Patent Publication Number: US-2020280655-A1

Title: Image forming apparatus and image forming method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2018-126021, filed Jul. 2, 2018, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to an image forming apparatus and an image forming method. 
     BACKGROUND 
     In an image forming apparatus, the risk of information leakage of printed confidential documents is a problem. As a countermeasure to information leakage, a method in which a user pays attention to storage of confidential documents, a method of printing with an image forming apparatus provided with a paper discharge destination that can be locked, and the like are used. 
     However, it is necessary to prepare an image forming apparatus provided with a paper discharge unit with a key. Further, there are problems in that the apparatus cannot be used when there is no available space in the paper discharge unit with a key, and it takes time and effort to take out confidential documents printed from the paper discharge unit with a key. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an external view illustrating an overall arrangement example of an image forming apparatus according to at least one embodiment; 
         FIG. 2  is a diagram illustrating a schematic arrangement of the image forming apparatus; 
         FIG. 3  is a block diagram illustrating a hardware arrangement example of the image forming apparatus; 
         FIG. 4  is a flowchart illustrating an operation example of the image forming apparatus; 
         FIG. 5  is a diagram illustrating an example of a display screen of a preview image; 
         FIG. 6  is a first diagram illustrating a secure overlay setting screen; 
         FIG. 7  is a second diagram illustrating a secure overlay setting screen; 
         FIG. 8  is a third diagram illustrating a secure overlay setting screen; 
         FIG. 9  is a diagram for schematically explaining examples of reducing (ACT  17 ) and image repeating (ACT  19 ); 
         FIG. 10  is a diagram illustrating a first example of a printed image on which an overlay image is formed; and 
         FIG. 11  is a diagram illustrating a second example of a printed image on which an overlay image is formed. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments provide an image forming apparatus and an image forming method capable of reducing the risk of information leakage of printed documents. 
     In general, according to at least one embodiment, an image forming apparatus includes a control unit (controller) and an image forming unit. The control unit is configured to generate image data of a first image based on a designated image or a designated character string designated by a user. The image forming unit is configured to form the first image in a predetermined area based on the image data using a decolorizing recording agent. The image forming unit is configured to form the first image on at least one of a front surface, on which a second image is formed, of a medium with the second image formed thereon using a non-decolorizing recording agent or a surface opposite to the front surface. 
     An image forming apparatus and an image forming method according to at least one embodiment will be described hereinafter with reference to the drawings. 
       FIG. 1  is an external view illustrating an overall arrangement example of an image forming apparatus  100  according to at least one embodiment. For example, the image forming apparatus  100  is a multi-function peripheral. The image forming apparatus  100  includes a display  110 , a control panel  120 , a printer  130 , a sheet accommodation unit  140 , and an image reading unit  200 . The printer  130  of the image forming apparatus  100  may be an electrophotographic device for fixing an image using a toner or may be an ink jet device. 
     The image forming apparatus  100  forms an image on a sheet using a developer such as a toner. The sheet is, for example, paper or label paper. The sheet may be any material as long as the image forming apparatus  100  can form an image on a front surface of the sheet. 
     The display  110  is an image display device such as a liquid crystal display or an organic electro luminescence (EL) display. The display  110  displays various pieces of information about the image forming apparatus  100 . 
     The control panel  120  includes a plurality of buttons. The control panel  120  receives an operation of a user. The control panel  120  outputs a signal according to an operation performed by the user to the control unit of the image forming apparatus  100 . The display  110  and the control panel  120  may be configured as an integral touch panel. 
     The printer  130  forms an image on a sheet based on image information generated by the image reading unit  200  or image information received via a communication path. For example, the printer  130  forms an image by the following processing. Details of a configuration of the printer  130  will be described later according to  FIG. 2 . 
     The sheet accommodation unit  140  accommodates sheets to be used for image formation in the printer  130 . The sheet on which an image is to be formed may be a sheet accommodated in the sheet accommodation unit  140  or may be a sheet which is manually fed. 
