JP6296521B2 - Image processing apparatus, image forming apparatus, and image processing method - Google Patents

Description

  The present invention relates to an image processing apparatus, an image forming apparatus, and an image processing method capable of maintaining the image quality of an image including a plurality of regions composed of dots of different sizes.

In an electrophotographic image forming apparatus, the density of the toner image changes due to deterioration of the photoreceptor or changes in the usage environment. Therefore, a developing bias or a grid bias (charging bias) is provided so that an appropriate solid density can be stably obtained. The image density is controlled by adjusting the image density. However, in the adjustment of the development bias and the grid bias, it is difficult to adjust the thin line and the dot thickness.
Therefore, Patent Document 1 proposes a technique for realizing a clear image quality even for fine lines and dots. The invention disclosed in Patent Document 1 detects the density of a test pattern toner image formed on a photoconductor, sets the optimum light amount of each test pattern based on the detection result, In this configuration, the image is formed with the optimum light amount of the test pattern corresponding to the image form. Thereby, in Patent Document 1, high image quality can be realized even when the photoreceptor is deteriorated.

  By the way, in recent years, using a terminal (multifunction device) installed in a convenience store, etc., it is connected to the city hall database via a communication line, the Internet, etc., and downloads confidential document data such as a resident card or seal certificate. Printing (print output). Further, when printing such a confidential document, a copy-forgery-inhibited pattern image is synthesized with the original confidential document in order to suppress forgery and falsification due to unauthorized copying (for example, Patent Document 2).

A copy-forgery-inhibited pattern image includes a latent image portion in which dots remain or a decrease in dots is small when a document (original) is copied, and a background portion in which dots disappear or a large number of dots decrease when a document (original) is copied. And two regions. Since the latent image portion and the background portion are adjusted to the same density, it is difficult to identify the latent image portion in the original, but since the latent image portion clearly appears in the copy, the copy was illegally copied. It can be easily determined that it is. For example, when a copy-forgery-inhibited pattern image (original) in which the characters “copy” are configured as a latent image portion is copied, the characters “copy” appear clearly in the copy.
As described above, the latent image portion of the copy-forgery-inhibited pattern image is composed of large dots, the background portion is composed of small dots, and the latent image portion and the background portion have different characteristics with respect to stability and reproducibility. That is, when copied, the latent image portion has a small density change and the original image is reproduced as it is, but the background portion has a large density change due to copying and the original image disappears. Therefore, a density difference is generated between the latent image portion and the background portion in the copy, and it becomes easy to distinguish characters and images constituting the latent image portion.

JP 2004-306590 A Japanese Patent No. 3918414

  However, the difference in characteristics between the latent image portion and the background portion does not occur only when the original is copied, but also occurs when an image (original) in which the copy-forgery-inhibited pattern image is combined is printed. Accordingly, the density mismatch between the latent image portion and the background portion may occur due to individual variations of the image forming apparatus, changes with time such as deterioration of each part constituting the image forming apparatus, changes in use environment, and the like. In the original, it is necessary to form the latent image portion and the background portion with the same density so that the tint block image is not conspicuous, but when there is a mismatch in density between the latent image portion and the background portion, the image quality of the original image is reduced. The problem of affecting it arises.

  The present invention has been made in view of such circumstances, and an object of the present invention is to optimally adjust the density of dots and fine lines typified by the latent image portion and background portion of a tint block image. Accordingly, it is an object of the present invention to provide an image processing apparatus, an image forming apparatus, and an image processing method capable of improving image reproducibility.

  The image processing apparatus according to the present invention is an image processing apparatus that performs processing on an image that includes a plurality of regions each composed of dots of different sizes, and that is arranged in two directions where a plurality of pixels intersect. The values assigned to the pixels included in one pixel group and the values assigned to the pixels included in the other pixel group for two sets of pixel groups arranged in the direction intersecting the two directions related to the arrangement of Unlike the above, the image processing apparatus includes a processing unit that performs halftone processing using a dither matrix pattern in which a fixed value is assigned to each pixel included in at least one of the two sets of pixel groups.

  The image processing apparatus according to the present invention is an image processing apparatus configured to process an image formed of lines having different thicknesses and arranged in two directions where a plurality of pixels intersect, and the two directions related to the arrangement of the pixels For two sets of pixel groups arranged in a direction intersecting with each other, a value assigned to a pixel included in one pixel group is different from a value assigned to a pixel included in the other pixel group. The image processing apparatus includes a processing unit that performs halftone processing using a dither matrix pattern in which a fixed value is assigned to each pixel included in at least one of the set of pixel groups.

  An image forming apparatus according to the present invention includes the above-described image processing apparatus and an image forming unit that forms an image, and the image forming unit forms an image processed by the image processing apparatus. It is characterized by that.

  An image processing method according to the present invention is an image processing method by an image processing apparatus that performs processing on an image that includes a plurality of regions each composed of dots of different sizes and in which a plurality of pixels are arranged in two intersecting directions. In the above, with respect to two sets of pixel groups arranged in a direction intersecting two directions related to the arrangement of the pixels, a value assigned to a pixel included in one pixel group and a pixel included in the other pixel group And a halftone process using a dither matrix pattern in which a constant value is assigned to each pixel included in at least one of the two sets of pixel groups. To do.

  According to the present invention, by performing halftone processing using one dither matrix pattern, it is possible to optimally adjust dots, fine lines, etc., improving image reproducibility and realizing high image quality stably. it can.

1 is a longitudinal sectional view illustrating an internal configuration example of an image forming apparatus according to an embodiment. 2 is a block diagram illustrating a configuration example of a control system of the image forming apparatus according to the present exemplary embodiment. FIG. It is a flowchart which shows the procedure of the image process by an image process part. It is a schematic diagram which shows the example of a tint block image. It is a schematic diagram which shows the example of the mask pattern used for the latent image part and background part of a tint block image. It is a schematic diagram which shows the example of the dither matrix pattern used for a halftone process. It is a schematic diagram which shows the specific example of a dither matrix pattern. It is a schematic diagram which shows the example of a 5 * 5 dither matrix pattern. It is a schematic diagram which shows the example of the mask pattern for background parts after a halftone process. It is a schematic diagram which shows the example of a 4x4 dither matrix pattern. It is a schematic diagram which shows the example of the mask pattern for latent image parts after a halftone process. It is a schematic diagram which shows the example of the mask pattern for background parts after a halftone process. FIG. 10 is a characteristic diagram showing a relational characteristic between a gradation value of a tint block image before halftone processing and an output density of the tint block image after halftone processing. It is a schematic diagram showing a mask pattern for a latent image portion having a resolution of 1200 dpi. It is a schematic diagram which shows the mask pattern for background parts whose resolution is 1200 dpi. It is a schematic diagram which shows the example of the dither matrix pattern used for the halftone process with respect to the image whose resolution is 1200 dpi. It is a schematic diagram which shows the example of the mask pattern for latent image parts after a halftone process. It is a schematic diagram which shows the example of the mask pattern for background parts after a halftone process.

