JP2006279293A - Image forming apparatus and method of concealing information - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To add information to be concealed to a printed matter by a safe and positive method. <P>SOLUTION: A storage section 40 of an image forming apparatus 1 stores a compounding ratio table TBL in which the combination of mixtures of toner having a similar spectrum reflection ratio is described. When forming an image having embedded confidential information, a control section 30 refers to the mixture ratio table TBL to instruct an image forming section 20 to generate an image at a toner mixture ratio in which the color of the confidential information is a color having a spectrum reflection rate similar to that of a certain color of the background of the confidential information, where a certain color is given to the background of the confidential information. The image forming section 20 generates an image in accordance with this instruction. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a technique for adding information to be concealed to a printed matter.

  Digital contents such as image data and music data are very easy to duplicate, process and redistribute. Therefore, there is an urgent need to take measures to protect the copyright and the like of digital content and to guarantee its originality. As this type of technology, so-called digital watermarking in which invisible information is embedded in digital content is well known.

  By the way, “steganography” has recently attracted attention as a technique similar to this digital watermark. Steganography is similar to the above-mentioned digital watermark in the sense of embedding information in digital content, but the purpose of digital watermark is to protect the digital content itself, whereas in steganography the embedded information is Is important information. In other words, in steganography, it can be said that digital content itself is “dummy data” for blinding third parties.

  Embedding information to be concealed (confidential information) in image data using steganography or digital watermark has been conventionally performed. However, it is not easy to embed confidential information in an image formed on a printed matter such as paper. This is because it is difficult to embed confidential information with high accuracy in a printed matter output by a printer.

  The embedding of the confidential information in the image data is mainly realized by adding redundancy to the image data. Redundant data is mixed as noise that cannot be perceived by the human eye. However, it has been extremely difficult for a printer to faithfully reproduce even this noise. For example, an ink jet printer causes ink bleeding, and an electrophotographic printer causes minute dot fluctuations due to so-called “crushing” or “washout”. Could not be reproduced.

In such a situation, an attempt is made to add confidential information to the printed matter. An example of this type of trial is Patent Document 1. Japanese Patent Application Laid-Open No. 2004-151561 discloses a technique for reading a printed matter with confidential information added by a scanner and extracting the confidential information from image data read by the scanner.
JP 2001-274980 A

However, the technique described in Patent Document 1 has the following problems.
First, in the technique described in Patent Document 1, information called a “customization key” corresponding to a “key” for extracting (restoring) secret information is required. In other words, there is a problem that if the customized key is lost or forgotten, confidential information cannot be extracted. Further, if this customized key is stolen by a malicious third party, there is a risk that confidential information may be extracted.

  Furthermore, in the technique described in Patent Document 1, a printed matter to which confidential information is added is read by a scanner. In other words, it can be duplicated by a scanner. If the printed material can be reproduced, even if the original owner owns the original, there is a risk that the printed material with the confidential information will be exchanged without knowing the owner. there were.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a technique for adding information to be concealed to a printed matter in a safe and reliable manner.

To achieve the above object, the present invention provides an image forming unit that forms an image on a recording material using a plurality of color materials having different colors, and a color that can form an image of a specific color by the image forming unit. Associating the first blending information indicating the blending ratio of the material with the second blending information indicating the blending ratio of the colorant capable of forming an image of a color whose color difference from the specific color is equal to or less than a predetermined threshold. A storage unit that stores the first color image in the image of the specific color at a combination ratio indicated by the stored first combination information with respect to the image forming unit when the image of the specific color is formed; An image is formed in the area, and a second area different from the first area in the image of the specific color at a composition ratio indicated by the second composition information stored in association with the first composition information Color instruction means for giving an instruction to form an image on To provide an image forming apparatus to obtain.
According to such an image forming apparatus, confidential information (image) can be added to a printed matter (recording material) using a color that is visually almost the same as the color used for the background. Therefore, it is possible to exchange confidential information in a state that cannot be known from a third party.

In the image forming apparatus according to the aspect of the invention, it is preferable that the threshold value is 1 or less when the color difference is represented by a color difference ΔE in the CIELAB color system.
If the color difference ΔE is 1 or less, it is known that the human eye can hardly perceive the color difference. Therefore, in this way, it is possible to improve the security of confidential information. In addition, when the color difference ΔE is 1 or less, the spectral reflectances of the two colors are almost equal. Therefore, it is possible to prevent so-called metamerism (conditional color) and prevent confidential information from being observed under a specific light source.