     The image reading unit  200  reads an image to be read as a brightness and darkness of light and converts the read image into image information such as RGB values. The image reading unit  200  includes, for example, a scanner lamp, a scanning optical system, a condenser lens, a charge coupled device (CCD) sensor, and the like. The scanner lamp illuminates the image to be read. In the scanning optical system, a mirror that changes the optical path of the light reflected from the image to be read is mounted. The condenser lens converges the light reflected from the image to be read and forms an image. The CCD sensor converts the formed image light into an electric signal. 
     The image reading unit  200  records the image information of the read image. The recorded image information may be transmitted to another information processing apparatus via a network. The recorded image information may be used in the image forming on the sheet by the printer  130 . 
       FIG. 2  is a diagram illustrating a schematic configuration of the image forming apparatus  100  according to at least one embodiment. The image forming apparatus  100  includes an auto document feeder (ADF)  201  on the upper part of the image reading unit  200 . The sheet accommodation unit  140  includes a plurality of sheet feeding cassettes  141 . 
     The image forming apparatus  100  includes a carrying unit  150  configured to carry the sheet accommodated in the sheet accommodation unit  140  to the transfer position of the printer  130 . The sheet carried by the carrying unit  150  is carried to the transfer position of the printer  130  by a pair of register rollers  151  at predetermined timing. 
     The printer  130  includes a first developer  132 , a second developer  133 , a laser exposure unit  134 , a cleaning device  135 , and the like around a photoconductive drum  131 . The photoconductive drum  131  is an electrostatic latent image carrier. The laser exposure unit  134  emits image exposure light to a surface of the photoconductive drum  131  based on the image information to form an electrostatic latent image on the surface of the photoconductive drum  131 . The electrostatic latent image is developed by the toner of the first developer  132  or the second developer  133 . 
     The toner image developed by the toner is transferred to the sheet by a transfer roller  136  at the transfer position. The sheet passed through the transfer position is heated and pressurized by a fixer  137 . The sheet passed through the fixer  137  is discharged to a paper discharge unit  153  by a pair of paper discharge rollers  152 . Alternatively, the sheet passed through the fixer is carried again to the transfer position via a double-side carrying unit and is discharged to the paper discharge unit  153  after completion of double-side printing. 
     The fixer  137  includes, for example, a heating roller  138 , a pressure roller  139 , a heating source, a temperature sensor (not illustrated), and the like. The pressure roller  139  comes into pressure contact with the heating roller  138 . The heating source includes, for example, a halogen lamp (not illustrated) or the like for heating the heating roller  138 . The temperature sensor detects the surface temperature of the heating roller  138 . The fixer  137  controls the heating temperature by controlling the energization of the halogen lamp. 
     A non-decolorizing toner is accommodated in the first developer  132 . A decolorizing toner is accommodated in the second developer  133 . The decolorizing toner is a recording agent containing a decolorable coloring agent. For example, by heating the sheet on which the image is formed using the decolorizing toner at a predetermined temperature or higher, the recording agent on the sheet is decolored. The non-decolorizing toner is a coloring agent which is not easy to decolorize. 
     The first developer  132  and the second developer  133  provide a predetermined potential difference between the developing roller and the photoconductive drum  131 . Thus, the first developer  132  and the second developer  133  electrostatically hold the accommodated toner on the photoconductive drum  131 . 
       FIG. 3  is a block diagram illustrating a hardware arrangement example of the image forming apparatus  100 . The image forming apparatus  100  includes a central processing unit (CPU)  301 , a memory  302 , a storage device  303 , and an operation panel and display unit  304 . The image forming apparatus  100  further includes a print unit  305 , a scanner unit  306 , an image processing unit  307 , and a preview image generation unit  308 . The image forming apparatus  100  further includes an overlaying unit  309 , an image repeating unit  310 , and a print control unit  311 . A bus  312  transmits data between each of the components  301  to  311  of the image forming apparatus  100 . 
     The CPU  301  operates as a control unit and controls the operation of each of the components  301  to  311  of the image forming apparatus  100 . The CPU  301  controls each of the components  301  to  311  by executing the program stored in the storage device  303 . 
     The memory  302  is, for example, a random access memory (RAM). The memory  302  temporarily stores the data used by each of the components  301  to  311  of the image forming apparatus  100 . The storage device  303  uses, for example, a hard disk drive (HDD), a solid state drive (SSD), or the like. The storage device  303  stores the image data acquired or generated by each component. 