  Hereinafter, an image processing apparatus, an image forming apparatus, and an image processing method according to the present invention will be described in detail with reference to the drawings illustrating embodiments thereof.

  FIG. 1 is a longitudinal sectional view showing an example of the internal configuration of the image forming apparatus according to the present embodiment. An image forming apparatus (image processing apparatus) 1 according to the present embodiment is a printer installed in a store such as a convenience store, for example, and is acquired from an information processing apparatus (not shown) such as a personal computer connected to the outside. Based on the obtained image data, it has a print function for forming multicolor and single color images on sheets such as copy paper and OHP (Over Head Projector) film. The image forming apparatus 1 also has a function of receiving image data such as characters, graphics, and photographs from an external information processing apparatus in a page description language format, and generating printable print execution data (image data).

  At the bottom of the image forming apparatus 1 of the present embodiment, a drawer-type paper feed cassette 20 having a tray for storing sheets (recording paper) used for image formation is disposed. The paper feed cassette 20 can be opened by the user pulling it toward the front side, and sheets can be replenished with the tray open. Further, a manual feed tray 26 on which a small amount of sheets is placed is provided on the right side surface of the image forming apparatus 1 in FIG. 1, and the sheets placed on the manual feed tray 26 are also taken into the image forming apparatus 1. To form an image.

An exposure unit 11 is provided above the paper feed cassette 20. In the upper part of the exposure unit 11, in order to form a multicolor image using each color of black (BK), cyan (C), magenta (M), and yellow (Y), developing units 12a, 12b, 12c, 12d, photosensitive drums 13a, 13b, 13c, 13d, cleaner units 14a, 14b, 14c, 14d, chargers 15a, 15b, 15c, 15d, and the like.
The symbols a, b, c, and d attached to each symbol are described so as to correspond to, for example, each color of black (BK), cyan (C), magenta (M), and yellow (Y). In the following, except for the case where a member corresponding to a specific color is specified and described, the members provided for each color are collected together into a developing device 12, a photosensitive drum 13, a cleaner unit 14, and a charger. It is described as 15.

  As the chargers 15a to 15d, roller-type chargers configured to come into contact with the corresponding photosensitive drums 13a to 13d are used, and the surfaces of the photosensitive drums 13a to 13d are uniformly set to a predetermined potential. To charge. A configuration using a brush-type charger or a charger-type charger may be used instead of the roller-type charger. For example, the chargers 15a to 15d respectively charge the surfaces of the corresponding photosensitive drums 13a to 13d with a negative polarity.

  The exposure unit 11 is configured by a laser scanning unit (LSU) including a laser irradiation unit that irradiates laser light and a reflection mirror, and laser light emitted from the laser irradiation unit is irradiated onto the photosensitive drums 13a to 13d. As shown, a polygon mirror and a reflecting mirror are provided. In addition to the LSU, for example, a writing head in which light emitting elements such as EL (Electro Luminescence) and LED (Light Emitting Diode) are arranged in an array can be used as the exposure unit 11. .

  The exposure unit 11 emits a laser beam based on the print execution data, irradiates the surface of the photosensitive drums 13a to 13d charged by the chargers 15a to 15d with the laser beam, and negative charges on the photosensitive drums 13a to 13d. Remove. As a result, electrostatic latent images corresponding to print execution data (image data) are formed on the photosensitive drums 13a to 13d.

  Each of the developing devices 12a to 12d contains toners (developers) of black, cyan, magenta, and yellow, and the toner stored therein is charged to a negative polarity, and this toner is charged to the photosensitive drum 13a. It supplies with respect to the electrostatic latent image formed in the surface of -13d. The negatively charged toner is adsorbed on the surface of the photoconductive drums 13a to 13d where the negative charge is removed by the laser beam, so that the developing devices 12 are respectively on the corresponding photoconductive drums 13. A toner image in which the electrostatic latent image is visualized is formed.

  Around the photosensitive drums 13a to 13d, in addition to the developing units 12a to 12d and the charging units 15a to 15d, a toner image visualized on the surface of the photosensitive drums 13a to 13d is transferred to a sheet and then the photosensitive member. Cleaner units 14a-14d for collecting and removing toner remaining on the surfaces of the drums 13a-13d are provided.

  The image forming apparatus 1 according to the present embodiment is configured to transfer toner images on the photosensitive drums 13a to 13d onto a sheet by an intermediate transfer method, and an intermediate transfer belt unit is provided above the photosensitive drums 13a to 13d. 18 is provided. The intermediate transfer belt unit 18 includes an intermediate transfer belt 17, an intermediate transfer belt driving roller 171, an intermediate transfer belt driven roller 172, intermediate transfer rollers 16a, 16b, 16c, and 16d, an intermediate transfer belt cleaning unit 19, and the like. The intermediate transfer belt unit 18 includes an intermediate transfer belt tension mechanism (not shown) that maintains the tension state of the intermediate transfer belt 17. Hereinafter, the intermediate transfer rollers 16a, 16b, 16c, and 16d are collectively referred to as an intermediate transfer roller 16.

  The intermediate transfer belt drive roller 171, the intermediate transfer belt tension mechanism, the intermediate transfer rollers 16 a to 16 d, the intermediate transfer belt driven roller 172, etc. stretch the intermediate transfer belt 17, and intermediate transfer is performed by the driving force of the intermediate transfer belt drive roller 171. The belt 17 is configured to rotate in the direction indicated by the arrow in the figure. The intermediate transfer rollers 16a to 16d are rotatably supported by an intermediate transfer roller mounting portion provided in the intermediate transfer belt tension mechanism. Further, the intermediate transfer rollers 16a to 16d are connected to a power supply unit (not shown) of the intermediate transfer belt tension mechanism, and the intermediate transfer belt 17 is brought into contact with the intermediate transfer belt 17 by a charging potential from the power supply unit. Charge to a predetermined potential (transfer bias). As a result, the toner images transferred from the photosensitive drums 13 a to 13 d are attracted to the intermediate transfer belt 17.

  The intermediate transfer belt 17 is formed in an endless shape using, for example, a film having a thickness of about 1100 μm to 1150 μm, and the surface thereof is provided in contact with each of the photosensitive drums 13 a to 13 d. A color toner image (multicolor toner image) is formed on the intermediate transfer belt 17 by sequentially superimposing and transferring the respective color toner images formed on the photosensitive drums 13a to 13d onto the intermediate transfer belt 17. Is done.

