Patent Publication Number: US-10332263-B2

Title: Image processing device, image processing method, and non-transitory computer-readable recording medium

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2016-211597, filed on Oct. 28, 2016, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to an image processing device, an image processing method, and a non-transitory computer-readable recording medium. 
     BACKGROUND 
     In recent years, there are many chances to capture documents as a camera. In addition, in many case, optical character recognition (OCR) is performed on the captured document image. 
     Examples of related-art are described in Japanese Laid-open Patent Publication No. 2016-127350 and Japanese Laid-open Patent Publication No. 2010-191952. 
     If the document is captured by a camera, in some cases, color spots or shadow occur in the background in the captured document image, and thus, a degree of recognition for content such as pictures or characters in the document image is deteriorated. 
     SUMMARY 
     According to an aspect of an embodiment, an image processing device includes a division unit which divides an input image into a first content area and a first background area, a generation unit which corrects a first content area pixel included in the first content area by using a first peripheral pixel existing in the vicinity of the first content area among first background area pixels included in the first background area to generate a first background image formed with the after-correction first content area pixel and the first background area pixel, and a removal unit which removes the first background image from the input image. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating a configuration example of an image processing device according to a first embodiment; 
         FIG. 2  is a diagram illustrating an operation example of the image processing device according to the first embodiment; 
         FIG. 3  is a diagram illustrating an operation example of the image processing device according to the first embodiment; 
         FIG. 4  is a diagram illustrating an operation example of the image processing device according to the first embodiment; 
         FIG. 5  is a diagram illustrating an operation example of the image processing device according to the first embodiment; 
         FIG. 6  is a diagram illustrating an operation example of the image processing device according to the first embodiment; 
         FIG. 7  is a diagram illustrating an operation example of the image processing device according to the first embodiment; 
         FIG. 8  is a diagram illustrating an operation example of the image processing device according to the first embodiment; 
         FIG. 9  is a diagram illustrating an operation example of the image processing device according to the first embodiment; 
         FIG. 10  is a diagram illustrating an operation example of the image processing device according to the first embodiment; 
         FIG. 11  is a diagram illustrating an operation example of the image processing device according to the first embodiment; 
         FIG. 12  is a diagram illustrating an operation example of the image processing device according to the first embodiment; 
         FIG. 13  is a diagram illustrating an operation example of a generation unit according to the first embodiment; 
         FIG. 14  is a diagram illustrating an operation example of the generation unit according to the first embodiment; 
         FIG. 15  is a diagram illustrating an operation example of the generation unit according to the first embodiment; 
         FIG. 16  is a diagram illustrating an operation example of the generation unit according to the first embodiment; 
         FIG. 17  is a diagram illustrating an operation example of the generation unit according to the first embodiment; 
         FIG. 18  is a diagram illustrating an operation example of the generation unit according to the first embodiment; 
         FIG. 19  is a diagram illustrating an operation example of a compensation unit according to the first embodiment; 
         FIG. 20  is a flowchart illustrating a process example of the image processing device according to the first embodiment; 
         FIG. 21  is a flowchart illustrating an example of an area dividing process according to the first embodiment; 
         FIG. 22  is a flowchart illustrating an example of a background image generating process according to the first embodiment; 
         FIG. 23  is a diagram illustrating a configuration example of a mobile terminal according to a second embodiment; and 
         FIG. 24  is a diagram illustrating a configuration example of a computer device according to a third embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Preferred embodiments of the present invention will be explained with reference to accompanying drawings. In addition, the image processing device, the image processing method, and the non-transitory computer-readable recording medium to the present disclosure are not limited to the embodiments. In addition, in the embodiments, configurations having the same functions and steps performing the same process are denoted by the same reference numerals. 
     [a] First Embodiment 
     Configuration of Image Processing Device 
       FIG. 1  is a diagram illustrating a configuration example of an image processing device according to a first embodiment. In  FIG. 1 , an image processing device  1  is configured to include a division unit  11 , a generation unit  12 , a removal unit  13 , a restoration unit  14 , and a compensation unit  15 . 
     The division unit  11 , the generation unit  12 , the removal unit  13 , the restoration unit  14 , and the compensation unit  15  may be embodied as hardware, for example, by a processor. As an example of the processor, there may be exemplified a central processing unit (CPU), a digital signal processor (DSP), a field programmable gate array (FPGA), or the like. In addition, the division unit  11 , the generation unit  12 , the removal unit  13 , the restoration unit  14 , and the compensation unit  15  may be embodied by a large scale integrated circuit (LSI) including a processor and peripheral circuits. In addition, the division unit  11 , the generation unit  12 , the removal unit  13 , the restoration unit  14 , and the compensation unit  15  may be embodied by using a graphics processing unit (GPU), an application specific integrated circuit (ASIC), or the like. 