     The operation panel and display unit  304  corresponds to the display  110  and the control panel  120  illustrated in  FIG. 1 . The operation panel and display unit  304  uses a display device and an input device. As described above, the display device is, for example, an image display device. The input device is an input device such as a keyboard or a touch panel. The operation panel and display unit  304  receives input of information by the user via the input device. The operation panel and display unit  304  receives the input of the designation information for generating a secure overlay. 
     The scanner unit  306  corresponds to the image reading unit  200  illustrated in  FIG. 1 . The scanner unit  306  reads the image on the sheet with the image sensor and generates image data. The scanner unit  306  outputs the generated image data to the image processing unit  307 . 
     The image processing unit  307  performs image processing on the image data of an image to be printed (hereinafter, referred to as an original document image) and generates print data. The image processing unit  307  performs image processing on the image data using the image data output from the scanner unit  306  as input. The image processing unit  307  performs image processing by inputting image data received from an external terminal device via a network and image data input from a recording medium as input. The image processing includes filtering and gradation of image data, and the like. The image processing unit  307  outputs the generated print data to the print unit  305 . 
     The print unit  305  and the print control unit  311  correspond to the printer  130  illustrated in  FIG. 1 . The print unit  305  controls the print control unit  311  to form an image based on the print data. The print control unit  311  uses a non-decolorizing toner or a decolorizing toner to form an image on the sheet. 
     The preview image generation unit  308  generates preview image data of the original document image in response to the preview instruction. When there is an instruction to form a secure overlay, the preview image generation unit  308  generates preview image data in which an overlay image is combined with the original document image. The secure overlay will be described later. The preview image generation unit  308  displays the generated preview image data on the operation panel and display unit  304 . 
     The overlaying unit  309  generates overlay image data based on designation information. The designation information is inputted via the operation panel and display unit  304 . The overlaying unit  309  gives an instruction to the image repeating unit  310  and forms overlay image data. 
     The image repeating unit  310  generates designated image data satisfying a predetermined image density based on the designation information. The image repeating unit  310  generates overlay image data in which the designated image data is repeated in a main scanning direction or a sub scanning direction. 
       FIG. 4  is a flowchart illustrating an operation example of the image forming apparatus  100 . 
     The image forming apparatus  100  receives a print job (ACT  11 ). The CPU  301  of the image forming apparatus  100  receives a print job in response to the operation of the user on the operation panel and display unit  304 . The CPU  301  receives the print job from a terminal device connected via a network via a printer driver operated by the terminal device. 
     The print job includes image data of the original document image to be printed. The print job may further include designation of a printing surface (one surface, both surfaces) of a sheet forming the original document image, designation of a sheet size, and the like. 
     Upon receiving the print job, the CPU  301  causes the operation panel and display unit  304  to display a preview image of the image data included in the print job (ACT  12 ). 
     Specifically, the CPU  301  outputs the image data to the preview image generation unit  308  to cause generation of a preview image data. The preview image generation unit  308  processes the image data based on a layout of the operation panel and display unit  304  and generates the preview image data. The CPU  301  causes the operation panel and display unit  304  to display the preview image. 
       FIG. 5  is a diagram illustrating an example of a display screen of a preview image. A display screen SC 1  includes a preview image M 1  of the original document image and a check box  401  related to designation of the secure overlay. 
     As illustrated in  FIG. 5 , the original document image exemplified in the embodiment is, for example, “Report on product X of ABC”. For example, “Product X” in the original document image indicates a product name and “ABC” indicates a name of a company producing the “Product X”. “ABC” in the original document image is represented by a logo. “YYYY/MM/DD” in the original document image indicates a date when the report was generated, and “XYZ Corporation” indicates a name of a company that generated the report. Note that the original document image illustrated in  FIG. 5  further includes a table and a graph illustrating a trend of the product X beside sentences. 
     For example, “Product X”, “YYYY/MM/DD”, “XYZ Corporation”, and the graph illustrating the trend of product X in the original document image is information on a confidential content. When a third party can easily browse the information, it is possible to easily grasp the confidential content included in “Report on product X of ABC”. In other words, when these pieces of information are not easily browsed by a third party, the confidentiality of the “Report on product X of ABC” is kept. 
