JP2019009621A - Image forming apparatus, control method thereof, and program - Google Patents

Abstract

A mechanism for improving reproducibility and graininess of an extreme density region while suppressing generation of a pseudo contour by suitably correcting a minimum dot size in a dither method is provided. The image forming apparatus forms a dot size correction chart expressing a predetermined density with a plurality of dot sizes on a sheet. In addition, the image forming apparatus determines the minimum dot size by comparing the density value of the dot size correction chart calculated by reading the dot size correction chart and the density value of the predetermined density. Further, the image forming apparatus generates a threshold matrix for performing dither processing using the determined minimum dot size. [Selection] Figure 5

Description

  The present invention relates to an image forming apparatus, a control method thereof, and a program.

  An electrophotographic system is known as an image recording system 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, and the latent image is developed with a charged color material (hereinafter referred to as toner). The image is recorded by transferring the developed toner image onto a transfer sheet and fixing it. The output image at that time may be multi-tone image data including halftones. However, in the above electrophotographic method, it is difficult to obtain halftone images. It is necessary to create an image by a method, and the conversion is necessary.

  For example, the principle of image binarization by the dither method will be described with reference to FIG. 3 for a printer having two gradations that can be reproduced by one pixel. In the dither method, an input multi-valued image (for example, an 8-bit gradation image) is divided into N × M (5 × 5 in the figure) blocks, and the gradation values of pixels in the block are the same size N × M. Compared with the threshold matrix, black is output if the pixel value is larger than the threshold, and white is output otherwise. By performing this for all pixels for each matrix size, the entire image can be binarized.

  In general, in the above-described dither method, if the dots generated in the low density region are small, the toner is difficult to adhere to the recording paper and becomes unstable, and the reproducibility and graininess in the low density region are deteriorated. Therefore, in the low density region, the dots are grown by using a threshold matrix that concentrates the dots and forms stable dots. However, depending on the size of the dot, the resolution of the original multi-gradation image data is impaired, so it is necessary to enlarge the dot at a level that does not impair the resolution. In other words, the dot size of the printer is determined in consideration of the reproducibility of the low density region and the trade-off between the granularity and the resolving power.

  Regarding dot growth from this low density region, Japanese Patent Laid-Open No. 2004-26883 discloses a technique in which the dot size is defined below the low density and the gradation is expressed by the density of the dots (hereinafter referred to as FM: Frequency-Modulation). Proposed. Further, in the low density region or higher, a technique is used in which gradation is expressed by dot size (hereinafter referred to as AM: Amplitude-Modulation). Thereby, the reproducibility and granularity of a low concentration region can be improved.

JP-A-10-145593

  However, the above prior art has the following problems. For example, in the above-described conventional electrophotographic image forming apparatus, the reproducible dot size changes depending on the usage environment (temperature / humidity) and changes with time, and the disappearance or missing of dots occurs. For this reason, even if a threshold matrix created in advance by the dither method is used, stable dots cannot be formed, and there is a problem that the reproducibility and graininess of the low density region deteriorate. Further, as shown in FIG. 4, in the FM area, the target density cannot be reproduced due to the disappearance of dots, and the density suddenly increases after switching to the AM system. Therefore, the boundary between the FM system and the AM system ( There is also a risk that a pseudo contour is generated in 401) of FIG. However, in the above prior art, a dither method for growing dots by switching between the FM method and the AM method has been proposed, but there is no mention of a correction method when the dot size changes, and the above problem is solved. It is not possible.

  The present invention has been made in view of at least one of the above-mentioned problems, and by appropriately correcting the minimum dot size in the dither method, the reproducibility of the extreme density region can be reduced while suppressing the occurrence of pseudo contours. An object is to provide a mechanism for improving graininess.

  The present invention is an image forming apparatus, wherein a dot size correction chart expressing a predetermined density with a plurality of dot sizes is formed on a sheet, and the dot size correction chart formed on the sheet by the forming means By the reading means, a determining means for determining the minimum dot size by comparing the density value of the dot size correction chart calculated by reading with the reading means and the density value of the predetermined density, and the determining means And generating means for generating a threshold matrix for executing dither processing using the determined minimum dot size.