     The division unit  11  divides an image (hereinafter, sometimes referred to as an “input image”) input to the image processing device  1  into an area (hereinafter, sometimes referred to as a “content area”) where a content image exists and an area (hereinafter, sometimes referred to as a “background area”) where no content image exists and information (hereinafter, sometimes referred to as “area information”) indicating the content area and the background area to the generation unit  12 . The input image is, for example, an image captured by a camera. Details of the process in the division unit  11  will be described later. 
     The generation unit  12  corrects pixels (hereinafter, sometimes referred to as “content area pixels”) included in the content area by using pixels (hereinafter, sometimes referred to as “peripheral pixels”) existing in the periphery of the content area among pixels (hereinafter, sometimes referred to as “background area pixels”) included in the background area in the input image on the basis of area information. The generation unit  12  generates an image (hereinafter, sometimes referred to as a “background image”) formed by the after-correction content area pixels (hereinafter, sometimes referred to as “corrected content area pixels”) and the background area pixels by correcting the content area pixels by using the peripheral pixels. The generation unit  12  output the background image generated in this manner to the removal unit  13 . In addition, the generation unit  12  outputs the corrected content area pixels to the compensation unit  15 . Details of the process in the generation unit  12  will be described later. 
     The removal unit  13  removes the background image from the input image. The removal unit  13  outputs an image (hereinafter, sometimes referred to as a “removed image”) obtained by removing the background image from the input image to the restoration unit  14 . Details of the process in the removal unit  13  will be described later. 
     The restoration unit  14  detects an edge of the content image included in the removed image to restore the edge, so that the edge of the content image included in the removed image is emphasized. The restoration unit  14  outputs the removed image including the after-edge-restoration content image to the compensation unit  15 . The detection of the edge in the content image may be performed by using, for example, a Sobel filter, a Prewitt filter, a LOG filter, a Canny method, or the like. 
     The compensation unit  15  compensates for gradation values of the content pixels forming the after-edge-restoration content image by using gradation values of the corrected content area pixels. The compensation unit  15  outputs the after-gradation-value-compensation image as an output image. Details of the process in the compensation unit  15  will be described later. 
     The image processing device  1  does not necessarily include the restoration unit  14 , and in the image processing device  1 , the removed image obtained by the removal unit  13  may be directly input to the compensation unit  15 . 
     In addition, the input image may be an after-noise-removal input image. Noise removal may be performed by using, for example, a moving average filter, a Gaussian smoothing filter, or the like. 
     Operations of Image Processing Device 
       FIGS. 2 to 12  are diagrams illustrating operation examples of the image processing device according to the first embodiment. Among  FIGS. 2 to 12 ,  FIGS. 2, 4, 5, and 7 to 12  are schematic diagrams illustrating images. 
       FIG. 2  illustrates the input image IM input to the image processing device  1 . The input image IM as a content image includes, for example, Characters  1  to  4  and Pictures  1  to  3 . In addition, for example, in the input image IM, a shadowed area (hereinafter, sometimes referred to as a “shadow area”) SD exists. 
     As illustrated in  FIG. 3 , the division unit  11  obtains a histogram HG 1  of the entire input image IM and acquires peaks P 1  and P 2  of the histogram HG 1 . Next, the division unit  11  selects the peak positioned on the rightmost side in the histogram HG 1 , that is, the peak P 2  which is a peak having the maximum gradation value among the acquired peaks P 1  and P 2 . Next, the division unit  11  calculates a gradation value corresponding to a bottom edge of the histogram HG 1  on the left side (namely, the side having a smaller gradation value) as viewed from the peak P 2  as a first background slice BS 1 . For example, the division unit  11  may also calculate a gradation value that first reaches the number of pixels that is less than 1/N (N is an integer) of the number of pixels of the peak P 2  as viewed on the left side from the peak P 2  in the histogram HG 1  as the bottom edge of the histogram HG 1 . In addition, for example, the division unit  11  may also calculate a gradation value of the point where a change in slope of the histogram HG 1  is equal to or more than a threshold value as viewed on the left side from the peak P 2  in the histogram HG 1  as the bottom edge of the histogram HG 1 . 
     Next, as illustrated in  FIG. 4 , the division unit  11  binarizes the input image IM with reference to the first background slice BS 1  and divides the input image IM into a first content area CA 1  and a first background area BA 1  according to the binarization of the input image IM. For example, in the input image IM, the division unit  11  binarizes the input image IM as the pixels having gradation values equal to or more than the first background slice BS 1  as ‘0’ and the pixels having gradation values less than the first background slice BS 1  as ‘1’. In  FIG. 4 , the first content area CA 1  is an area having the gradation values less than the first background slice BS 1  according to the binarization with reference to the first background slice BS 1  and corresponds to Characters  1  and  2 , Pictures  1  and  2 , and the shadow area SD. In addition, in  FIG. 4 , the first background area BA 1  is an area having the gradation values equal to or more than the first background slice BS 1  according to the binarization with reference to the first background slice BS 1 . 