In the image forming apparatus of the present invention, it is preferable that the image forming unit forms an image using at least five kinds of color materials.
For example, in a configuration in which the chromatic colors are only cyan, magenta, and yellow, which are the three primary colors, and an achromatic color, black, is added thereto, similar colors such that the color difference ΔE is 1 or less are used. Combinations are limited. Therefore, by increasing the chromatic colors and forming an image using five or more kinds of color materials in total, it is possible to increase the number of color combinations such that the color difference ΔE is 1 or less.

In one embodiment of the image forming apparatus of the present invention, the image forming means expresses an image with a plurality of halftone dots, and the number of screen lines representing the interval between the halftone dots is 200 lines or more. There may be.
In this way, the halftone dot shape becomes inconspicuous, so even if two images with different halftone dot area ratios are juxtaposed, they will not stand out if their colors are similar.

In the image forming apparatus according to the aspect of the invention, it is preferable that the second combination information is configured without including at least one of the color materials constituting the first combination information.
In this way, the confidential information can be confirmed based on the presence or absence of the color material and the dots, not the density of the color material and the size of the dots, so that the confidential information can be easily read.

In the image forming apparatus of the present invention, it is preferable that the color material is a toner, and the image forming unit coats the surface of the recording material with a transparent toner.
In this way, it is possible to make the difference in gloss and level difference for each color of the formed halftone dots inconspicuous, so that the security of confidential information can be further improved.

Further, the image forming apparatus of the present invention may be configured to include an input unit that specifies the shape of the first or second region.
In this way, the operator can embed confidential information of an arbitrary shape at an arbitrary position. Here, the shape is not limited to a shape such as a figure, and may be a character or a symbol.

The image forming apparatus according to the present invention preferably includes a sheet feeding unit that supplies a recording material having a layer in which a color material is embedded on the surface thereof to the image forming unit.
In this way, the gloss difference and level difference for each color of the color material can be made less noticeable.

  Further, the present invention provides a second color image having a color difference from the first color equal to or less than a predetermined threshold value as confidential information on the surface of the recording material on which the first color image is formed as a background. It is also specified as a method of concealing information characterized by formation. In short, the present invention is characterized in that information to be concealed is embedded using images of two types of colors having substantially the same spectral reflectance.

  As described above, according to the present invention, it is possible to add information to be concealed to a printed matter in a safe and reliable manner.

[1: Configuration]
FIG. 1 is a diagram showing a device configuration of an image forming apparatus 1 according to an embodiment of the present invention. The image forming apparatus 1 includes an image reading unit 10 that reads an object to be imaged O on which an image is formed to generate image data, and an image forming unit that forms a toner image on a recording material such as paper based on the image data. It is roughly divided into 20. The image reading unit 10 corresponds to a so-called scanner, and the image forming unit 20 corresponds to a so-called printer. That is, the image forming apparatus 1 is a so-called multi-function machine having functions of a scanner and a printer.

  The image reading unit 10 includes a full rate carriage 110, a half rate carriage 120, an imaging lens 130, a line sensor 140, a platen glass 150, and a platen cover 160. The full-rate carriage 110 includes a light source 111, a mirror 112, and the like, and irradiates light onto the object to be imaged O placed on the platen glass 150 while being moved at a speed v in the direction of arrow C in the drawing by a driving unit (not shown). The reflected light from the object to be imaged O is guided to the half rate carriage 120.

The half-rate carriage 120 includes mirrors 121 and 122 and guides light from the full-rate carriage 110 to the imaging lens 130. The half-rate carriage 120 is driven by a drive unit (not shown) and is moved in the same direction as the full-rate carriage 110 at half the speed (ie, v / 2) of the full-rate carriage 110.
The imaging lens 130 is an imaging means including, for example, an fθ lens. The imaging lens 130 is provided on an optical path connecting the mirror 122 and the line sensor 140, and forms an image of light from the object to be imaged O at the position of the line sensor 140.

  The line sensor 140 generates and outputs an image signal corresponding to the intensity of the imaged light. The line sensor 140 is an image pickup unit that can simultaneously receive light of different wavelengths, and is, for example, a multi-line CCD image sensor (image pickup element array) including an on-chip color filter. This CCD image sensor images an object to be imaged with different spectral sensitivities for each line. In the present embodiment, an image sensor capable of imaging with three colors of R (red), G (green), and B (blue) is used. The line sensor 140 of this embodiment outputs the image signals of these three colors with 8 bits for each color.