  Transfer of the toner image from the photosensitive drums 13 a to 13 d to the intermediate transfer belt 17 is performed by intermediate transfer rollers 16 a to 16 d that are in contact with the back side of the intermediate transfer belt 17. To the intermediate transfer rollers 16a to 16d, a high voltage transfer bias, that is, a high voltage having a polarity (+) opposite to the charging polarity (−) of the toner is applied to transfer the toner image. The intermediate transfer rollers 16a to 16d are rollers whose base is a metal (for example, stainless steel) shaft having a diameter of 18 to 110 mm and whose surface is covered with a conductive elastic material (for example, EPDM, foamed urethane, etc.). . The intermediate transfer rollers 16 a to 16 d can uniformly apply a high voltage to the intermediate transfer belt 17 by the conductive elastic material. In the present embodiment, a roller-shaped electrode is used as the transfer electrode, but a brush-shaped electrode can also be used.

As described above, the toner images visualized in accordance with the respective colors on the respective photosensitive drums 13a to 13d are stacked on the intermediate transfer belt 17, and an image for printing is formed on the intermediate transfer belt 17 by a multicolor toner image. It is reproduced. The multicolor toner image thus transferred onto the intermediate transfer belt 17 is transferred onto the sheet by the transfer roller 21 disposed at the contact position between the sheet and the intermediate transfer belt 17 by the rotation of the intermediate transfer belt 17. .
At this time, the intermediate transfer belt 17 and the transfer roller 21 are pressed against each other at a predetermined nip, and a voltage for transferring the multicolor toner image onto the sheet, that is, the charging polarity (−) of the toner is applied to the transfer roller 21. Is applied with a high voltage of reverse polarity (+). Here, in order to constantly obtain a nip between the intermediate transfer belt 17 and the transfer roller 21, either the transfer roller 21 or the intermediate transfer belt drive roller 171 is made of a hard material such as metal, and the other is It is made of a soft material such as elastic rubber or foamable resin.

  Further, the toner attached to the intermediate transfer belt 17 by the contact with the photosensitive drums 13a to 13d as described above, or the toner remaining on the intermediate transfer belt 17 without being transferred onto the sheet by the transfer roller 21, Since it causes toner color mixing in the next step, it is removed and collected by the intermediate transfer belt cleaning unit 19 provided in the vicinity of the intermediate transfer belt driven roller 172. The intermediate transfer belt cleaning unit 19 is provided with a cleaning blade as a cleaning member that comes into contact with the intermediate transfer belt 17, and the portion where the cleaning blade comes into contact with the intermediate transfer belt 17 starts from the back side of the intermediate transfer belt 17. It is supported by a driven roller 172.

  The sheet feeding cassette 20 and the manual feed tray 26 have pickup rollers 28a and 28b in the vicinity of the leading end portions of the stacked sheets, and the sheets separated and fed one by one by the pickup rollers 28a and 28b are as follows. The toner is supplied into the image forming apparatus 1 through the transport path S. A plurality of conveyance rollers 27 for promoting and assisting conveyance of the sheet are provided at appropriate positions on the conveyance path S, and are separately fed by the pickup rollers 28a and 28b and conveyed to the conveyance roller 27. The sheet is conveyed through the conveyance path S by the conveyance roller 27 to the registration roller 23.

  The registration roller 23 is provided below the transfer roller 21 and the intermediate transfer belt driving roller 171 described above. The registration roller 23 temporarily holds the sheet conveyed from the paper feed cassette 20 or the manual feed tray 26, and the sheet is transferred to the transfer roller 21 at a timing when the leading edge of the sheet and the leading edge of the toner image on the intermediate transfer belt 17 are aligned. The toner image on the intermediate transfer belt 17 is transferred onto the sheet by being conveyed.

The sheet onto which the toner image has been transferred is conveyed substantially vertically and reaches a fixing unit 22 provided on the upper side of the transfer roller 21. The fixing unit 22 includes a heat roller 24, a pressure roller 25, and the like, and controls the heater (not shown) based on a detection value from a temperature detector (not shown) to set the heat roller 24 to a predetermined fixing temperature. Keep on. Further, the fixing unit 22 rotates the sheet on which the multicolor toner image is transferred between the heat roller 24 and the pressure roller 25 and heat-fixes the multicolor toner image on the sheet by the heat of the heat roller 24. The multicolor toner image transferred to the sheet is melted, mixed, and pressed to be thermally fixed on the sheet.
The sheet on which the multicolor toner image is thermally fixed is discharged by a conveyance roller provided near the exit of the fixing unit 22.

  In the case of a single-side printing request, the sheet that has passed through the fixing unit 22 is discharged face-down onto a discharge tray 30 provided on the upper surface of the image forming apparatus 1 through a discharge roller 29. In the case of double-sided printing request, the sheet that has passed through the fixing unit 22 is once chucked by the paper discharge roller 29, and then guided to the double-sided document transport path S 1 by the reverse rotation of the paper discharge roller 29. Is again conveyed to the registration roller 23. Then, after the toner image is transferred and thermally fixed on the back side, the sheet is discharged onto the discharge tray 30 by the discharge roller 29.

  In addition to the configuration described above, the image forming apparatus 1 includes, for example, a paper feed cassette that can store a plurality of types of sheets having different sizes, a large capacity paper feed cassette that can store thousands of sheets, and a plurality of paper discharge trays. In addition, it is possible to provide a transport mechanism for transporting the image-formed sheet to each paper discharge tray, and it is also possible to adopt a configuration in which these can be retrofitted as an optional function.

  FIG. 2 is a block diagram illustrating a configuration example of a control system of the image forming apparatus 1 according to the present embodiment. An image forming apparatus 1 according to the present embodiment includes a CPU (Central Processing Unit) 2, a RAM (Random Access Memory) 3, an HDD (Hard Disk Drive) 4, an operation panel 5, a network controller 6, an image processing unit 7, and an image forming unit. 8 etc. The units included in the image forming apparatus 1 are connected to each other via a bus.

The CPU 2 controls the operation of the above-described hardware units connected via the bus, and executes various software functions according to a control program stored in the HDD 4.
The HDD 4 is a large-capacity memory device, and stores in advance a control program for the CPU 2 to control each hardware unit as described above. The RAM 3 is a volatile semiconductor memory, and temporarily stores data generated during execution of the control program by the CPU 2. The CPU 2 reads the control program stored in the HDD 4 into the RAM 2 and sequentially executes it, thereby causing the image forming apparatus 1 to operate as the image processing apparatus and the image forming apparatus of the present invention.

Further, the HDD 4 receives a print job received from an external device via the network controller 6, print execution data obtained by developing the print job (data to be printed), and predetermined image processing performed by the image processing unit 7. Store data etc. Data (data to be printed) stored in the HDD 4 is read to the image processing unit 7 or the image forming unit 8 at a timing instructed by the CPU 2.
Further, the HDD 4 stores various data used for image processing by the image processing unit 7, characteristic data of the image forming unit 8 (information such as density, gradation, and dot reproduction), pre-generated tint block data, and data by the image processing unit 7. Stores the dither matrix pattern used for halftone processing.