     In this manner, the division unit  11  divides the input image IM into the first content area CA 1  and the first background area BA 1  on the basis of the peaks P 1  and P 2  of the histogram HG 1  of the input image IM. 
     By doing so, it is possible to correctly divide the input image IM into the first content area CA 1  and the first background area BA 1 . 
     In addition, as illustrated in  FIG. 4 , in a case where an area (hereinafter, sometimes referred to as an “edge area”) EA being in contact with an image edge of the input image IM exists in the first content area CA 1 , as illustrated in  FIG. 5 , the division unit  11  extracts an image (hereinafter, sometimes referred to as an “edge image”) EI which exists in the edge area EA from the input image IM. 
     Next, as illustrated in  FIG. 6 , the division unit  11  obtains a histogram HG 2  only in the edge image EI in the input image IM and acquires a peak P 3  of the histogram HG 2 . Since only the peak P 3  exists as the peak of the histogram HG 2 , next, the division unit  11  calculates a gradation value corresponding to a bottom edge of the histogram HG 2  on the left side (namely, the side having a smaller gradation value) as viewed from the peak P 3  as a second background slice BS 2 . For example, the division unit  11  may also calculate a gradation value that first reaches the number of pixels that is less than 1/N (N is an integer) of the number of pixels of the peak P 3  as viewed on the left side from the peak P 3  in the histogram HG 2  as the bottom edge of the histogram HG 2 . In addition, for example, the division unit  11  may also calculate a gradation value of the point where a change in slope of the histogram HG 2  is equal to or more than a threshold value as viewed on the left side from the peak P 3  in the histogram HG 2  as the bottom edge of the histogram HG 2 . 
     Next, as illustrated in  FIG. 7 , the division unit  11  binarizes the edge image EI with respect to the second background slice BS 2  and divides the edge image EI into a second content area CA 2  and a second background area BA 2  according to the binarization of the edge image EI. For example, in the edge image EI, the division unit  11  binarizes the edge image EI as the pixels having gradation values equal to more than the second background slice BS 2  as ‘0’ and the pixels having gradation values less than the second background slice BS 2  as ‘1’. In  FIG. 7 , the second content area CA 2  is an area having the gradation values less than the second background slice BS 2  According to the binarization with reference to the second background slice BS 2  and corresponds to Characters  3  and  4  and Picture  3 . In addition, in  FIG. 7 , the second background area BA 2  is an area having the gradation values equal to or more than the second background slice BS 2  according to the binarization with reference to the second background slice BS 2 . 
     In this manner, in a case where the edge area EA exist in the first content area CA 1 , the division unit  11  further divides the edge image EI into the second content area CA 2  and the second background area BA 2  on the basis of the peak P 3  of the histogram HG 2  of only the edge image EI in the input image IM. 
     By doing so, it is possible to correctly divide the edge image EI into the second content area CA 2  and the second background area BA 2  in the edge area EA corresponding to the shadow area SD. Therefore, the removal unit  13  can remove the shadow from the input image IM irrespective of the position of the shadow occurring in the input image IM. 
     Next, as illustrated in  FIG. 8 , with respect to the area excluding the edge area EA of the input image IM, the generation unit  12  corrects first content area pixels included in the first content area CA 1  by using first peripheral pixels existing in the vicinity of the first content area CA 1  among first background area pixels included in the first background area BA 1 . By the correction, the generation unit  12  generates a first background image formed with first corrected content area pixels CP 1 , CP 2 , and CP 3  and first background area pixels. In addition, in a case where the edge area EA exists in the input image IM, as illustrated in  FIG. 9 , with respect to the edge area EA, the generation unit  12  further corrects second content area pixels included in the second content area CA 2  by using second peripheral pixels existing in the vicinity of the second content area CA 2  among the second background area pixels included in the second background area BA 2 . By the correction, the generation unit  12  generates a second background image formed with second corrected content area pixels CP 4  and CP 5  and second background area pixels. 
     Next, the removal unit  13  removes the first background image from the input image IM. In addition, in a case where the edge area EA exist in the input image IM, the removal unit  13  further removes the second background image from the input image IM. A removed image obtained by removing the first background image and the second background image from the input image IM is illustrated in  FIG. 10 . The removed image illustrated in  FIG. 10  includes content images of Characters  1  to  4  and Pictures  1  to  3 . As a result of the removing process of the removal unit  13 , in the removed image illustrated in  FIG. 10 , the gradation values of the pixels included in the area corresponding to the first background area BA 1  and the second background area BA 2  become the maximum gradation value of 255. Namely, as a result of the removing process of the removal unit  13 , the background of the input image IM becomes white. 
     For example, the removal unit  13  obtains the gradation values of the removed image according to Mathematical Formula (1) every pixel between the input image IM and the first background image and between the input image IM and the second background image and acquires the removed image by subtracting the first background image and the second background image from the input image IM. In a case where a result of calculation of Mathematical Formula (1) is larger than 255, the gradation value of the removed image is set to 255, and in a case where a result of calculation Mathematical Formula (1) is smaller than 0, the gradation value of the removed image is set to 0.
 