     The check box  401  accepts designation about whether or not to form a secure overlay during the printing of the original document image. The secure overlay is an image (hereinafter, referred to as an overlay image) to be formed for a part or all of the area of the original document image. In other words, the overlay image is an image that masks part or all of the area of the original document image. According to the example of the display screen SC 1  illustrated in  FIG. 5 , the formation of the secure overlay is designated. 
     Returning to the flowchart of  FIG. 4 , the CPU  301  acquires designation of the presence or absence of formation of a secure overlay. When it is designated to form a secure overlay (YES in ACT  13 ), the overlaying unit  309  acquires information on the overlay area (ACT  14 ). The overlay area is an area where an overlay image is to be formed in the area of the original document image. The designation of the overlay area will be described with reference to  FIG. 6 . 
     On the other hand, when it is designated not to form a secure overlay (NO in ACT  13 ), the CPU  301  gives an instruction to print the original document image. The printing will be described later in ACT  24 . 
       FIG. 6  is a first diagram illustrating a secure overlay setting screen. When the button “Next” on the display screen SC 1  ( FIG. 5 ) is selected, a setting screen SC 2  is displayed on the operation panel and display unit  304 . The setting screen SC 2  includes the preview image M 1  of the original document image, a check box  402 , and an area designation menu  403 . 
     The check box  402  receives the designation of a type of the area forming the overlay image. Specifically, the check box  402  includes buttons for selecting “Whole area” or “Partial area”. “Whole area” indicates that an overlay image is formed on a whole area of the original document image. “Partial area” indicates that an overlay image is formed in a part of the area of the original document image. 
     The check box  402  further receives the designation of the printing surface of the sheet forming the overlay image. Specifically, the check box  402  includes buttons for selecting “Front surface only”, “Rear surface only”, and “Both surfaces” “Front surface only” indicates that an overlay image is formed only on the front surface of the sheet on which the original document image is formed. “Rear surface only” indicates that an overlay image is formed only on the opposite surface of the front surface of the sheet. In other words, “Rear surface only” indicates that an overlay image is formed only on the surface of the sheet where the original document image is not formed. “Both surfaces” indicates that an overlay image is formed on both the front and rear surfaces. 
     According to the example of the setting screen SC 2  illustrated in  FIG. 6 , it is designated that an overlay image is formed on a partial area of the original document image area on both surfaces of the sheet. 
     When “Partial area” is selected in the check box  402 , the area designation menu  403  receives the designation of coordinates defining each area of “Partial area”. The user designates a predetermined area on the preview image M 1  displayed on the operation panel and display unit  304  by an operation such as touching or the like. Thus, coordinate information that defines the “Partial area” in the original document image is acquired. 
     The setting screen SC 2  illustrated in  FIG. 6  exemplifies a case where three areas (“Area 1”, “Area 2”, “Area 3”) er 1  to er 3  are designated as a “Partial area”. Hereinafter, an overlay area er 1 , an overlay area er 2 , and an overlay area er 3  are also referred to as an overlay area er when the areas are not distinguished. 
     In the example of the setting screen SC 2 , “Area 1” indicates the overlay area er 1  on the upper left side of the preview image M 1 . The overlay area er 1  is an area where “Product X of ABC, vol. 1” is displayed. As described above, “Product X” corresponds to information on confidential content. 
     “Area 2” indicates the overlay area er 2  on the upper right side of the preview image M 1 . The overlay area er 2  is an area where the character strings “XYZ Corporation, YYYY/MM/DD” are displayed. As described above, “XYZ Corporation” and “YYYY/MM/DD” correspond to information on confidential content. “Area 3” indicates the overlay area er 3  on the lower right side of the preview image M 1 . The overlay area er 3  is an area where a graph illustrating the trend of the product X is posted. As described above, the graph illustrating the trend of the product X corresponds to the information on the confidential content. 
     Returning to the flowchart of  FIG. 4 , when the information of the overlay area er forming the overlay is acquired (ACT  14 ), the overlaying unit  309  receives designation of information that is source of the overlay image. 