The operation panel 5 includes an operation unit having various operation buttons for receiving operation instructions from the user, and a display unit such as an LED display or a liquid crystal display that displays information to be notified to the user. . In addition, you may comprise the operation panel 5 with the touchscreen which can input by touching a display screen.
The network controller 6 is a communication interface for communicating with an information processing apparatus such as an external personal computer via a communication line, and transmits / receives data to / from the external information processing apparatus. The external information processing apparatus transmits page description language format data (print job), and the network controller 6 receives the print job transmitted from the external information processing apparatus. The network controller 6 transfers the received print job to the HDD 4 or the image processing unit 7. The image processing unit 7 develops the transferred print job to generate print execution data, and transfers the generated print execution data to the HDD 4. The HDD 4 stores the print job transferred from the network controller 6 and the print execution data transferred from the image processing unit 7 in units of pages, for example.

The image processing unit 7 performs a process of expanding the print job received by the network controller 6 and generating print execution data. Also, the color conversion process, the gradation correction process, the halftone process, and the like are performed on the print execution data. Various image processes are executed. In particular, the image processing unit 7 of the present embodiment performs synthesis processing for adding copy-forgery-inhibited pattern data to print execution data, and halftone processing using a dither matrix pattern. Details of processing performed by the image processing unit 7 will be described later. The image processing unit 7 outputs the image-processed data (image data) to the HDD 4.
The image processing unit 7 may be a software function realized by the CPU 2 executing a control program for executing predetermined image processing, or may be realized by a dedicated hardware circuit.

The image forming unit 8 is an image forming unit that includes the units illustrated in FIG. 1 and forms an image based on data to be printed on a sheet. The image forming unit 8 of the present embodiment is an electrophotographic printer, but an ink jet printer or the like may be used.
In addition to the configuration described above, the image forming apparatus 1 includes a density sensor, a temperature / humidity sensor, and the like as information acquisition means used for color calibration performed to improve color reproducibility by the image forming unit 8.

  The image forming apparatus 1 having the above-described configuration generates print execution data by the image processing unit 7 from a print job received from an external information processing apparatus via the network controller 6, and displays an image based on the generated print execution data. It forms on a sheet | seat by the formation part 8. FIG. The image forming apparatus 1 according to the present embodiment adds (combines) copy-forgery-inhibited pattern data to print execution data when performing image output based on the print execution data. In this case, since the copy-forgery-inhibited pattern image based on the copy-forgery-inhibited pattern data is included in the image formed on the sheet based on the print execution data, it is possible to generate a printed matter that has been subjected to forgery suppression processing. If the data to be printed is confidential document data, illegal copying can be suppressed by forming a copy-forgery-inhibited pattern image on the printed matter, which is effective.

  Below, the process which the image process part 7 performs is demonstrated. FIG. 3 is a flowchart showing a procedure of image processing by the image processing unit 7. Here, various image processes performed on print execution data obtained by the image processing unit 7 developing a print job received via the network controller 6 will be described. The print job is, for example, print processing of confidential document data such as a resident card or a seal certificate stored in a city hall database (server device). Further, the network controller 6 of the image forming apparatus 1 connects to the city hall database via a communication line or the Internet, and acquires a print job of confidential document data. The image processing unit 7 of the image forming apparatus 1 receives print execution data (confidential document data) such as RGB (R: red, G: green, B: blue) data or BK (monochrome) data based on the print job. Generate.

The image processing unit 7 combines the copy-forgery-inhibited pattern data generated in advance and stored in the HDD 4 with the print execution data (S1). The copy-forgery-inhibited pattern data is also RGB data or BK data, and the RGB data copy-forgery-inhibited pattern data is combined with the RGB data print execution data, and the BK-data copy-forgery-inhibited data is combined with the print execution data for the BK data. . The tint block data is synthesized by adding or replacing pixel values of the pixels.
Details of the tint block data and the tint block image based on the tint block data will be described later. The copy-forgery-inhibited pattern data may be generated every time confidential document data is output (for each confidential document data) or may be generated in advance, but the generated copy-forgery-inhibited pattern data is stored in the HDD 4 and has a predetermined timing. And is input to the image processing unit 7.

The image processing unit 7 determines whether or not the print execution data to be processed is RGB data (S2). If the print execution data is RGB data (S2: YES), the print processing data synthesized with the copy-forgery-inhibited pattern data is processed. Then, color conversion processing is performed (S3), and CMYK (C: cyan, M: magenta, Y: yellow, K: black) data is generated. In the color conversion process, RGB data is converted into a complementary color CMY color space to generate CMY data, K data is generated from the CMY data, and new CMY data is subtracted from the original CMY data. Is a process for generating
On the other hand, when the print execution data to be processed is not RGB data (S2: NO), that is, when the print execution data is BK data, the image processing unit 7 skips the process of step S3 and performs the color conversion process. Absent. The color conversion process is performed based on the color table of the ICC profile, for example, by a color management module mounted on firmware (not shown) included in the image forming apparatus 1.

  The image processing unit 7 performs gradation correction processing on the print execution data combined with the copy-forgery-inhibited pattern data (S4). As a result, the spatial frequency characteristics of the print execution data (image data) are corrected, and blurring or graininess deterioration of the output image can be prevented. In addition, the background pattern data (background pattern data gradation value) synthesized with the print execution data by the gradation correction processing has the following halftones in which the density of the latent image portion and the background portion of the background pattern image based on the background pattern data are the following halftones. The gradation value (input gradation value) is changed to be the same after processing. If the print execution data is RGB data, the image processing unit 7 performs gradation correction processing for each color component.

  Next, the image processing unit (processing unit) 7 performs halftone processing using a dither matrix pattern on the print execution data subjected to tone correction processing (S5). Details of the halftone process will be described later. The halftone process is also performed for each color component if the print execution data is RGB data.

  The image processing unit 7 temporarily stores the print execution data subjected to the halftone processing in the HDD 4 or the RAM 3, reads it at a predetermined timing for image formation, outputs it to the image forming unit 8, and forms an image in the image forming unit 8. (S6). In this way, by forming an image after synthesizing the copy-forgery-inhibited pattern data, a confidential document can be generated with a printed matter on which forgery suppression processing is performed. Further, the density of the latent image portion and the density of the background portion of the copy-forgery-inhibited pattern image formed on the sheet are equalized by the above-described tone correction processing and halftone processing. Thereby, the copy-forgery-inhibited pattern image does not interfere with the visual recognition of the confidential document as the foreground.

Next, a background pattern data and a background pattern image based on the background pattern data will be described.
FIG. 4 is a schematic diagram showing an example of a tint block image. The copy-forgery-inhibited pattern image has a latent image portion that becomes visible (visualized) when copied, and a background portion other than the latent image portion. In the copy-forgery-inhibited pattern image shown in FIG. 4, the character portion of “copy” is a latent image portion, and the other region is a background portion.