Gradation value of removed image=255−{(255−Gradation value of input image)−(255−Gradation value of background image)}  (1)
 
     In comparison with the input image IM, sometimes, in the removed image, the sharpness of the content image may be lost, and the color of the content image may light. 
     Therefore, the restoration unit  14  restores edges of Characters  1  to  4  and Pictures  1  to  3  that are the content images included in the removed image. The restoration unit  14  restores the edges of the content images, for example, by overlapping the previously-generated edge images on the removed image. By the restoration of the restoration unit  14 , the sharpness of the content image is restored in the removed image. An after-edge-restoration content image is illustrated in  FIG. 11 . 
     In addition, the compensation unit  15  compensates for the gradation values of the content pixels forming Characters  1  and  2  in the after-edge-restoration content image by using the gradation values of the first corrected content area pixels CP 1 , compensates for the gradation values of the content pixels forming Picture  1  by using the gradation values of the first corrected content area pixels CP 2 , and compensates for the gradation values of the content pixels forming Picture  2  by using the gradation values of the first corrected content area pixels CP 3 . In addition, in a case where the edge area EA exists in the input image IM, the compensation unit  15  further compensates for the gradation values of the content pixels forming Picture  3  in the after-edge-restoration content image by using the gradation values of the second corrected content area pixels CP 4  and compensates for the gradation values of the content pixels forming Characters  3  and  4  by using the gradation values of the second corrected content area pixels CP 5 . 
     According to the compensation by the compensation unit  15 , Characters  1  to  4  in the removed image are compensated for to be sharper, and the color tones of Pictures  1  to  3  are compensated for to be close to the color tone of the input image IM. An after-gradation-value-compensation image (that is, an output image) is illustrated in  FIG. 12 . 
     Operation of Generation Unit 
       FIGS. 13 to 18  are diagrams illustrating operation examples of the generation unit according to the first embodiment. Hereinafter, examples of the generation unit  12  will be described with reference to the following divided Operation Examples 1 to 8. Hereinafter, among the content area pixels, pixels which are to be corrected by the generation unit  12  are sometimes referred to as “target pixels”. 
     Operation Example 1: FIG.  13   
     Operation Example 1 is an operation example of a case where the first content area CA 1  is surrounded by only the first background area BA 1 , and namely, an operation example of a case where the first peripheral pixels existing in the vicinity of the first content area CA 1  are included in only the first background area BA 1  and are not included in the second background area BA 2 . 
     In Operation Example 1, the generation unit  12  calculates first distances from the first target pixel D st   1 ( x, y ) to the boundary of the first content area CA 1  among the first content area pixels included in the first content area CA 1 , namely, first distances a 1 , b 1 , c 1 , and d 1  between the first target pixel D st   1 ( x, y ) and the first peripheral pixels adjacent to the first content area CA 1 . When the first target pixel D st   1 ( x, y ) is set as the origin, the first distance a 1  is a distance in the +Y direction (upward direction), the first distance b 1  is a distance in the −Y direction (downward direction), the first distance c 1  is a distance in the −X direction (leftward direction), and the first distance d 1  is a distance in the +X direction (rightward direction). 
     In addition, the generation unit  12  sets predetermined ranges adjacent to the first content area CA 1  in the four directions of upward, downward, leftward, and rightward. In  FIG. 13 , the generation unit  12  set ranges of, for example, 5 pixels×5 pixels as the predetermined ranges Up 1 , Down 1 , Left 1 , and Right 1 . 
     The generation unit  12  calculates first average values Src Up   1 ( x, y , Src Down   1 ( x, y ), Src Left   1 ( x, y ), and Src Right   1 ( x, y ) of the gradation values of a plurality of the first peripheral pixels included in the respective predetermined ranges Up 1 , Down 1 , Left 1 , and Right 1  in the respective upward, downward, leftward, and rightward directions. Src Up   1 ( x, y ) is a first average value of the gradation values of the first peripheral pixels in the predetermined range Up 1  in the upward direction, Src Down   1 ( x, y ) is a first average value of the gradation values of the first peripheral pixels in the predetermined range Down 1  in the downward direction, Src Left   1 ( x, y ) is a first average value of the gradation values of the first peripheral pixels in the predetermined range Left 1  in the leftward direction, and Src Right   1 ( x, y ) is a first average value of the gradation values of the first peripheral pixels in the predetermined range Right 1  in the rightward direction. 
     The generation unit  12  corrects the gradation value of the first target pixel D st   1 ( x, y ) on the basis of the first distances a 1 , b 1 , c 1 , and d 1  and the first average values Src Up   1 ( x, y ), Src Down   1 ( x, y ), Src Left   1 ( x, y ), and Src Right   1 ( x, y ) according to Mathematical Formula (2). The generation unit  12  sequentially sets each of all the first content area pixels included in the first content area CA 1  as the first target pixel D st   1 ( x, y ) and corrects the gradation values of the first target pixel D st   1 ( x, y ) according to Mathematical Formula (2), so that all the first content area pixels included in the first content area CA 1  are corrected. 
     
