magenta pixels with lightness (L * = 49). It includes 4 pixels and 248 pixels of lightness (L * = 93) white. In this case, the local average value is 91.7656.

  In FIG. 22B, in other blocks other than the region 2205, the local average value is obtained in the same manner as in the region 2205, and the reciprocal of the variance value of the local average values of all divided regions is calculated as 8.2281. It becomes. In the present embodiment, in order to facilitate calculation, the brightness of the halftone dot formed last is used for the pixel where the halftone dots overlap. However, in this case, for example, the lowest brightness among the overlapping halftone dots can be used, or the brightness when a plurality of color materials are overlapped can be measured and used.

  FIG. 23 is a diagram showing the execution result (halftone image data) of halftone processing in the image forming apparatus according to the present embodiment. In FIG. 23, the execution result of the halftone processing in the present embodiment is shown as FIG. 23C, and in order to more clearly show the effect of suppressing the occurrence of moire in the present embodiment (that is, comparison) For reference), FIGS. 23 (a) and 23 (b) are shown for reference.

  FIG. 23A shows a dot cell cycle similar to that of the present embodiment. The minimum halftone dot unit is one pixel, that is, a threshold matrix that does not concentrate halftone dots in a low density region is used to change four colors of CMYK. The halftone image data of the gradation of the low density area which overlapped is shown. FIG. 23 (b) shows a threshold matrix in which the halftone dot unit is 4 pixels in the same halftone cell cycle as in this embodiment, and the arrangement of halftone dots when the colors are overlapped is locally concentrated. FIG. 5 shows low-density gradation halftone dot image data in which four colors of CMYK are superimposed. However, the threshold value matrix applied in FIG. 23B is one in which threshold values are set (arranged) without considering the degree of dispersion in the generation thereof. Further, FIG. 23C shows a halftone dot image data of a low density region in which four colors of CMYK are overlaid using the threshold value matrix 1800, 1900, 2000, 2100 described in the present embodiment as described above. Is shown.

  In FIG. 23A, moire (rosetta) generated when colors are superimposed can be visually recognized at the same period, and in FIG. 23B, moire having a lower frequency than that in FIG. 23A is visually recognized. be able to. That is, the frequency gradually becomes lower toward a lower density region (left side of FIG. 23B). On the other hand, in FIG. 23C, the low-frequency moire visually recognized in the low density region of FIG. 23B is improved, and the arrangement of halftone dots is more uniform when the colors are superimposed. Recognize.

  As described above, according to the present embodiment, by calculating the degree of dispersion using lightness instead of binary, it is lower that occurs when colors are superimposed according to the brightness of the color material. Generation of frequency moiré can be suppressed.

(Embodiment 4)
In the first embodiment described above, the degree of dispersion is calculated for binary image data obtained by superimposing the four colors of CMYK, and the halftone dots when the four colors of CMYK are superimposed are evenly dispersed, so that Improved the occurrence of low-frequency moire in overlapping. In Embodiment 2 described above, the occurrence of moiré at low frequencies is improved in the overlap of three CMK colors except yellow (Y), which is difficult for humans to visually recognize. Further, in the above-described third embodiment, in the overlapping of four colors of CMYK, the occurrence of low frequency moiré is improved by calculating the degree of dispersion for the image data obtained by converting the halftone dot where the four colors of CMYK are superimposed into lightness. did.

  Therefore, in this embodiment (Embodiment 4), the degree of dispersion is calculated with respect to image data obtained by converting halftone dots overlaid with three CMK colors into the brightness of the color material, excluding yellow (Y), which is difficult for humans to visually recognize, A method for improving the generation of low-frequency moire will be described. Note that the procedure for setting the formation order of the minimum halftone dot unit of the halftone dot cell is approximately the same as that in Embodiment 2 except that the degree of dispersion is calculated for the image data obtained by converting the halftone dots into the brightness of the color material. That is, it is the same as the process shown in FIG.

  That is, in the same procedure as in FIG. 16, in the overlap of three CMK colors, the degree of dispersion is calculated for image data in which halftone dots are converted to lightness of the color material, and the low density halftone network is used to maximize the degree of dispersion. Determine the order of dot formation. As in FIG. 22, the dispersity is obtained by dividing the image data converted to lightness by a unit of a predetermined size, obtaining a local average value obtained by calculating the average of the pixel values by the unit of the divided area, The value obtained by taking the reciprocal from the variance of the local average value of the region is used.

  In the present embodiment, the method of using the threshold value matrix in which the degree of dispersion is calculated by superimposing halftone dots of the CMK three colors and the order in which the halftone dots are formed is described. However, the present invention is not necessarily limited thereto. Therefore, for example, in the superposition of different colors such as CMY3 colors and CM2 colors, the degree of dispersion is obtained and the halftone dot formation order is set (determined).

  As described above, according to the present embodiment, by calculating the degree of dispersion using lightness instead of binary, in three colors of CMK that are easy to be visually recognized by humans, according to the brightness of the color material, It is possible to suppress the occurrence of moire with a lower frequency that occurs when colors are superimposed.

(Other embodiments)
The object of the present invention can also be achieved by executing the following processing. That is, a storage medium that records a program code of software that realizes the functions of the above-described embodiments is supplied to a system or apparatus, and a computer (or CPU, MPU, etc.) of the system or apparatus is stored in the storage medium. This is the process of reading the code. In this case, the program code read from the storage medium itself realizes the functions of the above-described embodiments, and the program code and the storage medium storing the program code constitute the present invention. Become.

Claims (9)

An image forming apparatus that performs halftone processing using a threshold matrix having a plurality of halftone cells for each recording material to generate a halftone image,
In the threshold value matrix, among the plurality of halftone cells, the halftone dot cell having the maximum degree of dispersion of halftone dots calculated by superimposing the halftone dot images generated for each recording material of at least two colors or more is used. An image forming apparatus, wherein a threshold value is assigned in a predetermined unit so that a halftone dot of a predetermined size is formed in order.   The image forming apparatus according to claim 1, wherein the threshold value assigned in the predetermined unit is a value that continues in ascending order.   The threshold values are set in ascending order in order from the halftone cell in which the degree of dispersion of the halftone dots is maximized, in a portion other than the portion to which the threshold value of the halftone cell is assigned in a predetermined unit. The image forming apparatus according to claim 1.   The degree of dispersion of the halftone dots is calculated from a local average value of pixel values in a predetermined divided region of a binarized image generated from the halftone dot image. The image forming apparatus according to claim 1.   The degree of dispersion of the halftone dots is calculated from a local average value of lightness in a predetermined divided area of a binarized image generated from the halftone dot image. The image forming apparatus according to claim 1.   6. The image forming apparatus according to claim 4, wherein the predetermined divided area is dynamically changed according to the number of halftone cells to which the threshold is assigned in a predetermined unit. .   The degree of dispersion of the halftone dots is calculated by superimposing the halftone dot images generated by the halftone processing for each of the four color recording materials of CMYK. The image forming apparatus described in the item.   The degree of dispersion of the halftone dots is calculated by superimposing the halftone dot images generated by the halftone processing for each of the three color recording materials of CMK. The image forming apparatus described in the item. For each recording material, including halftone processing using a threshold matrix having a plurality of halftone cells, and generating a halftone image,
In the threshold value matrix, among the plurality of halftone cells, the halftone dot cell having the maximum degree of dispersion of halftone dots calculated by superimposing the halftone dot images generated for each recording material of at least two colors or more is used. An image forming method in an image forming apparatus, wherein a threshold value is assigned in a predetermined unit so that halftone dots of a predetermined size are formed in order.