       FIG. 7  is a second diagram illustrating a secure overlay setting screen. When the button “Next” on the setting screen SC 2  ( FIG. 6 ) is selected, a setting screen SC 3  is displayed on the operation panel and display unit  304 . The setting screen SC 3  includes the preview image M 1  of the original document image and a check box  404 . 
     The check box  404  receives the designation of the type of information which is the source of the overlay image. Hereinafter, the information which is the source of the overlay image is referred to as designation information. The check box  404  includes buttons for selecting “Image designation” or “Character string designation”. “Image designation” indicates that an overlay image is generated based on a part of the image of the original document image. “Character string designation” indicates that an overlay image is generated based on an arbitrary character string designated by the user. 
       FIG. 7  exemplifies the setting screen SC 3  when “Image designation” is selected as the type of designation information. When “Image designation” is selected, the setting screen SC 3  receives the designation of an area of a part of the image of the original document image (hereinafter, also referred to as partial image). The user designates a predetermined area on the preview image M 1  displayed on the operation panel and display unit  304  by an operation such as touching or the like. Thus, the coordinate information that defines the partial image is acquired. 
     According to the setting screen SC 3  illustrated in  FIG. 7 , the partial image M 2  on the upper left side of the preview image M 1  is designated. The partial image M 2  includes “ABC” represented by the logo and the character string “vol. 1”. The partial image M 2  is not limited to the example of  FIG. 7 . The partial image M 2  may include a part of the table in the original document image, another character string, and the like. The partial image M 2  may include images such as illustrations and photographs. 
       FIG. 8  is a third diagram illustrating a secure overlay setting screen.  FIG. 8  exemplifies a setting screen SC 4  when “Character string designation” is selected as the type of designation information. When “Character string designation” is selected, a text input section  405  receives the designation of the character string which is the source of the overlay image. By operating the operation panel and display unit  304 , the user inputs an arbitrary character string in the text input section  405 . 
     The character string is designated by the user who recognizes the contents of the original document image. Therefore, for example, the recognizable character string that makes it possible for the user who recognizes the contents of the original document image to determine the contents of the original document image is designated. According to the setting screen SC 4  illustrated in  FIG. 8 , the character string “ABC2018vol1” is designated. For example, the character string “ABC2018vol1” indicates that the original document image is a document of “vol1” with respect to the company “ABC” in “2018”. The character string may include information indicating the classification, purpose, organization, and the like of the original document image. Further, the character string may include an arbitrary date, a time stamp, and the like. 
     Returning to the flowchart of  FIG. 4 , when “Image designation” is selected as designation information (YES in ACT  15 ), the overlaying unit  309  trims the partial image M 2  (ACT  16 ). The overlaying unit  309  extracts the image data of the partial image M 2  from the image data of the original document image based on the coordinate information of the partial image M 2  and generates the designated image data. 
     The image repeating unit  310  reduces the generated designated image data (ACT  17 ). The image repeating unit  310  reduces the designated image data according to a predetermined reduction rate at which the image density of the designated image data becomes equal to or higher than a predetermined image density. The image density is, for example, the coverage of toner per unit area or the image concentration. The predetermined image density indicates a value capable of appropriately masking the information of the original document image. The predetermined reduction ratio is set according to, for example, verification and the like. The image repeating unit  310  stores the designated image data in the memory  302  after the reducing (ACT  18 ). 
     On the other hand, when “Character string designation” is selected as the designation information (NO in ACT  15 ), the image repeating unit  310  generates designated image data based on the input character string. The image repeating unit  310  generates designated image data representing the character string “ABC2018vol1” input from the setting screen SC 4 . 
     The image repeating unit  310  generates designated image data representing a character string of a predetermined font size at which the image density of the designated image data is equal to or lower than the predetermined image density. The image density is as described above. For example, the predetermined font size is set according to verification and the like based on the predetermined image density. The image repeating unit  310  stores the generated designated image data in the memory  302  (ACT  18 ). 