  According to the present invention, by appropriately correcting the minimum dot size in the dither method, it is possible to improve the reproducibility and graininess of the extreme density region while suppressing the occurrence of a pseudo contour.

1 is a configuration diagram of an image forming system including an image processing apparatus according to an embodiment. FIG. 2 is a block diagram illustrating configurations of a scanner image processing unit and a printer image processing unit 110 according to an embodiment. The figure explaining the principle of dither. The figure explaining the false outline of the boundary part of FM area | region and AM area | region. 9 is a flowchart showing processing for correcting a minimum dot size of a threshold matrix according to an embodiment. 6 is a flowchart illustrating threshold value matrix generation according to an embodiment. The figure which shows the chart for gradation correction which concerns on one Embodiment. The figure explaining a mode that the gradation expression of the low density area | region which concerns on one Embodiment is grown using the number of dots. The figure which shows the chart for dot size correction | amendment which concerns on one Embodiment. The figure which shows the update of the threshold value matrix which concerns on one Embodiment. The figure which shows the update of the threshold value matrix which concerns on one Embodiment. The figure which shows the chart for dot size correction | amendment which concerns on one Embodiment. 6 is a flowchart illustrating threshold value matrix generation according to an embodiment. The figure which shows the update of the threshold value matrix which concerns on one Embodiment.

  An embodiment of the present invention is shown below. The individual embodiments described below will help to understand various concepts, such as superordinate concepts, intermediate concepts and subordinate concepts of the present invention. Further, the technical scope of the present invention is established by the claims, and is not limited by the following individual embodiments.

<First Embodiment>
<System configuration>
Below, the 1st Embodiment of this invention is described using drawing. First, the configuration of a system according to the present embodiment will be described with reference to FIG. The system includes a printer that constitutes the image forming apparatus 100. The image forming apparatus according to the present embodiment is a color image forming apparatus represented by four colors of C (cyan), M (magenta), Y (yellow), and K (black). The image forming apparatus 100 includes a CPU 101, ROM 102, RAM 103, HDD 104, display unit 105, operation unit 106, scanner unit 107, scanner image processing unit 108, network I / F 109, printer image processing unit 110, and printer unit 111. These components are connected to each other via a system bus 112 so that data can be transmitted and received. A computer 114 is connected to the image forming apparatus 100, and a monitor 113 is connected to the computer 114.

  The CPU 101 is a central processing unit that performs overall control of the entire apparatus, arithmetic processing, and the like, and executes image processing described below based on a program stored in the ROM 102. The ROM 102 is a read-only memory, and is a storage area for a system activation program, a program for controlling the scanner unit 107 and the printer unit 111, character data, character code information, and the like. A RAM 103 is a random access memory, and is used when a program and data stored in the ROM 102 are loaded and executed by the CPU 101 for each of various processes. The RAM 103 is used as a data storage area for image files received from the scanner unit 107 or the network I / F 109. The HDD 104 is composed of, for example, a hard disk or the like, and is used for storing results of processing executed by the CPU 101, programs, information files, print images, and the like. The HDD 104 is also used as a work area when the CPU 101 executes processing.

  The display unit 105 displays a screen using, for example, a liquid crystal, and is used to display a setting state of the device, processing of each part of the device such as the CPU 101, and an error state. The operation unit 106 receives an input of various commands such as a setting change and reset by the user. Information on various commands input via the operation unit 106 is stored in the RAM 103 and used when the CPU 101 executes processing.

  The scanner unit 107 functions as a reading unit, irradiates light on a document, converts the reflected light into an electrical signal using a CCD or the like equipped with an RGB color filter, and handles the document via a parallel (or serial) cable or the like. RGB image data obtained is obtained. Further, the scanner unit 107 transmits the RGB image data to the scanner image processing unit 108. The scanner image processing unit 108 is a part that performs image processing such as shading processing and image area separation processing on the image data read by the scanner unit 107.