  With the above configuration, in the image reading unit 10, the light source 111 irradiates the object to be imaged O placed on the platen glass 150, and the reflected light is read by the line sensor 140. The line sensor 140 supplies image signals of three colors R, G, and B to the image processing unit 50 described later based on the read reflected light. The image processing unit 50 generates image data based on the image signal and supplies the image data to the image forming unit 20.

  Next, the configuration of the image forming unit 20 will be described. The image forming unit 20 includes developing mechanisms 210a, 210b, 210c, and 210d, an intermediate transfer belt 220, primary transfer rolls 230a, 230b, and 230c, a secondary transfer roll 240, a backup roll 250, and a paper feed mechanism 260. The first fixing mechanism 270, the switching mechanism 280, and the second fixing mechanism 290 are provided.

FIG. 2 is a diagram showing the configuration of the developing mechanism 210a in more detail. The developing mechanism 210a is a so-called rotary developing device, and includes a photosensitive drum 211a, a charger 212a, an exposure device 213a, and developing units 214a, 215a, 216a, and 217a. The photosensitive drum 211a is an image carrier having a photoconductive layer made of OPC (Organic Photo Conductor) as a charge acceptor on the surface thereof, and is rotated in the direction of arrow A in the figure. The charger 212a includes, for example, a charging roller, and uniformly charges the surface of the photosensitive drum 211a. The exposure device 213a irradiates light to the photosensitive drum 211a with, for example, a laser diode, and forms an electrostatic latent image with a predetermined potential. The developing units 214a, 215a, 216a, and 217a store toners of different colors, and form toner images by attaching the toner to the electrostatic latent image formed on the surface of the photosensitive drum 211a. The toners stored in the developing units 214a, 215a, 216a, and 217a are, for example, four color toners of Y (yellow), G (green), O (orange), and I (transparent). Here, the transparent toner is a toner that does not contain a color material. For example, SiO ₂ (silicon dioxide) or TiO ₂ (titanium dioxide) is externally added to a low molecular weight polyester resin. The transparent toner does not form a colored image. For example, the transparent toner is used to increase the gloss and smoothness of the paper surface by forming a uniform toner image on the paper surface. By forming the transparent toner uniformly on the surface of the paper, it is possible to make the step formed at the boundary between the portion where the toner image is formed and the portion where the toner image is not conspicuous.

On the other hand, the developing mechanisms 210b, 210c, and 210d include single-color developing devices 214b, 214c, and 214d. Since the photosensitive drums 211b, 211c, and 211d, the chargers 212b, 212c, and 212d, and the exposure devices 213b, 213c, and 213d have substantially the same configuration as the developing mechanism 210a described above, description thereof is omitted. To do. The developing mechanisms 210b, 210c, and 210d respectively create three color toner images, for example, M (magenta), C (cyan), and K (black).
The developing mechanisms 210a, 210b, 210c, and 210d are not particularly limited in the color of toner accommodated, and the order thereof is not particularly limited.

  Here, with reference to FIG. 1 again, another configuration of the image forming unit 20 will be described. The intermediate transfer belt 220 is an endless belt member that is moved in the direction of arrow B in the drawing by a driving unit (not shown). The intermediate transfer belt 220 transfers the toner image (primary transfer) at a position facing the photosensitive drums 211a, 211b, 211c, and 211d, and moves the toner image to transfer the toner image onto the paper P (secondary transfer) again. The primary transfer rolls 230a, 230b, 230c, and 230d urge the intermediate transfer belt 220 with appropriate pressure at positions where the intermediate transfer belt 220 faces the photosensitive drums 211a, 211b, 211c, and 211d, and transfer the toner image to the intermediate transfer belt 220. . The secondary transfer roll 240 and the backup roll 250 are urged with an appropriate pressure at a position where the intermediate transfer belt 220 faces the paper P, and transfer the toner image onto the paper P.