     The image repeating unit  310  repeats image of the designated image data to generate overlay image data (ACT  19 ). Specifically, the image repeating unit  310  generates overlay image data in which the designated image data is repeated in either or both of the main scanning direction and the sub scanning direction. The image repeating unit  310  stores the generated overlay image data in the memory  302  (ACT  20 ). 
       FIG. 9  is a diagram for schematically explaining examples of the reducing (ACT  17 ) and the image repeating (ACT  19 ).  FIG. 9  exemplifies a case where the partial image M 2  ( FIG. 7 ) is selected as designation information. 
     The image repeating unit  310  reduces the image data of the partial image M 2  according to the predetermined reduction ratio to generate designated image data M 21  illustrated in  FIG. 9 . The image repeating unit  310  repeats the designated image data M 21  in the main scanning direction and the sub scanning direction to generate overlay image data M 22  illustrated in  FIG. 9 . As the designated image data M 21  has the predetermined image density, an overlay image capable of appropriately masking the information of the original document image is generated. 
     The image repeating unit  310  may generate overlay image data M 23  that is tilted. In this case, the image repeating unit  310  tilts the image of the designated image data M 21  according to a predetermined angle. The image repeating unit  310  generates overlay image data M 23  in which the tilted designated image data M 21  is repeated in the main scanning direction and the sub scanning direction. The same applies to the case where “Character string designation” is selected as designation information. 
     The direction of rows and columns in the overlay image data M 23  that is tilted is different from the direction of rows and columns in the original document image. Therefore, by using the overlay image data M 23 , the information of the original document image can be more appropriately masked. 
     Returning to the flowchart of  FIG. 4 , when the overlay image data is generated, the image processing unit  307  performs image processing of the image data of the original document image (ACT  21 ). Thus, print data of the original document image is generated. The image processing includes, for example, filtering, gradation, and the like. The image processing may include other processes. 
     The overlaying unit  309  overlays image (ACT  22 ). The overlaying unit  309  combines the overlay image data with the image data of the original document image. Specifically, the overlaying unit  309  combines the overlay image data with the overlay area er of the image data of the original document image. 
     The overlaying unit  309  generates image data for overlay in which the overlay image is arranged in the overlay area er. The image processing unit  307  performs image processing of the image data for overlay and generates print data for overlay. 
     The preview image generation unit  308  generates preview image data in which the overlay image is combined based on the result of the image overlaying (ACT  23 ). The CPU  301  causes the operation panel and display unit  304  to display the generated preview image. 
     Although not illustrated, the operation panel and display unit  304  may further display a button that makes regeneration of the overlay image selectable. Thus, for example, when the image density of the overlay image is not sufficient, it is possible to regenerate the overlay image by changing the reduction ratio or the font size. When it is desired to change the overlay area er or designation information, the overlay image can be regenerated. When regeneration is selected, the processes of ACT  13  to ACT  23  are executed again. 
     Based on the generated print data, the print unit  305  forms an image on the printing surface designated by the print job (ACT  24 ). The print unit  305  controls the non-decolorizing toner print control unit  311  and forms an image using non-decolorizing toner based on the print data of the original document image generated by ACT  21 . 
     The print unit  305  forms an image of the overlay image on the sheet, on which the image of the original document image is formed using the non-decolorizing toner, using the decolorizing toner. The print unit  305  forms an image of the overlay image on the printing surface designated on the setting screen SC 2  ( FIG. 6 ). Specifically, the print unit  305  controls the print control unit  311  to form an image based on the overlay print data. Thus, an overlay image is formed using the decolorizing toner on the designated printing surface. Hereinafter, the sheet on which the image is formed is also referred to as a printed image. 
       FIG. 10  is a diagram illustrating a first example of a printed image on which the overlay image is formed.  FIG. 10  exemplifies a printed image when the part image M 2  ( FIG. 7 ) is designated as designation information.  FIG. 10  illustrates a printed image when the overlay image is formed on the “both surfaces” of the sheet.  FIG. 10  illustrates a front surface ST 11  and a rear surface ST 12  of the printed image. 
     According to the front surface ST 11 , an overlay image based on the partial image M 2  is formed on the overlay areas er 1  to er 3  while overlapping with the original document image. That is, the information of the overlay areas er 1  to er 3  in the original document image is masked by the overlay image. The user who recognizes the contents of the original document image can determine the kind of original document image based on the logo “ABC” and the character string “vol. 1” included in the overlay image. 