  A network I / F 109 connects the image forming apparatus 100 to a network such as an intranet via the network I / F 109. PDL data (Page Description Language) is input to the image forming apparatus 100 from the computer 114 via the network I / F 109. PDL is a language for describing an image output to a printer and instructing the printer when printing a sentence or an image created on a computer. A typical example is PostScript from Adobe System.

  The printer image processing unit 110 is a part that performs image processing suitable for the printer on the image data subjected to image processing by the scanner image processing unit 108 or PDL data received via the network I / F 109. The printer unit 111 functions as a forming unit, and exposes, latent images, development, transfer, and fixing electrophotographic CMYK image data including four types of cyan, magenta, yellow, and black processed by the printer image processing unit 110. It is a part for forming an image on a recording medium by a process.

  The monitor 113 and the computer 114 constitute a host device of the image forming apparatus 100. That is, the computer 114 holds image data for printing in order to print an image displayed on the monitor 113 by the printer unit 111 and supplies the image data to the image forming apparatus 100 at the time of printing.

<Image processing unit>
Next, the configuration of the scanner image processing unit 108 and the printer image processing unit 110 according to the present embodiment will be described with reference to FIG. The scanner image processing unit 108 includes a luminance density conversion unit 201, a density determination unit 202, a gamma generation unit 203, a threshold matrix generation unit 204, and an order table holding unit 205. Each module unit is executed using an instruction received via the operation unit 106 as a trigger. That is, when the scan operation instruction information is input to the CPU 101 via the operation unit 106, the CPU 101 loads a program for starting the scan operation from the ROM 102 and executes it. Thereafter, the document is read by the scanner unit 107 based on the program, and each module unit is executed.

  The luminance density conversion unit 201 converts the luminance value read by the scanner unit 107 into a density value using a 1D-LUT. The 1D-LUT indicates a one-dimensional LUT (Look Up Table). The density determination unit 202 determines whether or not a density value is within a predetermined range with respect to the density value processed by the luminance density conversion unit 201, and determines the dot size. The gamma generation unit 203 generates a 1D-LUT for correcting gradation using the density value processed by the luminance density conversion unit 201 and the target value held in advance, and the generated 1D-LUT is stored in the gamma holding unit 206. Hold. The threshold matrix generation unit 204 generates a threshold matrix using the dot size determined by the density determination unit 202 and the order table held in the order table holding unit 205, and the generated threshold matrix is used as the threshold matrix holding unit 207. Hold on. The order table holding unit 205 holds an order table indicating the number and angle of dither lines and the dot growth order.

  Next, the printer image processing unit 110 will be described. The printer image processing unit 110 includes a gamma holding unit 206, a threshold matrix holding unit 207, a chart holding unit 208, a color conversion unit 209, a gamma correction unit 210, and an image forming unit 211. Each module unit is executed with an instruction from the PC received via the network I / F 109 as a trigger. That is, when instruction information for a printing operation via the network I / F 109 is input to the CPU 101, the CPU 101 loads a program for starting the printing operation from the ROM 102, and each module unit is executed based on the program.

  The gamma holding unit 206 holds a 1D-LUT that corrects the gradation generated by the gamma generation unit 203. The threshold matrix holding unit 207 holds the threshold matrix generated by the threshold matrix generating unit 204. The chart holding unit 208 holds chart data (hereinafter referred to as a gradation correction chart) used when correcting gradation, and the chart data is C, M, Y, K gradation data. Consists of signal values. In addition, a chart used for correcting the minimum dot size (hereinafter referred to as a dot size correction chart) is held, and the chart data includes a plurality of dot size patches for each of C, M, Y, and K. Consists of. The color conversion unit 209 converts the input RGB data into CMYK data based on the color conversion table. At this time, the color conversion table has, for example, 16 × 16 × 16 grid points defined by the RGB values of the input data, and CMYK values corresponding to the grid points are stored as table data. The RGB data is converted into CMYK data by performing an interpolation calculation based on the grid point data. As the interpolation method, any known interpolation method such as tetrahedral interpolation or cube interpolation can be used.