FIG. 5 is a schematic diagram showing an example of a mask pattern used for a latent image portion and a background portion of a tint block image. FIG. 5A shows a mask pattern used for a latent image portion, and FIG. 5B shows a mask pattern used for a background portion. Show. In FIG. 5, each square shown by a broken line represents a pixel. In the figure, the right direction is the main scanning direction of the image, and the downward direction is the sub-scanning direction of the image.
As shown in FIG. 5A, the latent image portion in the copy-forgery-inhibited pattern image is composed of a mask pattern in which each dot is large (coarse), and the background portion is composed of a mask pattern in which each dot is small (fine) as shown in FIG. 5B. Composed. The mask pattern for the latent image portion shown in FIG. 5A has 16 pixels of 4 × 4 pixels as one dot for each block of 18 × 18 (sub scanning direction × main scanning direction) pixels, and the upper left corner of each dot is In this pattern, two dots are arranged in an oblique direction so as to coincide with the upper left corner and the central portion of each block. In other words, the mask pattern for the latent image portion is composed of dots having a large number of collective pixels (the number of pixels constituting the dots).

  The mask pattern for the background shown in FIG. 5B arranges each dot evenly with two pixels of 2 × 1 pixels as one dot, and equally among 32 dots arranged in 24 × 24 pixels. This is a pattern in which four arranged dots are added by one pixel each to form three pixels. That is, the mask pattern for the background portion is composed of dots with a small number of collective pixels. In addition, each dot constituting the mask pattern for the background portion is arranged 5 pixels apart in the left-right direction (main scanning direction) in FIG. 5B, and the row of dots adjacent in the up-down direction (sub-scanning direction) is The pixels are arranged one pixel apart in the vertical direction and are shifted by two pixels in the horizontal direction. In addition, in 4 dots composed of 3 pixels, each pixel arranged in the vicinity has one pixel added at a different position with respect to 2 pixels of 2 × 1 pixels. In the example shown in FIG. 5B, in 4 dots in the upper left 24 × 24 pixels, the upper left dot and the lower right dot are 2 pixels of 2 × 1 pixels, and one pixel is added on the right side of the upper pixel. In the upper right dot and lower left dot, one pixel is added to the right side of the lower pixel with respect to two pixels of 2 × 1 pixels.

The mask pattern shown in FIG. 5 is merely an example. For example, the latent image portion only needs to be composed of dots of a predetermined number j (natural number) or more, and the background portion has a predetermined number i (natural number). , I <j) as long as it is composed of dots made up of pixels. Note that i is, for example, 3, which is the number of pixels having a size that disappears when copied. Further, j is, for example, 6, which is the number of pixels having a size that does not disappear even when copied. In this way, by providing (ji) gaps between the number of pixels constituting the latent image portion and the number of pixels constituting the background portion, each dot can be clearly distinguished.
Specifically, the dots used in the mask pattern for the background portion may be composed of, for example, three or less pixels. When a dot is formed by three pixels, two pixels adjacent in the main scanning direction (or sub-scanning direction) and one of the two pixels in the sub-scanning direction (or main scanning direction). A dot is composed of three pixels, one adjacent pixel. In addition, in each dot located in the vicinity, another pixel is arranged at a different position with respect to two adjacent pixels. As described above, by changing the position of the pixel to be added in each neighboring dot, the output density with respect to the input gradation value can be smoothly changed.

  In the copy-forgery-inhibited pattern image, the amount of change in the output density with respect to the gradation value (input gradation value) is slightly different between a dot having a collection pixel number of 2 pixels and a dot having a collection pixel number of 3 pixels. Therefore, the amount of change in the output density with respect to the gradation value can be finely adjusted by changing the mixing ratio of the dot having the collective pixel number of 2 pixels and the dot having the collective pixel number of 3 pixels. It should be noted that dots having an odd number of collective pixels (for example, three pixels) need to be arranged so that the amount of change in output density is not biased when a checkered dither matrix pattern is applied in halftone processing described later. There is. For example, for 4 dots that are arranged in 24 × 24 pixels and the number of collective pixels is 3 pixels, half of the 2 dots are located at positions where 2 of the 3 pixels have a small pixel value by halftone processing. The remaining two dots are arranged at positions where only one of the three pixels has a small pixel value by halftone processing.

  Here, a description will be given of a mechanism in which the latent image portion becomes visible (visualized) when copied. When a copy-forgery-inhibited pattern image having a latent image portion composed of large dots as shown in FIG. 5A and a background portion composed of small dots as shown in FIG. 5B is copied, the background portion composed of small dots is copied during copying. When the density is lowered by various processes, copying is stopped. As a result, the dots in the background portion disappear and the latent image portion appears clearly. When a copy-forgery-inhibited pattern image in which the characters “copy” are configured with small dots and the other background area is configured with large dots, the characters configured with small dots are not copied. As a result, the dot of the character composed of small dots disappears, and the “copy” character becomes apparent in a white state.

  The image processing unit 7 synthesizes the bitmap data of the characters used in the latent image portion (“copy” in FIG. 4), the mask pattern for the latent image portion, and the mask pattern for the background portion, as described above. A tint block data for forming a tint block image is generated. Specifically, the image processing unit 7 arranges characters to be used for the latent image unit at predetermined one or a plurality of positions with respect to an area of a size of one original when the image forming unit 8 outputs an image. The latent image portion mask pattern is assigned to each latent image portion, and the background portion mask pattern is assigned to the background portion. As a result, copy-forgery-inhibited pattern data for forming a copy-forgery-inhibited pattern image as shown in FIG. 4 is generated using the mask pattern shown in FIG. The bitmap data, the latent image portion mask pattern, and the background portion mask pattern are stored in advance in the HDD 4.

  As described above, the copy-forgery-inhibited pattern data includes a latent image portion mask pattern made up of small dots and a background portion mask pattern made up of large dots. However, since each pattern is designed to be formed with the same density when it is output, there is almost no difference between the patterns in the appearance of the background pattern, and the image data (foreground) ( (Confidential document data) is not affected.

  Next, halftone processing by the image processing unit 7 will be described. FIG. 6 is a schematic diagram illustrating an example of a dither matrix pattern used for halftone processing, and FIG. 7 is a schematic diagram illustrating a specific example of a dither matrix pattern. The dither matrix pattern shown in FIGS. 6 and 7 has a size of 4 × 4. The dither matrix pattern is a series of patterns that change the print output (output gradation value) according to the gradation value (input gradation value) of the image to be subjected to halftone processing. In the dither matrix pattern of the present embodiment, a value within a predetermined range is assigned to each of the pixels 1a and 1b to 1h, and a value within a predetermined range is assigned to each of the pixels 2a and 2b to 2h. ing. The halftone process is a process of comparing each pixel value of the input image data (print execution data) with each value of the dither matrix pattern and determining the pixel value of each pixel.