       
         
           
             
               
                 
                   
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     In this manner, in Operation Example 1, the generation unit  12  corrects the first content area pixels by replacing the gradation values of the first content area pixels with weighted average values of the gradation values of the first peripheral pixels according to the first distances a 1 , b 1 , c 1 , and d 1  between the first content area pixels and the first peripheral pixels. 
     By doing so, since it is possible to smoothly connect the color tone of the first content area CA 1  after the correction of the first content area pixels and the color tone of the first background area BA 1 , it is possible to suppress unnatural connection in the boundary between the first content area CA 1  after the correction of the first content area pixels and the first background area BA 1 . 
     In addition, at the time of correction of the first content area pixels, the generation unit  12  calculates the first average values of the gradation values of a plurality of the first peripheral pixels existing in the predetermined ranges Up 1 , Down 1 , Left 1 , and Right 1  adjacent to the first content area CA 1  and obtains weighted averages of the gradation values of the first peripheral pixels by using the calculated first average values. 
     By doing so, it is possible to further smoothly connect the color tone of the first content area CA 1  after the correction of the first content area pixels and the color tone of the first background area BA 1 . 
     Operation Example 2: FIG.  14   
     Operation Example 2 is an example of a case where the first content area CA 1  is surrounded by both of the first background area BA 1  and the second background area BA 2 , and namely, an operation example of a case where the first peripheral pixels existing in the vicinity of the first content area CA 1  are included in both of the first background area BA 1  and the second background area BA 2 . 
     As illustrated in  FIG. 14 , Operation Example 2 is different from Operation Example 1 in that the entire predetermined range Down 1  in the downward direction is included in the second background area BA 2  (namely, the shadow area SD). The generation unit  12  corrects the gradation value of the first target pixel D st   1 ( x, y ) according to Mathematical Formula (3). In Mathematical Formula (3), the first distance b 1  in the downward direction and the first average value Src Down   1 ( x, y ) in the downward direction do not exist. Therefore, the generation unit  12  calculates the first distances a 1 , c 1 , and d 1  and the first average values Src Up   1 ( x, y ), Src Left   1 ( x, y ), and Src Right   1 ( x, y ), but it does not calculate the first distance b 1  and the first average value Src Down   1 ( x, y ). Namely, Operation Example 2 is the same as Operation Example 1 in that the generation unit  12  corrects the gradation values of the first target pixel D st   1 ( x, y ) by using the 1 distances a 1 , c 1 , and d 1  and the first average values Src Up   1 ( x, y ), Src Left   1 ( x, y ), and Src Right   1 ( x, y ), but it is different from Operation Example 1 in that the generation unit  12  corrects the gradation values of the first target pixel D st   1 ( x, y ) not by using the first distance b 1  and the first average value Src Down   1 ( x, y ). 
     
       
         
           
             
               
                 
                   
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                           ( 
                           
                             
                               1 
                               / 
                               d 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                           ) 
                         
                       
                     
                     × 
                     
                       [ 
                       
                         
                           { 
                           
                             
                               1 
                               / 
                               a 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                             × 
                             
                               Src 
                               Up 
                             
                             ⁢ 
                             1 
                             ⁢ 
                             
                               ( 
                               
                                 x 
                                 , 
                                 y 
                               
                               ) 
                             
                           
                           } 
                         
                         + 
                         
                           { 
                           
                             
                               1 
                               / 
                               c 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                             × 
                             
                               Src 
                               Left 
                             
                             ⁢ 
                             1 
                             ⁢ 
                             
                               ( 
                               
                                 x 
                                 , 
                                 y 
                               
                               ) 
                             