     On the rear surface ST 12 , only the overlay image is formed. Specifically, in the rear surface ST 12 , the overlay image is formed in the areas on the rear side of the overlay areas er 1  to er 3  in the front surface ST 11 . In other words, on the rear surface ST 12 , the overlay image is formed in the linearly symmetric areas of the overlay areas er 1  to er 3  of the front surface ST 11 . Thus, the information of the overlay areas er 1  to er 3  of the original document image is also masked from the rear surface ST 12 . 
     In this manner, the overlay image based on the information designated by the user is formed so as to overlap with the original document image. In the example of  FIG. 10 , the image forming apparatus  100  generates an overlay image based on a partial image of the image of the original document image. Therefore, the user recognizing the contents of the original document image can determine the kind of original document image based on the overlay image. In other words, even when the overlay image is formed, the user can determine the kind of original document image. 
     On the other hand, the third party does not recognize the contents of the original document image. Therefore, the third party cannot determine the kind of original document image when image is not decolorized due to the overlay image being formed. In this manner, even when the discharged printed image is seen by a third party, the confidentiality of the original document image can be kept. 
     Therefore, according to the embodiment, it is possible to reduce the risk of leakage of the contents of the original document without providing a discharge destination that can be locked in the image forming apparatus  100 . When not decolorized, it is not necessary to cut or paint the printed image when discarding the printed image. Since the overlay image is formed as an image with the decolorizing toner, by decolorizing, the user can browse the contents of the original document image. 
     The image forming apparatus  100  discharges, for example, the printed image with the front surface on which the original document image is formed directed downward. That is, the image forming apparatus  100  discharges the printed image with the rear surface on which the original document image is not formed directed upward. At this time, the original document image formed on the front surface may be transparent from the rear surface and the third party may recognize the contents of the original document image. 
     On the other hand, in the embodiment, the confidential information of the original document image on the front surface is masked by the overlay image from the rear surface. Thus, it is possible to reduce the risk that the third party can easily recognize the contents of the original document image from the rear surface. When the overlay image is formed on both surfaces, even when the third party picks up and looks through the printed image, the third party cannot easily recognize the contents of the original document image. 
       FIG. 11  is a diagram illustrating a second example of the printed image on which an overlay image is formed.  FIG. 11  exemplifies a printed image when the character string “ABC2018vol1” ( FIG. 8 ) is designated as designation information.  FIG. 11  illustrates a printed image when the overlay image is formed on the “Both surfaces” of the sheet as in  FIG. 10 .  FIG. 11  illustrates a front surface ST 21  and a rear surface ST 22  of the printed image. 
     According to the front surface ST 21 , the overlay image based on the character string “ABC2018vol1” is formed in the overlay areas er 1  to er 3  while overlapping with the original document image. The information on the overlay areas er 1  to er 3  of the original document image is masked by the overlay image. The user who recognizes the contents of the original document image can determine the kind of original document image based on the character string “ABC2018vol1” included in the overlay image. 
     On the rear surface ST 22 , only the overlay image based on the character string “ABC2018vol1” is formed. Specifically, on the rear surface ST 22 , the overlay image is formed in an area on the rear side of the overlay areas er 1  to er 3  on the front surface ST 21 . In other words, in the rear surface ST 22 , the overlay image is formed in linearly symmetric areas of the overlay areas er 1  to er 3  of the front surface ST 21 . Thus, the information on the overlay areas er 1  to er 3  of the original document image is also masked from the rear surface ST 22 . 
     Even in the example of  FIG. 11 , the same effect as the effect described in  FIG. 10  is exerted. According to the example of  FIG. 11 , the image forming apparatus  100  generates an overlay image based on an arbitrary character string designated by the user. For example, the user designates a character string that makes it possible to determine the kind of original document image, if the user recognizes the contents of the original document image. Therefore, the user can determine the kind of original document image based on the overlay image even when the overlay image is formed. On the other hand, since the overlay image is formed, the third party cannot determine the kind of original document image when not decolorizing. 