  The gamma correction unit 210 uses the 1D-LUT data held in the gamma holding unit 206 for the CMYK data processed by the color conversion unit 209 to keep the gradation characteristics of the printer unit 111 constant. This is the part where correction processing is performed. The image forming unit 211 uses the threshold matrix held in the threshold matrix holding unit 207 to convert the CMYK image data corrected by the gamma correction unit 210 into an N (integer) bit halftone image suitable for the printer. This is the part sent to the printer unit 111. A halftone dot used for gradation reproduction of a normal AM screen has an arbitrary shape such as a circle, an ellipse, a rectangle, a rectangle, and the dot size is changed depending on the density as shown in FIG. Gradation is expressed by area modulation.

  Therefore, in FIG. 8A, the density increases as 801 <802 <803, and the dot size also increases as 801 <802 <803. In this embodiment, the conventional AM screen is used in the low density region or higher, but in the low density region or lower, the dot size for forming the dither is fixed, and the gradation expression is performed by the number of dots. Therefore, as shown in FIG. 8B, the density increases as 804 <805 <806, whereas the dot size is the same as 804 = 805 = 806. After dots having a fixed size are arranged at all the growth points of the halftone dots (806), gradation is expressed by increasing the dot size in the same manner as the conventional AM screen.

<Minimum dot size correction>
Next, a processing procedure for correcting the minimum dot size of the threshold value matrix according to the present embodiment will be described with reference to FIG. The processing flow in FIG. 5 is realized by the CPU 101 loading a program for executing this processing flow from the ROM 102 to the RAM 103 and executing the program. The acquired threshold value matrix is held in the threshold value matrix holding unit 207. In addition, an instruction to the user is displayed on the UI by the display unit 105, and a user instruction is received from the operation unit 106.

  First, in step S <b> 501, the image forming unit 211 performs dither processing on the gradation correction chart held in the chart holding unit 208 using the threshold matrix held in the threshold matrix holding unit 207. The data is transferred to the printer unit 111. Thereafter, the printer unit 111 outputs a gradation correction chart. At this time, for example, as shown by reference numeral 701 in FIG. 7, the gradation correction chart is composed of patches of 64 gradations in total for each color of C, M, Y, and K, 4 columns and 16 rows. It is not limited to.

  In step S502, the CPU 101 reads the gradation correction chart using the scanner unit 107, and calculates the average luminance value by averaging the luminance values of the read patches. The luminance density conversion unit 201 converts the average luminance value of each patch into a density value using a 1D-LUT, and extracts a density value corresponding to a predetermined low-density patch (for example, 702 in FIG. 7). .

  Next, in S503, the density determination unit 202 determines whether or not the density value in the low density area extracted in S502 is within a predetermined density value range. Here, not only the density value but also the dot area ratio may be used. The halftone dot area ratio is calculated using the Murray Davis equation by comparing the halftone dot density and the solid density shown in the following equation (1).

Dot percent ^{= ((1-10 -DT) / (} 1-10 -DS)) × 100 (%) ... Equation (1)
Here, DT is a halftone dot density, and DS is a solid density.

  In the present embodiment, the density determination is performed using one density value in the low density area of the gradation correction chart. However, the density determination may be performed using a plurality of density values in the low density area, for example. In S503, if Yes, the process proceeds to S508, and if No, the process proceeds to S504.

  In step S <b> 504, the image forming unit 211 passes the chart information to the printer unit 111 without performing dither processing on the dot size correction chart held in the chart holding unit 208. The printer unit 111 outputs a dot size correction chart. At this time, as shown in FIG. 9, the dot size correction chart is composed of a total of 36 patch groups of C, M, Y, K colors, 1 row and 9 rows, and the dot size is 1-9. Composed. In this embodiment, a square dot shape is used, but the present invention is not limited to this.