                           
                           } 
                         
                         + 
                         
                           { 
                           
                             
                               1 
                               / 
                               d 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                             × 
                             
                               Src 
                               Right 
                             
                             ⁢ 
                             1 
                             ⁢ 
                             
                               ( 
                               
                                 x 
                                 , 
                                 y 
                               
                               ) 
                             
                           
                           } 
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     In this manner, in Operation Example 2, in a case where the first peripheral pixels are included in both of the first background area BA 1  and the second background area BA 2 , the generation unit  12  corrects the first content area pixels by using the first peripheral pixels included in the first background area BA 1  but not by using the first peripheral pixels included in the second background area BA 2 . 
     By doing so, even in a case where the first content area CA 1  is surrounded by both of the first background area BA 1  and the second background area BA 2 , it is possible to smoothly connect the color tone of the first content area CA 1  after the correction of the first content area pixels and the color tone of the first background area BA 1 . 
     Operation Example 3: FIG.  15   
     Similarly to Operation Example 2, Operation Example 3 is an example of a case where the first content area CA 1  is surrounded by both of the first background area BA 1  and the second background area BA 2 , and namely, an operation example of a case where the first peripheral pixels existing in the vicinity of the first content area CA 1  are included in both of the first background area BA 1  and the second background area BA 2 . 
     As illustrated in  FIG. 15 , Operation Example 3 is different from Operation Example 1 in that the entire predetermined range Down 1  in the downward direction is included in the second background area BA 2  (namely, the shadow area SD) and a portion of the predetermined range Left 1  in the leftward direction and a portion of the predetermined range Right 1  in the rightward direction are included in the second background area BA 2 . Similarly to Operation Example 2, the generation unit  12  corrects the gradation value of the first target pixel D st   1 ( x, y ) according to the above-described Mathematical Formula (3). Operation Example 3 is different from Operation Example 2 in that, when the generation unit  12  calculates the first average value Src Left   1 ( x, y ) in the leftward direction and the first average value Src Right   1 ( x, y ) in the rightward direction, the generation unit uses the first peripheral pixels included in the first background area BA 1 , but it does not use the first peripheral pixels included in the second background area BA 2 . 
     In this manner, in Operation Example 3, similarly to Operation Example 2, in a case where the first peripheral pixels are included in both of the first background area BA 1  and the second background area BA 2 , the generation unit  12  corrects the first content area pixels by using the first peripheral pixels included in the first background area BA 1  but not by using the first peripheral pixels included in the second background area BA 2 . 
     Operation Example 4: FIG.  14  and FIG.  15   
     Similarly to Operation Example 2, Operation Example 4 is an example of a case where the first content area CA 1  is surrounded by both of the first background area BA 1  and the second background area BA 2 , and namely, an operation example of a case where the first peripheral pixels existing in the vicinity of the first content area CA 1  are included in both of the first background area BA 1  and the second background area BA 2 . 
     Operation Example 4 is different from Operation Example 1 in that the generation unit  12  corrects the gradation value of the first target pixel D st   1 ( x, y ) according to Mathematical Formula (4). In Mathematical Formula (4), W Up   1  is a weighted value with respect to the first average value Src Up   1 ( x, y ) in the upward direction, W Down   1  is a weighted value with respect to the first average value Src Down   1 ( x, y ) in the downward direction, W Left   1  is a weighted value with respect to the first average value Src Left   1 ( x, y ) in the leftward direction, and W Right   1  is a weighted value with respect to the first average value Src Right   1 ( x, y ) in the rightward direction. 
     
       
         
           
             
               
                 
                   
                     Graduation 
                     ⁢ 
                     
                       
                           
                       
                       ⁢ 
                       
                           
                       
                     
                     ⁢ 
                     value 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     of 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       D 
                       st 
                     
                     ⁢ 
                     1 
                     ⁢ 
                     
                       ( 
                       
                         x 
                         , 
                         y 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           a 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                         + 
                         
                           b 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                         + 
                         
                           c 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                         + 
                         
                           d 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                       
                       
                         
                           ( 
                           
                             
                               1 
                               / 
                               a 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                           ) 
                         
                         + 
                         
                           ( 
                           
                             
                               1 
                               / 
                               b 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                           ) 
                         
                         + 
                         
                           ( 
                           
                             
                               1 
                               / 
                               c 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                           ) 
                         
                         + 
                         
                           ( 
                           
                             
                               1 
                               / 
                               d 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                           ) 
                         
                       
                     
                     × 
                     
                         
                       
                         [ 
                         
                           
                             { 
                             
                               
                                 W 
                                 Up 
                               
                               ⁢ 
                               
                                 1 
                                 / 
                                 a 
                               
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                               × 
                               