The dither matrix pattern of the present embodiment has 16 points (0, 17, 34, 51, 68, 85, 102, 119, 136, 153) that are evenly allocated among 8-bit values (0 to 255). 170, 187, 204, 221, 238, 255). Therefore, image data in which the pixel value of each pixel is controlled to one of 16 values is obtained by halftone processing using such a dither matrix pattern.
More specifically, for example, the dither matrix pattern shown in FIG. 7A is a pattern used for an image with an input gradation value of 64. The pixels 1a to 1d have 136, the pixels 1e to 1h have 119, and the pixel 2a. 0 is assigned to ~ 2h. The dither matrix pattern shown in FIG. 7B is a pattern used for an image with an input gradation value of 128, and 255 is assigned to pixels 1a to 1h and 0 is assigned to pixels 2a to 2h. Furthermore, the dither matrix pattern shown in FIG. 7C is a pattern used for an image having an input gradation value of 191. The pixels 1a to 1h have 255, the pixels 2a to 2d have 136, and the pixels 2e to 2h have 119. Each is assigned. Thus, in the dither matrix pattern of the present embodiment, the same value or difference of 17 is assigned to the pixels 1a to 1h, and the same value or difference of 17 is also assigned to the pixels 2a to 2h. For convenience of explanation, such a pattern is referred to as a checkered dither matrix pattern. The pixels 1a to 1h and the pixels 2a to 2h are pixels arranged in a direction intersecting the main scanning direction and the sub-scanning direction, that is, an oblique direction, and 1 in each of the main scanning direction and the sub-scanning direction. This is a pixel arranged every other pixel.

  In the present embodiment, a 4 × 4 dither matrix pattern will be described as an example. However, if each value constituting the pattern is a dither matrix pattern having a halftone density and a checkered pattern, the pattern is limited to a 4 × 4 dither matrix pattern. Not. For example, when a 1 × 2 dither matrix pattern is used, halftone processing may be performed by shifting one pixel at a time in the sub-scanning direction (vertical direction) of the document, or when a 2 × 1 dither matrix pattern is used. Alternatively, halftone processing may be performed by shifting one pixel at a time in the main scanning direction (horizontal direction) of the document.

There is no problem if the dither matrix pattern is an even size such as 2 × 2, 4 × 4, 6 × 6. However, when using an odd-size dither matrix pattern such as 3 × 3, 5 × 5..., A copy-forgery-inhibited pattern image pattern (latent image portion mask pattern and background portion mask pattern) and a dither matrix pattern are used. May interfere. Therefore, when using an odd-size dither matrix pattern, it is necessary to use a dither matrix pattern that does not interfere with the pattern of the tint block image.
FIG. 8 is a schematic diagram illustrating an example of a 5 × 5 dither matrix pattern, and FIG. 9 is a schematic diagram illustrating an example of a mask pattern for a background portion after halftone processing. FIG. 9 shows a state after the dither matrix pattern shown in FIG. 8 is applied to the mask pattern for the background portion shown in FIG. 5B. Note that the dither matrix pattern shown in FIG. 8 is also configured so that each value constituting the pattern has a checkered pattern. As shown in FIG. 9, when processing based on a 5 × 5 dither matrix pattern is performed, the dots may not be evenly arranged. In FIG. 9, pixels that are lighter than black (gray) are pixels whose density has been reduced by halftone processing using a dither matrix pattern. Therefore, it is necessary to use a dither matrix pattern in which dots after halftone processing are evenly arranged without interfering with the pattern of the tint block image.

  Further, the dither matrix pattern is not limited to a pattern having an even pixel × even pixel size or a pattern having an odd pixel × odd pixel size that does not interfere with the tint block image pattern. In addition, the dither matrix pattern may be a pattern of an odd pixel × even pixel size or a pattern of an even pixel × odd pixel size. A pattern that does not interfere with the pattern of images having different areas may be used.

10 is a schematic diagram showing an example of a 4 × 4 dither matrix pattern, FIG. 11 is a schematic diagram showing an example of a mask pattern for a latent image portion after halftone processing, and FIG. 12 is for a background portion after halftone processing. It is a schematic diagram which shows the example of this mask pattern. 11 shows a state after the dither matrix pattern shown in FIG. 10 is applied to the mask pattern for the latent image portion shown in FIG. 5A, and FIG. 12 shows the mask for the background portion shown in FIG. 5B. The state after applying the dither matrix pattern shown in FIG. 10 to the pattern is shown.
The dither matrix pattern shown in FIG. 10 is a pattern used for an image with an input gradation value of 196. 255 is assigned to the pixels 1a to 1h, and 136 is assigned to the pixels 2a to 2h. When such a dither matrix pattern is applied to a pixel having an input gradation value of 196, halftone processing is performed in which each pixel is corrected to a gradation value of 136 or 255.

  When the dither matrix pattern as shown in FIG. 10 is applied to the copy-forgery-inhibited pattern image, as shown in FIG. 11, the pixel values of the pixels constituting each dot (large dot) in the latent image portion are in a checkered pattern. descend. However, since there is a pixel having the maximum pixel value (255) around the pixel whose pixel value has decreased, the apparent density of the pixel whose pixel value has decreased does not decrease. On the other hand, as shown in FIG. 12, each dot (small dot) in the background portion has a lower pixel value because there is only one pixel with the maximum pixel value (255) around the pixel with the lower pixel value. The apparent density of the pixel decreases.

The present invention utilizes the difference in the output density characteristics of the latent image portion and the background portion, and performs halftone processing using a checkered dither matrix pattern on the copy-forgery-inhibited pattern image. The density of the parts is adjusted simultaneously and individually.
FIG. 13 is a characteristic diagram showing the relationship between the tone value of the tint block image before halftone processing and the output density of the tint block image after halftone processing. The horizontal axis in FIG. 13 indicates the tone value (input tone value) of the tint block image before halftone processing, the vertical axis indicates the output density of the tint block image after halftone processing, and the solid line indicates the latent image of the tint block image. The relationship characteristic in the image portion is shown, and the broken line shows the relationship characteristic in the background portion of the tint block image. As shown in FIG. 13, the latent image portion has a small change in output density with respect to a change in input tone value, whereas the background portion has a change in output density with respect to a change in input tone value. large. Therefore, by processing using one dither matrix pattern, it is possible to simultaneously adjust the density of the latent image portion and the background portion of the tint block image. According to the characteristics shown in FIG. 13, when the halftone process using the dither matrix pattern shown in FIG. 10 is performed on the latent image portion and the background portion having the gradation value 196, the latent image portion and the background portion. It can be seen that the concentrations of are adjusted to be the same.

  Therefore, the image forming apparatus 1 according to the present embodiment performs, for example, the gradation correction processing by the image processing unit 7 so that the gradation value of the copy-forgery-inhibited pattern image (latent image portion and background portion) becomes 196. Halftone processing using a dither matrix pattern as shown is performed. Thereby, a tint block image in which the densities of the latent image portion and the background portion are adjusted to be the same can be formed on the sheet.