  In step S <b> 505, the CPU 101 reads the dot size correction chart using the scanner unit 107, and calculates the average luminance value by averaging the luminance values of the read patches. The luminance density conversion unit 201 converts the average luminance value of each patch into a density value using a 1D-LUT, and calculates the density value of each patch. The density determination unit 202 determines whether or not the calculated density value of each patch is within a predetermined density value range, and determines the dot size of the corresponding patch as the corrected dot size. Here, when there are a plurality of patches that fall within the predetermined density value range, the CPU 101 selects the one having the smallest dot size. The reason is that the smaller the dot size, the lower the dot visibility and the less likely the dither structure is seen. Therefore, the smaller dot size is suitable. Here, not only the density value but also the dot area ratio may be used.

  In step S <b> 506, the threshold value matrix generation unit 204 creates a threshold value matrix using the minimum dot size determined in step S <b> 505 and the order table held in the order table holding unit 205. Details will be described later. In step S <b> 507, the image forming unit 211 performs dither processing on the tone correction chart using the threshold value matrix created in step S <b> 506 and outputs the tone correction chart from the printer unit 111. Thereafter, as described above in S502, the CPU 101 and the luminance density conversion unit 201 read the gradation correction chart using the scanner unit 107, calculate the density value, and proceed to S508.

  In step S508, the gamma generation unit 203 generates a 1D-LUT that corrects gradation using the density value calculated in step S502 or S507 and the target value (predetermined density value) stored in advance. In step 206, the process ends.

<Generation of threshold matrix>
Next, with reference to FIG. 6, a detailed processing procedure for generating a threshold matrix in S506 according to the present embodiment will be described. The threshold value matrix generation unit 204 performs all the processes in FIG.

  In step S <b> 601, the threshold matrix generation unit 204 reads the order table, the number of sub-matrices (sub), the threshold bit number (bit), and the number of threshold matrixes (plane) held in the order table holding unit 205. As shown in FIG. 10A, the order table is a table in which the growth order of dither dots is described, and represents the gradation reproduction of a normal AM screen. The sub-matrix is a method of reducing an apparent matrix size (pattern) by combining a plurality of matrices in which threshold values at the same position are shifted little by little to create one threshold value matrix. In the example of FIG. 10A, 1001 is a main matrix, and 1002 to 1004 are sub-matrices in which threshold values at the same position are shifted little by little. The bit number (bit) of the threshold represents the number of gradations that can be reproduced by the threshold matrix. The number (plane) of the threshold matrix is determined based on the number of gradations that can be reproduced by one dot, and is 1 for 1 bit and 16 for 4 bits. In the example of FIG. 10A, one is 1 bit.

  In step S602, the threshold value matrix generation unit 204 reads the minimum dot size (dot) determined in step S505. In the example of FIG. 10, it is 7 dots, and in the example of FIG. 11, it is 5 dots. Subsequently, in step S603, the threshold value matrix generation unit 204 determines whether the threshold value in the order table is a threshold to be updated based on the order table read in step S601 and the minimum dot size read in step S602. To do. As a determination method, the number of dots (dot_max) to be updated is calculated by the following equation (2).

dot_max = sub × dot × plane (2)
Here, dot_max indicates the number of dots to be updated, sub indicates the number of sub-matrices, dot indicates the minimum dot size, and plane indicates the number of threshold matrixes.

  Specifically, the threshold value matrix generation unit 204 determines whether each threshold value in the order table is smaller than the number of dots to be updated (dot_max). In the example of FIG. 10, the threshold value painted in gray represents the threshold value to be updated, and the number of dots to be updated (dot_max) is 28. In the example of FIG. 11, the number of dots (dot_max) to be updated is 20. If Yes, the process proceeds to S604, and if No, the process proceeds to S607.

  In step S <b> 604, the threshold value matrix generation unit 204 modifies the threshold value of the order table by the number of sub-matrices (sub). Subsequently, in S605, the threshold value matrix generation unit 204 divides (div) the threshold value of the order table by the number of sub-matrices (sub). Further, in S606, the threshold value matrix generation unit 204 calculates the updated threshold value (mtx) according to the following expression (3) with respect to the threshold value of the order table.

mtx = (mod × plane × dot) + div (Equation 3)
Here, mtx indicates an updated threshold value, mod indicates a value obtained by dividing the threshold value of the order table by the number of sub-matrices, dot indicates a minimum dot size, and div divides the threshold value of the order table by the number of sub-matrices. And plane represents the number of threshold matrixes.