     In the examples of  FIGS. 10 and 11 , a case where the overlay image is formed in “partial area” of the original document image is exemplified, but there is no limitation thereto. The overlay image may be formed on the “whole area” of the original document image. In the embodiment, the overlay image is generated based on information designated by the user. When the overlay image is formed on the whole area, the original document image is covered by the overlay image. Also in this case, the user can determine the kind of original document image based on the overlay image. 
     In the examples of  FIGS. 10 and 11 , a case where the same overlay image is formed for a plurality of overlay areas er is exemplified, but there is no limitation thereto. The overlay image may be formed based on different designation information for each overlay area er. Alternatively, the type of the designation information (designated image and designated character string) may be different for each overlay area er. 
     In the examples of  FIGS. 10 and 11 , a case where “Both surfaces” are designated as printing surfaces is exemplified. However, the confidentiality of the printed image can be enhanced even when “Rear surface only” is designated. As described above, there is a case where the original document image formed on the front surface is transparent from the rear surface and the kind of original document image can be determined when the printed image is discharged. In such a case, since the overlay image is formed on the rear surface, it is not easy for the third party to determine the contents of the discharged original document image. 
     As described above, the image forming apparatus  100  of the embodiment includes the control unit (overlaying unit  309 ) and the image forming unit (print unit  305 ). The control unit generates the image data of the overlay image (first image) based on the designated image or the designated character string designated by the user. The image forming unit forms the overlay image in a predetermined area with the decolorizing toner based on the image data. The image forming unit forms the first image on at least one of the front surface and the surface opposite to the front surface. The front surface is the surface, on which the original document image is formed, of the sheet with the original document image (second image) formed using the non-decolorizing toner. 
     The image forming apparatus  100  in the embodiment forms the overlay image based on arbitrary information designated by the user. Thus, the user who recognizes the contents of the original document image can determine the kind of original document image based on the overlay image even when not decolorizing. On the other hand, the third party who does not recognize the contents of the original document image cannot determine the kind of original document image due to the overlay image being formed. 
     Thus, in the image forming apparatus  100 , the third party cannot easily recognize the contents of the printed image. Accordingly, it is possible to reduce the risk of leakage of the contents of the original document image without providing a discharge destination that can be locked in the image forming apparatus  100 . When not decolorizing, it is unnecessary to cut or paint the printed image when discarding the printed image. 
     In the image forming apparatus  100  of the embodiment, the designated image is a partial image of the second image (original document image). Since the overlay image is based on the partial image of the original document image, the user can more easily determine the kind of the original document image. By using the partial image of the original document image, there is no need to newly prepare an overlay image. Thus, the overlay image can be easily generated. 
     The predetermined area in the embodiment is one or more partial areas designated by the user in the area where the second image (original document image) is formed. The user can form an overlay image in an arbitrary area according to the arrangement and amount of confidential contents included in the original document image. The user can form an overlay image only in an area where information is desired to be masked. 
     The control unit of the embodiment generates the image data of the first image (overlay image) having a predetermined image density based on the designated image or the designated character string. Thus, the image forming apparatus  100  can form an overlay image that appropriately masks the original document image regardless of the kind of the designation information. 
     In the above embodiment, the image formation of the original document image and the overlay image is image formed as a series of processes. However, the embodiment is not limited thereto. The image forming apparatus  100  may form only an overlay image on the sheet on which the original document image is already formed. 
     In this case, the scanner unit  306  of the image forming apparatus  100  reads the original document image and generates image data. The overlaying unit  309  receives the designation of an overlay area and designation of a partial image based on the generated image data of the original document image. The print unit  305  forms only an overlay image on the sheet, on which an original document image is already formed, by a decolorizing toner. 
     In the embodiment described above, a case where the user inputs the designation information via the operation panel and display unit  304  is exemplified. However, the designation information may be designated on a terminal device connected to the image forming apparatus  100  via a network. For example, the printer driver which operates by the terminal device receives input of the designation information during transmission of the print job. The printer driver transmits the print job including the designation information to the image forming apparatus  100 . Thus, the user can save the effort of inputting the information via the operation panel and display unit  304 . 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.