                                 Src 
                                 Up 
                               
                               ⁢ 
                               1 
                               ⁢ 
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                             } 
                           
                           + 
                           
                             { 
                             
                               
                                 W 
                                 Down 
                               
                               ⁢ 
                               
                                 1 
                                 / 
                                 b 
                               
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                               × 
                               
                                 Src 
                                 Down 
                               
                               ⁢ 
                               1 
                               ⁢ 
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                             } 
                           
                           + 
                           
                             { 
                             
                               
                                 W 
                                 Left 
                               
                               ⁢ 
                               
                                 1 
                                 / 
                                 c 
                               
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                               × 
                               
                                 Src 
                                 Left 
                               
                               ⁢ 
                               1 
                               ⁢ 
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                             } 
                           
                           + 
                           
                             { 
                             
                               
                                 W 
                                 Right 
                               
                               ⁢ 
                               
                                 1 
                                 / 
                                 d 
                               
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                               × 
                               
                                 Src 
                                 Right 
                               
                               ⁢ 
                               1 
                               ⁢ 
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                             } 
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     For example, as illustrated in  FIG. 14 , in a case where the entire predetermined range Up 1  in the upward direction, the entire predetermined range Left 1  in the leftward direction, and the entire predetermined range Right 1  in the rightward direction are included in the first background area BA 1  and the entire predetermined range Down 1  in the downward direction is included in the second background area BA 2 , the generation unit  12  sets the values of W Up   1 , W Left   1 , and W Right   1  to be larger than the value of W Down   1  in Mathematical Formula (4) (W Up   1 , W Left   1 , W Right   1 &gt;W Down   1 ). 
     In addition, for example, as illustrated in  FIG. 15 , in a case where the entire predetermined range Up 1  in the upward direction is included in the first background area BA 1 , the entire predetermined range Down 1  in the downward direction is included in the second background area BA 2 , and a portion of the predetermined range Left 1  in the leftward direction and a portion of the predetermined range Right 1  in the rightward direction are included in the second background area BA 2 , the generation unit  12  sets the values of W Up   1 , W Left   1 , and W Right   1  to be larger than the value of W Down   1  and sets the value of W Up   1  to be larger than the values of W Left1  and W Right   1  (W Up   1 &gt;W Left   1 , W Right   1 &gt;W Down   1 ) in Mathematical Formula (4). 
     In this manner, in Operation Example 4, in a case where the first peripheral pixels are included in both of the first background area BA 1  and the second background area BA 2 , the generation unit  12  sets a weighting factor of the gradation values of the first peripheral pixels included in the first background area BA 1  to be larger than a weighting factor of the gradation values of the first peripheral pixels included in the second background area BA 2  and corrects the first content area pixels by replacing the gradation values of the first content area pixels with weighted average values of the gradation values of the first peripheral pixels according to the first distances a 1 , b 1 , c 1 , and d 1  between the first content area pixels and the first peripheral pixels. 
     By doing so, in a case where the first content area CA 1  is surrounded by both of the first background area BA 1  and the second background area BA 2 , by the correction of the first content area pixels, it is possible to allow a degree of influence of the first peripheral pixels included in the first background area BA 1  to be larger than a degree of influence of the first peripheral pixels included in the second background area BA 2 . Therefore, it is possible to suppress a change in color tone in the boundary between the first content area CA 1  after the correction of the first content area pixels and the first background area BA 1 . 
     Heretofore, Operation Examples 1 to 4 are described. 
     Next, Operation Examples 5 to 8 will be described. Operation Examples 5 to 8 are operation examples of a case where the edge area EA exists in the first content area CA 1 . 
     Operation Example 5: FIG.  16   
     Operation Example 5 is an operation example of a case where the second content area CA 2  is surrounded by only the second background area BA 2 , and namely, an operation example of a case where the second peripheral pixels existing in the vicinity of the second content area CA 2  are included in only the second background area BA 2  and are not included in the first background area BA 1 . 
     In Operation Example 5, the generation unit  12  calculates second distances from the second target pixel D st   2 ( x, y ) to the boundary of the second content area CA 2  among the second content area pixels included in the second content area CA 2 , namely, second distances a 2 , b 2 , c 2 , and d 2  between the second target pixel D st   2 ( x, y ) and the second peripheral pixels adjacent to the second content area CA 2 . When the second target pixel D st   2 ( x, y ) is set as the origin, the second distance a 2  is a distance in the +Y direction (upward direction), the second distance b 2  is a distance in the −Y direction (downward direction), the second distance c 2  is a distance in the −X direction (leftward direction), and the second distance d 2  is a distance in the +X direction (rightward direction). 
     In addition, the generation unit  12  sets predetermined ranges adjacent to the second content area CA 2  in the four directions of upward, downward, leftward, and rightward. In  FIG. 16 , the generation unit  12  set ranges of, for example, 5 pixels×5 pixels as the predetermined ranges Up 2 , Down 2 , Left 2 , and Right 2 . 
     The generation unit  12  generates second average values Src Up   2 ( x, y ), Src Down   2 ( x, y ), Src Left   2 ( x, y ), and Src Right   2 ( x, y ) of the gradation values of a plurality of the second peripheral pixels included in the respective predetermined ranges Up 2 , Down 2 , Left 2 , and Right 2  in the respective upward, downward, leftward, and rightward directions. Src Up   2 ( x, y ) is a second average value of the gradation values of the second peripheral pixels in the predetermined range Up 2  in the upward direction, Src Down   2 ( x, y ) is a second average value of the gradation values of the second peripheral pixels in the predetermined range Down 2  in the downward direction, Src Left   2 ( x, y ) is a second average value of the gradation values of the second peripheral pixels in the predetermined range Left 2  in the leftward direction, and Src Right   2 ( x, y ) is a second average value of the gradation values of the second peripheral pixels in the predetermined range Right 2  in the rightward direction. 
     The generation unit  12  corrects the gradation values of the second target pixel D st   2 ( x, y ) on the basis of the second distances a 2 , b 2 , c 2 , and d 2  and the second average values Src Up   2 ( x, y ), Src Down   2 ( x, y ), Src Left   2 ( x, y ), and Src Right   2 ( x, y ) according to Mathematical Formula (5). The generation unit  12  sequentially sets each of all the second content area pixels included in the second content area CA 2  as the second target pixel D st   2 ( x, y ) and corrects the gradation values of the second target pixel D st   2 ( x, y ) according to Mathematical Formula (5), so that all the second content area pixels included in the second content area CA 2  are corrected. 
     