  The example of FIG. 10B is the updated threshold value, and the threshold value painted in gray compared to FIG. 10A is updated because it is a threshold value to be updated. On the other hand, the threshold values other than gray are threshold values that are targets of the AM method, and thus there is no change between (a) and (b). The same applies to the example of FIG.

  In step S607, the threshold value matrix generation unit 204 normalizes the threshold value of the order table updated in step S608 based on the maximum value of the threshold value of the order table and the number of bits (bits) of the threshold value. Subsequently, in step S608, the threshold value matrix generation unit 204 holds the threshold value normalized in step S607 in the threshold value matrix holding unit 207 as a corrected threshold value matrix. Based on the minimum dot size determined for each color by the above procedure, the gradation is expressed by the number of dots that define the low density region (FM screen), and the lattice points of the halftone dot pattern (dither pattern) are arranged ( After filling, gradation is expressed on a normal AM screen.

  As described above, the image forming apparatus according to the present embodiment forms a dot size correction chart expressing a predetermined density with a plurality of dot sizes on a sheet. In addition, the image forming apparatus determines the minimum dot size by comparing the density value of the dot size correction chart calculated by reading the dot size correction chart and the density value of the predetermined density. Further, the image forming apparatus generates a threshold matrix for performing dither processing using the determined minimum dot size. The predetermined concentration is a concentration in a low concentration region (extreme concentration region). As a result, a chart composed of a plurality of dot sizes in the low density area is output, and the stability of the low density area is constantly maintained by selecting the dot size at the specified level and updating the threshold matrix. The reproducibility and graininess of the low concentration range can be improved.

<Second Embodiment>
Hereinafter, a second embodiment of the present invention will be described. In the first embodiment, a mechanism has been described in which a chart composed of a plurality of dot sizes in the low density region is output, and a threshold size matrix is updated by selecting a dot size that is a specified level. As a result, the stability of the low concentration range was constantly maintained, and the reproducibility and granularity of the low concentration range could be improved. On the other hand, in the present embodiment, this is applied to provide a mechanism for correcting (determining) the minimum dot size (in this case, white dots) in a dither that grows by switching between the AM method and the FM method even in a high density region. . That is, by combining this embodiment and the first embodiment, a mechanism for correcting the minimum dot size in the extreme density region including the low density region and the high density region can be provided. Note that the dot reproducibility in the high density region can be improved by defining the dot size in the high density region and switching between the AM method and the FM method for growth. A system configuration diagram showing a configuration example of an image processing system including an image processing apparatus according to the present embodiment is the same as the system configuration diagram shown in FIG. 1 of the first embodiment, and a description thereof will be omitted. The block diagram showing the configuration of the scanner image processing unit 108 and the printer image processing unit 110 in the present embodiment is the same as the block diagram shown in FIG. Further, the flowchart showing the process for correcting the minimum dot size of the threshold matrix according to the present embodiment is the same as the flowchart shown in FIG. 5 of the first embodiment, and pays attention to the low density region in each step. The above processing may be replaced with a high concentration region. In S504, the chart configured with a plurality of dot sizes is different in that a chart configured with a plurality of dot sizes (outline) is used in a patch having a solid background as shown in FIG. .

<Generation of threshold matrix>
With reference to FIG. 13, a detailed processing procedure for generating the threshold matrix in S506 according to the present embodiment will be described. All the processes in FIG. 13 are performed by the threshold matrix generation unit 204.

  Since S1301 is the same as S601 in the first embodiment, a description thereof will be omitted. Subsequently, in S1302, the threshold value matrix generation unit 204 reads the minimum dot size (dot) determined in S505. In the example of FIG. 14, it is 5 dots. Thereafter, in step S1303, the threshold value matrix generation unit 204 determines whether the threshold value in the order table is a threshold for updating the high density area based on the order table read in step S1301 and the minimum dot size read in step S1302. Determine whether. As a determination method, the number of dots (dot_min) to be updated in the high density region is calculated by the following equation (4).

dot_min = max−sub × dot × plane (Formula 4)
Here, dot_min indicates the number of dots to be updated, max indicates the maximum threshold value of the order table, sub indicates the number of sub-matrices, dot indicates the minimum dot size, and plane indicates the number of threshold matrixes. .