The above-described FIGS. 5 to 7 and FIGS. 10 to 12 describe the dither matrix pattern, the latent image portion mask pattern, and the background portion mask pattern used for an image having a resolution of 600 dpi. The present invention can also be applied to an image having a resolution of 1200 dpi.
For the background pattern image (background pattern data) having the mask pattern for the latent image portion shown in FIG. 5A and the mask pattern for the background portion shown in FIG. 5B and having a resolution of 600 dpi, FIG. It is suitable to use the checkered dither matrix pattern shown in FIG. On the other hand, for a background pattern image (background pattern data) having the mask pattern for the latent image portion shown in FIG. 14 and the mask pattern for the background portion shown in FIG. 15 and having a resolution of 1200 dpi, the dither matrix shown in FIG. It is better to use a pattern.

  14 is a schematic diagram showing a mask pattern for a latent image portion having a resolution of 1200 dpi, FIG. 15 is a schematic diagram showing a mask pattern for a background portion having a resolution of 1200 dpi, and FIG. 16 is a halftone process for an image having a resolution of 1200 dpi. It is a schematic diagram which shows the example of the dither matrix pattern used. The mask pattern for the latent image portion shown in FIG. 14 has 64 pixels of 8 × 8 pixels as one dot for each block of 36 × 36 pixels, and the upper left corner of each dot is located at the upper left corner and the central portion of each block. This is a pattern in which two dots are arranged in an oblique direction so as to match. Further, the mask pattern for the background portion shown in FIG. 15 is a pattern in which each dot is evenly arranged with 8 pixels of 4 × 2 pixels as one dot. In addition, each dot constituting the mask pattern for the background portion is arranged 10 pixels apart in the left-right direction (main scanning direction) in FIG. 15, and the row of dots adjacent in the up-down direction (sub-scanning direction) is In addition, the pixels are arranged apart from each other by 2 pixels in the vertical direction and are shifted by 4 pixels in the horizontal direction.

  In the dither matrix pattern shown in FIG. 16, a value within a predetermined range is assigned to each of the four pixels 1a and 1b to 1h, and a predetermined range is assigned to each of the four pixels 2a and 2b to 2h. The value in is assigned. Since the resolution of 1200 dpi is twice the resolution of 600 dpi, the dither matrix pattern shown in FIG. 16 has four times the number of pixels (1a to 1h, 2a to 2h) corresponding to each value in the dither matrix pattern shown in FIG. has been changed. In this way, by performing halftone processing using a dither matrix pattern in which the number of pixels corresponding to each value constituting the dither matrix pattern is changed according to the resolution of the image, even in an image with different resolutions, The density of each region composed of dots of different sizes can be adjusted simultaneously, and the density of dots, fine lines, etc. can be adjusted optimally. Similarly, for an image having a resolution of 2400 dpi, a dither matrix pattern in which the number of pixels (1a to 1h, 2a to 2h) corresponding to each value in the dither matrix pattern shown in FIG. That's fine.

The mask patterns shown in FIGS. 14 and 15 are merely examples. For example, the latent image portion has a predetermined number of pixels (four in the dither matrix pattern in FIG. 16) corresponding to each value constituting the dither matrix pattern. The background portion only needs to be composed of dots composed of pixels that are a predetermined number i (natural number, i <j) times or less. Note that i is 3, for example, and the background portion mask pattern is constituted by dots formed by 12 or less pixels. Further, j is 6, for example, and a latent image portion mask pattern is constituted by dots composed of 24 or more pixels.
The background mask pattern shown in FIG. 15 is composed of dots composed of twice as many pixels (that is, 8 pixels) as the pixels corresponding to the respective values constituting the dither matrix pattern. For example, when a dot is formed by three times as many pixels (that is, 12 pixels), the number of pixels (here, 2 × 2 four pixels) corresponding to each value forming the dither matrix pattern is set as one set of pixel groups, Two sets of pixel groups adjacent in the main scanning direction (or sub-scanning direction), and one set of pixel groups adjacent in the sub-scanning direction (or main scanning direction) with respect to either one of the two sets of pixel groups; The dot can be configured with. In addition, for each dot located in the vicinity, another set of pixel groups is arranged at different positions with respect to two adjacent sets of pixels. In this way, by changing the position of the pixel group to be added in each neighboring dot, the output density with respect to the input gradation value can be changed smoothly.

FIG. 17 is a schematic diagram illustrating an example of a mask pattern for a latent image portion after halftone processing, and FIG. 18 is a schematic diagram illustrating an example of a mask pattern for a background portion after halftone processing. 17 shows a state after the dither matrix pattern shown in FIG. 16 is applied to the latent image portion mask pattern shown in FIG. 14, and FIG. 18 shows the background portion mask shown in FIG. The state after applying the dither matrix pattern shown in FIG. 16 to the pattern is shown.
As described above, even if the background pattern image has a resolution of 1200 dpi, by using the difference in the output density characteristics of the latent image portion and the background portion and applying a checkered dither matrix pattern to the background pattern image, the latent image portion and The background density can be adjusted simultaneously and individually. Note that not only a process using a dither matrix pattern as shown in FIG. 16 but also a process using a dither matrix pattern shown in FIG. 16 and a dither matrix pattern shown in FIG. 6 are used for an image having a resolution of 1200 dpi. The processing may be switched as appropriate, or may be performed in combination as appropriate.

  In the embodiment described above, it is possible to generate a printed matter that has been subjected to forgery suppression processing by combining the copy-forgery-inhibited pattern data when printing confidential document data. Further, by performing halftone processing using a checkered dither matrix pattern, the density of the latent image portion and the background portion of the tint block image can be adjusted simultaneously, and can easily be adjusted to the same density. . In the copy-forgery-inhibited pattern data synthesized with the confidential document data, the density of the latent image portion and the density of the background portion are controlled to be the same, so the copy-forgery-inhibited pattern image is difficult to recognize in the printed matter (original) when the confidential document data is printed. , It does not hinder the visibility of confidential documents as the foreground.

  In the above-described embodiment, a configuration has been described in which a copy-forgery-inhibited pattern image having characters such as “copy” as a latent image portion is combined with a confidential document. In addition, a copy-forgery-inhibited pattern image may be formed with a character such as “copy” as a background portion (an area composed of small dots). It manifests itself in a missing state.