The paper feed mechanism 260 includes paper trays 261a and 261b that accommodate various paper P, and supplies the paper P during image formation. In the present embodiment, there are two types of paper P: plain paper P ₀ and coated paper P ₁ . Here, the plain paper P ₀ is a white high-quality paper such as PPC paper, and is a paper normally used in a printer or a copying machine. Further, the coated paper P _1, the normal to the surface of paper and equivalent groups such as cellulose material eg provided an image receiving layer by a thermoplastic resin such as polyethylene, toner this image-receiving recording sheet surface by heating and pressing Paper that is intended to be embedded in a layer. At this time, a paraffin wax or the like may be added to the image receiving layer in order to improve the releasability of the toner. By using such a recording sheet, it is possible to make the difference in gloss and unevenness on the surface of the sheet P caused by toner less noticeable. Here, it is assumed that the plain paper P ₀ is stored in the paper tray 261 a and the coated paper P ₁ is stored in the paper tray s 61 b.

The first fixing mechanism 270 includes a roll member for heating and pressurizing the paper P, and fixes the toner image transferred on the surface of the paper P with heat and pressure. The switching mechanism 280 changes the conveyance path of the paper P according to the type of toner image formed on the surface of the paper P. Conveying Specifically, the switching mechanism 280, when the coated paper P ₁ is used as the sheet P is conveyed in the arrow R direction in Fig this sheet P, the other paper P to L direction in FIG. And let it drain.

  The second fixing mechanism 290 includes a fixing belt 291, a heating device 292, and a cooling device 293. The second fixing mechanism 290 further heats the paper P once heated and pressure-fixed by the heating device 292 to cause the toner to be melted again, and then keeps the paper P in close contact with the fixing belt 291 having a smooth surface. The toner is fixed by cooling with the cooling device 293. By doing so, an image with a smooth surface and high glossiness can be formed. Hereinafter, the process of forming a high gloss image using the second fixing mechanism 290 is referred to as “high gloss process”.

  With the above configuration, the image forming unit 20 forms an image on the paper P using the above-described seven color toners based on the image data input from the image processing unit 50. At this time, the image forming unit 20 changes the image forming process in accordance with an instruction from the control unit 30 described later. Specifically, when the image forming unit 20 performs “secret printing” to be described later, the image forming unit 20 forms a toner image with transparent toner on the surface of the paper P, and then performs the above-described high gloss processing.

Next, the hardware configuration of the above-described image forming apparatus 1 will be described.
FIG. 3 is a block diagram illustrating a hardware configuration of the image forming apparatus 1. The image forming apparatus 1 includes a control unit 30, a storage unit 40, an image processing unit 50, an operation unit 60, and a data input / output unit 70 in addition to the image reading unit 10 and the image forming unit 20 described above. Yes.

  The control unit 30 is an arithmetic device including a CPU (Central Processing Unit), a RAM (Random Access Memory), a ROM (Read Only Memory), and the like (not shown), and executes various programs PRG stored in the storage unit 40. Thus, the operation of each part of the image forming apparatus 1 is controlled. The storage unit 40 is a large-capacity storage device such as an HDD (Hard Disk Drive), for example. In addition to various programs PRG for operating each unit of the image forming apparatus 1 described above, a combination of toner blends having similar spectral reflectances. Is stored in the blending ratio table TBL.

  FIG. 4 is a diagram showing an example of the blending ratio table TBL. In the figure, the blending information g1 indicates the blending ratio of toners of C, M, Y, K, G, and O colors when a green toner image is formed by the image forming apparatus 1 of the present embodiment. The blending information g2 indicates a blending ratio of toners of C, M, Y, K, G, and O colors that can form a toner image having a color substantially equal to the green color. The values shown in FIG. 4 are halftone dot area ratios (unit:%). These are derived using the toner actually used in the image forming apparatus 1 of the present embodiment.

  FIG. 5 is a graph showing the spectral reflectance when a toner image is formed according to the blending ratio shown in the blending information g1 and g2. In the figure, the horizontal axis of the graph is wavelength (unit: nm), and the vertical axis is reflectance (unit:%). FIG. 5A shows the spectral reflectance of green G1 indicated by the blending information g1, and FIG. 5B shows the spectral reflectance of green G2 indicated by the blending information g2. Comparing these graphs, the spectral reflectance of green G2 is slightly different from the spectral reflectance of green G1 in the vicinity of 540 nm, but can be said to be almost equal overall. That is, these two colors have almost the same spectral reflectance and are perceived as almost the same color by the human eye. Since the spectral reflectances are almost equal, these colors do not depend on the observation conditions such as the light source, and are always perceived as the same color. That is, no conditional color (metamerism) occurs between these two colors.