  It is determined whether each threshold value in the order table is larger than the number of dots to be updated (dot_min). In the example of FIG. 14, the threshold value painted in white represents the threshold value to be updated, and the number of dots to be updated (dot_min) is 20. If Yes, the process proceeds to S1304. If No, the process proceeds to S1307.

  In step S1304, the threshold value matrix generation unit 204 subtracts the threshold value in the order table by the number of dots to be updated (dot_min), and then performs a remainder calculation (mod) on the number of sub-matrices (sub). In step S1305, the threshold value matrix generation unit 204 subtracts the threshold value of the order table by the number of dots to be updated (dot_min) and then divides (div) by the number of sub-matrices (sub). In step S1306, the threshold value matrix generation unit 204 calculates an updated threshold value (mtx) according to the following equation (5) with respect to the threshold value of the order table.

mtx = (mod × plane × dot) + div + dot_min (5)
Here, mtx indicates an updated threshold value, mod indicates a value obtained by subtracting the threshold value in the order table by the number of dots to be updated, and then a remainder by the number of sub-matrices, and dot indicates a minimum dot size. In addition, div indicates a value obtained by subtracting the threshold of the order table by the number of dots to be updated and then divided by the number of sub-matrices, plane indicates the number of threshold matrices, and dot_min indicates the number of dots to be updated. .

  The example of FIG. 14B is the updated threshold value, and the threshold value painted in white compared to FIG. 14A is updated because it is a threshold value to be updated. On the other hand, the threshold values other than white are threshold values that are targets of the AM method, and thus there is no change between (a) and (b).

  Since S1307 is the same as S607 in the first embodiment, a description thereof will be omitted. Further, since S1308 is the same as S608 in the first embodiment, description thereof is omitted.

  As described above, according to the present embodiment, a chart composed of a plurality of dot sizes in a high density region (extreme density region) is output, and a threshold size matrix is updated by selecting a dot size that is a specified level. . Thereby, the stability of the high density region can be constantly maintained, and the gradation reproducibility of the high density region can be improved. In the present embodiment, the processing for correcting the minimum dot size of the threshold matrix has been described in dithering by switching between the AM method and the FM method in the high density region. However, by combining the first embodiment and the present embodiment, a threshold matrix is used in a dither in which dots are grown in the FM method in the low density region and the high density region and in the AM method in the medium density region. The minimum dot size may be corrected.

<Other embodiments>
The present invention supplies a program that realizes one or more functions of the above-described embodiments to a system or apparatus via a network or a storage medium, and one or more processors in a computer of the system or apparatus read and execute the program This process can be realized. It can also be realized by a circuit (for example, ASIC) that realizes one or more functions.

  100: image forming apparatus, 101: CPU, 102: ROM, 103: RAM, 104: HDD, 105: display unit, 106: operation unit, 107: scanner unit, 108: scanner image processing unit, 109: network I / F 110: Printer image processing unit 111: Printer unit 112: System bus 113: Monitor 114: Computer

Claims (14)