  In the above-described embodiment, the configuration has been described in which the density of the latent image portion and the background portion of the copy-forgery-inhibited pattern image is adjusted simultaneously and individually by performing halftone processing based on the checkered dither matrix pattern for the copy-forgery-inhibited pattern image. . In addition to the copy-forgery-inhibited pattern image, an input tone value in each area can be obtained by performing halftone processing based on a checkered dither matrix pattern for an image including a plurality of areas composed of dots of different sizes. Since the density of each region can be appropriately adjusted using the difference in the relationship between the output density and the output density, an appropriate image quality can be realized. Specifically, halftone processing based on a checkered dither matrix pattern is performed on an image having a plurality of areas composed of dots of different sizes and an image having lines of different thicknesses (thin lines and thick lines). If done, the dot diameter and line width can also be adjusted. As with the background portion (small dots) of the above-mentioned tint block image, the thin line greatly varies in line width due to changes over time or environmental changes, whereas the thick line indicates the latent image portion (large dot) of the above-mentioned tint block image. Similarly, the line width does not fluctuate very much due to changes over time and environmental changes. Therefore, by performing halftone processing based on a checkered dither matrix pattern for the lines, individual variations of the image forming apparatus, changes with time such as deterioration of each part constituting the image forming apparatus, changes in use environment, etc. Even when a dot occurs, the dot diameter and line width can be adjusted appropriately, so that the density of the latent image portion and background portion of the tint block image composed of dots and fine lines can be adjusted appropriately, and high image quality can be maintained.

Further, as the thin line, for example, in the number of pixels corresponding to each value constituting the dither matrix pattern, the width in the main scanning direction (or sub-scanning direction) (one pixel in the dither matrix pattern in FIG. 6), as shown in FIG. Examples of the dither matrix pattern include a line having a width equal to or less than a (natural number) times (for two pixels). Note that a is 2, for example. In addition, as a thick line, for example, in the number of pixels corresponding to each value constituting the dither matrix pattern, the width in the main scanning direction (or the sub scanning direction) (one pixel in the dither matrix pattern in FIG. 6, A line having a width of b (natural number, b> a) times or more of 2 pixels in the dither matrix pattern. Note that b is 4, for example.
Even in an image including such a thin line and a thick line, by performing halftone processing based on the checkered dither matrix pattern as described above, the width of each line can be adjusted appropriately, and an appropriate image quality can be obtained. realizable.

  In the above-described embodiment, an example in which the image forming apparatus of the present invention is applied to a printer has been described as an example. However, not only a print function but also a scanner function for reading an image of a document, and a number of images on a sheet based on read image data. The present invention can be applied to various image forming apparatuses having a copying function for forming color and single color images. The image processing apparatus of the present invention can also be applied to a PC (Personal Computer) that does not include an image forming unit. Further, the image forming apparatus 1 of the above-described embodiment is an example of a configuration that forms an intermediate transfer type color image. For example, a toner image formed on a photosensitive drum corresponding to each color arranged in a tandem method is used. The present invention can also be applied to an image forming apparatus configured to sequentially transfer onto a conveyed sheet.

  The preferred embodiments of the present invention have been specifically described above. However, each configuration, operation, and the like can be changed as appropriate, and are not limited to the above-described embodiments.

1 Image forming apparatus 2 CPU
7 Image processing unit (processing unit)
8 Image forming part

Claims (13)

In an image processing apparatus that includes a plurality of regions each composed of dots of different sizes and that processes an image that is arranged in two directions where a plurality of pixels intersect,
Regarding two sets of pixel groups arranged in a direction intersecting two directions related to the arrangement of the pixels, a value assigned to a pixel included in one pixel group and a pixel allocated to the other pixel group And a processing unit that performs halftone processing using a dither matrix pattern in which a fixed value is assigned to each pixel included in at least one of the two sets of pixel groups. Image processing device.   The processing unit performs halftone processing using a dither matrix pattern in which the number of pixels corresponding to each value constituting the dither matrix pattern is changed according to the resolution of the image. The image processing apparatus according to claim 1. The region composed of small dots of the plurality of regions is a background portion that does not become visible when copied,
The image processing apparatus according to claim 1, wherein the area composed of large dots is a latent image portion that is visualized when copied.   The image processing apparatus according to claim 3, wherein the dither matrix pattern is configured with values at which the density of the background portion and the latent image portion are equal by the halftone processing. A small dot constituting one of the plurality of regions is composed of pixels not more than i (natural number) times the number of pixels corresponding to each value constituting the dither matrix pattern,
The large dot is composed of pixels that are j (natural number, j> i) times or more of the number of pixels corresponding to each value constituting the dither matrix pattern. The image processing apparatus according to one.   The image processing apparatus according to claim 5, wherein the small dots are composed of pixels that are not more than three times the number of pixels corresponding to each value constituting the dither matrix pattern. The small dot is a pixel group of a number corresponding to each value constituting the dither matrix pattern, and any one of the two adjacent pixel groups in a direction intersecting the arrangement direction of the pixel group. The pixel group is composed of three pixel groups, one pixel group adjacent to the pixel group,
The image processing apparatus according to claim 6, wherein each of neighboring dots has a different positional relationship between the two sets of pixel groups and the one set of pixel groups. In an image processing apparatus that performs processing on an image that is configured with lines having different thicknesses and arranged in two directions in which a plurality of pixels intersect,
Regarding two sets of pixel groups arranged in a direction intersecting two directions related to the arrangement of the pixels, a value assigned to a pixel included in one pixel group and a pixel allocated to the other pixel group And a processing unit that performs halftone processing using a dither matrix pattern in which a fixed value is assigned to each pixel included in at least one of the two sets of pixel groups. Image processing device. The thin lines constituting one of the lines having different thicknesses are composed of lines having a width equal to or less than a (natural number) times the width in one direction of the pixel corresponding to each value constituting the dither matrix pattern,
A thick line thicker than the thin line is composed of a line having a width equal to or more than b (natural number, b> a) times the width in one direction of the pixel corresponding to each value constituting the dither matrix pattern. The image processing apparatus according to claim 8, wherein the apparatus is an image processing apparatus. The dither matrix pattern has a size of 2 pixels in one direction in which pixels are arranged and 1 pixel in the other direction,
The image processing according to any one of claims 1 to 9, wherein the processing unit shifts the dither matrix pattern pixel by pixel in the other direction to perform the halftone processing. apparatus. The dither matrix pattern has a size of n (even) pixels in one direction in which pixels are arranged and m (even) pixels in the other direction, or (n + 1) pixels in the one direction, and (m + 1) pixels in the other direction. of one of the size, before the image processing apparatus according to any one of claims 1, characterized in that it has a size that does not interfere with Kiga image to 9.
An image processing apparatus according to any one of claims 1 to 11,
An image forming unit for forming an image,
The image forming apparatus, wherein the image forming section forms an image processed by the image processing apparatus. In an image processing method by an image processing apparatus that performs processing on an image that includes a plurality of regions each composed of dots of different sizes and is arranged in two directions where a plurality of pixels intersect,
Regarding two sets of pixel groups arranged in a direction intersecting two directions related to the arrangement of the pixels, a value assigned to a pixel included in one pixel group and a pixel allocated to the other pixel group A halftone process using a dither matrix pattern in which a constant value is assigned to each pixel included in at least one of the two sets of pixel groups. Processing method.

Images