  The blending information g2 can be experimentally determined by, for example, the steepest gradient method (hill climbing method). For example, the ratio of the toner of a certain color (here, K) is largely changed, and then the blending ratio that minimizes the color difference is obtained while gradually increasing or decreasing the color of the other toner. That's it.

At this time, if the color difference ΔE of the CIELAB color system is used, it is desirable that the difference between the two colors is ΔE ≦ 1. Here, ΔE represents the metric brightness of green G1 as L ₁ *, the chromaticity coordinates as a ₁ * and b ₁ *, the metric brightness of green G2 as L ₂ *, and the chromaticity coordinates as a ₂ * and b ₂ *. Then, it is a value given by the following formula (1).
ΔE = {(L ₂ * −L ₁ *) ² + (a ₂ * −a ₁ *) ² + (b ₂ * −b ₁ *) ² } ^1/2 (1)

  The blending ratio table TBL of the storage unit 40 stores the blending information as described above for a plurality of colors. Then, the control unit 30 reads out the optimum blending information corresponding to the color of the image to be formed from the storage unit 40 and instructs the image forming unit 20 via the image processing unit 50 for this blending ratio.

  Returning to the description of FIG. The image processing unit 50 includes a plurality of image processing circuits such as an ASIC (Application Specific Integrated Circuit) and an LSI (Large Scale Integration), an image memory that temporarily stores image data, and the like. Predetermined image processing is executed. Here, image processing includes basic image processing such as AD conversion, shading correction, and gamma conversion, as well as processing such as color space conversion, image rotation, image enlargement / reduction, under color removal (UCR) processing, and screen processing. Contains.

  The image processing unit 50 performs such image processing on the image signal generated by the image reading unit 10 or the image data input to the data input / output unit 70, and each color binary image that can be processed by the image forming unit 20. Generate data. Here, the binary image data is two-gradation image data in which the gradation is expressed in a pseudo manner by the halftone dot area ratio. The image processing unit 50 supplies the generated binary image data to the image forming unit 20.

  The operation unit 60 includes, for example, a touch panel display, various buttons, and the like, and receives an input instruction from an operator of the image forming apparatus 1. This input instruction is supplied to the control unit 30. The data input / output unit 70 is an interface device for exchanging data with an external device. The image forming apparatus 1 can output image data to be supplied to the image forming unit 20 to an external device such as a computer or a printer, or can receive image data from the external device as necessary. ing.

[2: Operation]
Next, print processing performed by the image forming apparatus 1 configured as described above will be described. The image forming apparatus 1 according to the present embodiment performs two types of image formation, “normal printing” using the above-described plain paper P ₀ and “secret printing” using the coated paper P ₁ . Here, the confidential print is an operation for forming an image by embedding specific confidential information in a part of an image. The secret information may be a figure other than a code such as a character or a barcode. The confidential information may be embedded in the image data in advance, or an operator may input an instruction via the operation unit 60 of the image forming apparatus 1. At this time, the secret information may be in a format including the color forming the secret information, that is, the blending information. Also, the distinction between “normal print” and “secret print” may be performed by an instruction from the operator, or image data including confidential information may have an identifier such as a flag.

  FIG. 6 is a flowchart for explaining print processing of the image forming apparatus 1. Describing along the figure, first, the control unit 30 determines whether or not confidential information is included in the image data to be printed (step S1). If the image data includes confidential information (step S1; YES), the control unit 30 refers to the combination ratio table TBL in the storage unit 40 and specifies the combination information to be used for the confidential information (step S2). When the color is not particularly specified for the area where the confidential information is embedded, the control unit 30 selects appropriate blending information corresponding to the area.

When the blending information is specified, the control unit 30 supplies an instruction to the image processing unit 50 to form an image of the area including the confidential information and the background with the blending ratio indicated by the blending information. At this time, since the image data contains confidential information, the control unit 30 an instruction to the effect that using the coated paper P ₁ at the time of image formation, by the transparent toner to the surface of the image indicating that imaged the toner image Provide instructions simultaneously. Based on this instruction, the image processing unit 50 generates binary image data of each color and supplies it to the image forming unit 20. The image forming unit 20 forms an image according to the binary image data (step S3).