An image forming apparatus,
Forming means for forming a dot size correction chart expressing a predetermined density with a plurality of dot sizes on a sheet;
Reading means for reading the dot size correction chart formed on the sheet by the forming means;
A determining unit that determines the minimum dot size by comparing the density value of the dot size correction chart calculated by reading with the reading unit and the density value of the predetermined density;
An image forming apparatus comprising: a generating unit that generates a threshold matrix for performing dither processing using the minimum dot size determined by the determining unit. A tone correction chart is formed on the sheet by the forming unit, the tone correction chart is read by the reading unit to calculate a density value, and whether or not the density value is within a predetermined threshold range. A determination means for determining
2. The image according to claim 1, wherein the forming unit forms the dot size correction chart on a sheet when the determination unit determines that the density value is not within the predetermined threshold value range. Forming equipment.   The image forming apparatus according to claim 1, wherein the predetermined density is a density in an extreme density area including at least one of a low density area and a high density area.   The image forming apparatus according to claim 3, wherein the threshold value matrix is generated so that a gradation expression of the extreme density region is grown using the number of dots.   5. The threshold matrix is generated such that a gradation expression after a minimum dot of a fixed size is arranged at all lattice points of a dither pattern is grown by dot area modulation. Image forming apparatus.   6. The image forming apparatus according to claim 3, wherein the generation unit generates the threshold matrix based on a table representing a growth order of dots held in advance. The determining means includes
From the density value calculated by reading from the dot size correction chart, the dot area ratio in the extreme density region is calculated according to the following formula (A):
Dot percent ^{= ((1-10 -DT) / (} 1-10 -DS)) × 100 (%) ... Equation (A) (DT: Concentration of the halftone portion, DS: solid density)
7. The image forming apparatus according to claim 3, wherein a dot size having a halftone dot area ratio within a predetermined threshold range is determined as the minimum dot size.   The image forming apparatus according to claim 7, wherein the smallest dot size is determined as the minimum dot size among a plurality of dot sizes having a dot area ratio within the predetermined threshold range. The generating means is based on the minimum dot size determined by the determining means, according to the following formula (B):
dot_max = sub × dot × plane ... Formula (B) (dot_max: number of dots to be updated, sub: number of sub-matrix shifted from threshold matrix, dot: minimum dot size, plane: number of threshold matrix)
9. The image forming apparatus according to claim 3, wherein it is determined whether or not the threshold value is an update target. The generating means is according to the following formula (C):
mtx = (mod × plane × dot) + div (Expression (C)) (mtx: updated threshold value, mod: threshold value of the order table indicating the dot growth order, a value obtained by dividing the sub-matrix number, dot: minimum dot size Div: value obtained by dividing the threshold value of the order table by the number of sub-matrices, plane: number of threshold matrixes)
The image forming apparatus according to claim 9, wherein the updated threshold value is calculated. The generating means is based on the minimum dot size determined by the determining means, according to the following formula (D):
dot_min = max−sub × dot × plane... Formula (D) (dot_min: number of dots to be updated, max: maximum value of threshold in order table, sub: number of sub-matrix obtained by shifting threshold from threshold matrix, dot: Minimum dot size, plane: number of threshold matrixes)
9. The image forming apparatus according to claim 3, wherein it is determined whether or not the threshold value is an update target. The generating means is according to the following formula (E):
mtx = (mod × plane × dot) + div + dot_min (E) (mtx: threshold after update, mod: number of sub-matrixes after subtracting threshold of order table indicating dot growth order by number of dots to be updated , Remainder: the value obtained by subtracting the threshold of the order table by the number of dots to be updated and then dividing by the number of sub-matrixes, plane: number of threshold matrices, dot_min: target of update Number of dots)
The image forming apparatus according to claim 11, wherein the updated threshold value is calculated. An image forming apparatus control method comprising:
A forming step of forming a dot size correction chart in which a predetermined density is expressed by a plurality of dot sizes on a sheet; and
A reading step for reading the dot size correction chart formed on the sheet in the forming step;
A determining step in which a determining unit determines a minimum dot size by comparing the density value of the dot size correction chart read and calculated in the reading step with the density value of the predetermined density;
And a generation step of generating a threshold matrix for executing dither processing using the minimum dot size determined in the determination step. A program for causing a computer to execute each step in a control method of an image forming apparatus, wherein the control method includes:
A forming step of forming a dot size correction chart in which a predetermined density is expressed by a plurality of dot sizes on a sheet; and
A reading step for reading the dot size correction chart formed on the sheet in the forming step;
A determining step in which a determining unit determines a minimum dot size by comparing the density value of the dot size correction chart read and calculated in the reading step with the density value of the predetermined density;
And a generating step of generating a threshold value matrix for executing dither processing using the minimum dot size determined in the determining step.

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