At this time, since the paper on which the image is formed is the coated paper P ₁ and the surface of the transparent toner layer is formed on the surface of the image forming unit 20, the paper on which the toner is fixed by the first fixing mechanism 270. The sheet is conveyed in the R direction in FIG. Then, the image forming unit 20 performs high-gloss processing in which the toner image on the paper is melted again and fixed by the second fixing mechanism 290 (step S4). When the high gloss process is completed, the image forming unit 20 discharges the sheet (step S5) and ends the print process.

On the other hand, when the image data does not include confidential information, the control unit 30 causes the image forming unit 20 to perform normal image formation (step S6). That this time, the image forming unit 20 together with the use of plain paper P ₀ on the paper, without using the transparent toner, and forms an image without the high gloss process. When this image formation is completed, the image forming unit 20 discharges the sheet (step S5) and ends the printing process.

The print processing by the image forming apparatus 1 is as described above. Next, an image including confidential information formed by the above process will be described.
FIG. 7 is a schematic diagram showing a part of an image in which confidential information is embedded. In this image, an area a2 indicated by “F” with the area a1 as a background is embedded as confidential information. The color of the area a1 is, for example, the above-described green G1, and at this time, the color of the area a2 is the above-described green G2.

  When such an image is visually observed, it is almost impossible to perceive that the letter “F” is hidden here if it is a normal human eye. Therefore, it is difficult for anyone who knows in advance that such information is embedded to know that there is any information here.

  As a method of reading confidential information from this image, for example, there is a method of determining the halftone dot area ratio using a spectrophotometer and discriminating the background and the confidential information from the difference. For example, in the case of green G1 and green G2 described with reference to FIGS. 4 and 5, the halftone dot area ratio of cyan is approximately four times the background in the confidential information. Therefore, the difference in the halftone dot area ratio of this color is relatively easy to distinguish. Further, it can be said that the presence or absence of the halftone dot itself is easier to discriminate than the judgment of the density of the halftone dot area ratio. Therefore, in the case of green G1 and green G2, it may be determined using a color that does not form a halftone dot itself in either one of the areas, such as magenta or black.

[3: Effects and usage examples]
As described above, according to the image forming apparatus 1 of the present embodiment, it is possible to add information to be concealed to the printed matter. Next, an example of using an image formed by such an image forming apparatus 1 and its effect will be described.
By embedding the confidential information of this embodiment in an image, it is possible to exchange information that is not desired to be known to others using analog means such as paper. Since electronic means such as a network is not used for exchanging information, there is no possibility that a malicious third party will steal information without the knowledge of the parties concerned.

  Further, since this technique is not a technique for providing redundant data, the image quality of the formed image is not deteriorated. Furthermore, since the redundancy of the image data is not used, confidential information can be given to, for example, a solid region having a uniform color material density. Therefore, confidential information can be embedded in, for example, a logo mark of a company or the like, or a simple background color that does not seem to have anything.

  In addition, there is no secret information corresponding to a so-called “key”, and no data is required to extract the information. Therefore, there is no fear that the data corresponding to this key will be lost or forgotten or stolen by a malicious third party.

  Also, even if an image embedded with confidential information is viewed by a third party, unless it is well watched, it cannot be known that there is confidential information, and the area where the confidential information exists is preliminarily determined. Even if you know it, it is almost impossible to judge visually what is written there.

  As a further effect, the secret information has a feature that the information is lost when copied. As described above, since the secret information is formed with a color whose spectral reflectance is almost the same as the background color, it is extremely difficult to discriminate the light source using any light source. Even if this secret information is scanned using a scanner that is widely used, the secret information becomes assimilated with the background and cannot be read. That is, as long as the original is possessed by the parties, the confidentiality of the confidential information can be reliably maintained.

[4: Modification]
Although one embodiment of the present invention has been described above, the present invention is not limited to such an aspect, and various modifications can be made in the implementation. In the following, modifications applicable to the present invention will be exemplified.

  First, a modification of the image forming unit will be described. In the above-described embodiment, the rotary developing mechanism 210a including the four developing units 214, 215, 216, and 217 and the developing mechanisms 210b, 210c, and 210d including the single color developing devices 214b, 214c, and 214d are provided. Although described above, other configurations may be used. For example, a so-called tandem developing device in which all seven colors are arranged in series may be used, or a plurality of rotary developing mechanisms may be provided. At this time, of course, the number of toner colors may be increased or decreased. Further, instead of the intermediate transfer belt, a paper conveyance belt may be provided, and the transfer from the photosensitive drum to the recording paper may be performed directly without performing the transfer to the intermediate transfer body (intermediate transfer belt).

  Although it has been described that the number of colors of the toner may be increased or decreased, it is desirable that the number of colors of the toner is at least 5 or more (excluding the transparent toner). This is because there are limitations on combinations of colors having substantially the same spectral reflectance in the four types of primary colors C, M, and Y and achromatic K. The toner to be used is not limited to orange and green used in the above-described embodiment, and various colors such as red and blue may be used.

  The halftone dots used for image formation in the present invention are not particularly limited, but the number of screen lines is preferably 200 lines, that is, 200 lpi (lines per inch) or more. This is because when the number of screen lines is small and the halftone dots to be formed are large, the boundary between the area in which the confidential information is embedded and the background thereof is easily noticeable.

  Further, in the above-described embodiment, coated paper and transparent toner are used when embedding confidential information, but these are not essential components. For example, even with only one of coated paper and transparent toner, a certain effect can be obtained, and depending on the particle size of the toner and the quality of the paper, sufficient image quality can be obtained without using coated paper or transparent toner. Obtainable.

Further, in the above-described embodiment, the example in which the present invention is implemented by the multifunction machine having the functions of the scanner and the printer has been described. However, the present invention can of course be implemented in a printer having no scanner function. . In this case, however, the image data used is limited to that supplied from an external device.
Further, the type of printer is not limited to the electrophotographic system, and the present invention can be applied to, for example, a printing machine that expresses gradation with halftone dots as in the above-described embodiment.

1 is a diagram illustrating an apparatus configuration of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a diagram illustrating a configuration of a developing mechanism of the image forming apparatus according to the embodiment in more detail. FIG. 2 is a block diagram illustrating a hardware configuration of the image forming apparatus according to the embodiment. It is the figure which showed an example of the mixture ratio table in the same embodiment. It is the graph which showed the spectral reflectance of the toner image used for formation of secret information. 6 is a flowchart for explaining print processing of the image forming apparatus of the embodiment. It is the schematic diagram which showed a part of image in which confidential information was embedded.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 10 ... Image reading part, 110 ... Full-rate carriage, 120 ... Half-rate carriage, 130 ... Imaging lens, 140 ... Line sensor, 150 ... Platen glass, 160 ... Platen cover, 20 ... Image forming part, 210, 210a, 210b, 210c, 210d ... developing mechanism, 220 ... intermediate transfer belt, 230 ... primary transfer roll, 240 ... secondary transfer roll, 250 ... backup roll, 260 ... feed mechanism, 270 ... fixing mechanism, 30 ... Control unit 40 ... Storage unit 50 ... Image processing unit 60 ... Operation unit 70 ... Data input / output unit

Claims (9)

An image forming means for forming an image on a recording material using a plurality of color materials having different colors;
Color material capable of forming an image of a color having a color difference between the first blend information indicating the blend ratio of a color material capable of forming an image of a specific color by the image forming unit and a color difference equal to or less than a predetermined threshold. Storage means for associating and storing second blending information indicating the blending ratio of
When the image of the specific color is formed, the image forming unit is caused to form an image in a first area in the image of the specific color at a combination ratio indicated by the stored first combination information. And an instruction to form an image in a second region different from the first region in the image of the specific color at a blending ratio indicated by the second blending information stored in association with the first blending information. An image forming apparatus comprising: color instruction means for providing The image forming apparatus according to claim 1, wherein the threshold value is 1 or less when the color difference is expressed by a color difference ΔE in the CIELAB color system. The image forming apparatus according to claim 1, wherein the image forming unit forms an image using at least five kinds of color materials. The image forming apparatus according to claim 1, wherein the image forming unit expresses an image with a plurality of halftone dots, and the number of screen lines representing the interval between the halftone dots is 200 lines or more. The image forming apparatus according to claim 1, wherein the second combination information is configured without including at least one of the color materials constituting the first combination information. The colorant is toner;
The image forming apparatus according to claim 1, wherein the image forming unit covers a recording material surface with a transparent toner. The image forming apparatus according to claim 1, further comprising an input unit that specifies a shape of the first or second region. The image forming apparatus according to claim 1, further comprising a paper feeding unit that supplies a recording material having a layer in which a color material is embedded to the surface to the image forming unit. On the surface of the recording material on which an image of the first color is formed as a background,
A method for concealing information, wherein a second color image having a color difference with the first color equal to or less than a predetermined threshold is formed as the secrecy information.

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