       
         
           
             
               
                 
                   
                     Graduation 
                     ⁢ 
                     
                       
                           
                       
                       ⁢ 
                       
                           
                       
                     
                     ⁢ 
                     value 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     of 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       D 
                       st 
                     
                     ⁢ 
                     2 
                     ⁢ 
                     
                       ( 
                       
                         x 
                         , 
                         y 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           a 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           2 
                         
                         + 
                         
                           b 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           2 
                         
                         + 
                         
                           c 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           2 
                         
                         + 
                         
                           d 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           2 
                         
                       
                       
                         
                           ( 
                           
                             
                               1 
                               / 
                               a 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                           
                           ) 
                         
                         + 
                         
                           ( 
                           
                             
                               1 
                               / 
                               b 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                           
                           ) 
                         
                         + 
                         
                           ( 
                           
                             
                               1 
                               / 
                               c 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                           
                           ) 
                         
                         + 
                         
                           ( 
                           
                             
                               1 
                               / 
                               d 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                           
                           ) 
                         
                       
                     
                     × 
                     
                         
                       
                         [ 
                         
                           
                             { 
                             
                               
                                 1 
                                 / 
                                 a 
                               
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               2 
                               × 
                               
                                 Src 
                                 Up 
                               
                               ⁢ 
                               2 
                               ⁢ 
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                             } 
                           
                           + 
                           
                             { 
                             
                               
                                 1 
                                 / 
                                 b 
                               
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               2 
                               × 
                               
                                 Src 
                                 Down 
                               
                               ⁢ 
                               2 
                               ⁢ 
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                             } 
                           
                           + 
                           
                             { 
                             
                               
                                 1 
                                 / 
                                 c 
                               
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               2 
                               × 
                               
                                 Src 
                                 Left 
                               
                               ⁢ 
                               2 
                               ⁢ 
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                             } 
                           
                           + 
                           
                             { 
                             
                               
                                 1 
                                 / 
                                 d 
                               
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               2 
                               × 
                               
                                 Src 
                                 Right 
                               
                               ⁢ 
                               2 
                               ⁢ 
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                             } 
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     In this manner, in Operation Example 5, the generation unit  12  corrects the second content area pixels by replacing the gradation values of the second content area pixels with weighted average values of the gradation values of the second peripheral pixels according to the second distances a 2 , b 2 , c 2 , and d 2  between the second content area pixels and the second peripheral pixels. 
     By doing so, since it is possible to smoothly connect the color tone of the second content area CA 2  after the correction of the second content area pixels and the color tone of the second background area BA 2 , it is possible to suppress unnatural connection in the boundary between the second content area CA 2  after the correction of the second content area pixels and the second background area BA 2 . 
     In addition, at the time of correction of the second content area pixels, the generation unit  12  calculates the second average values of the gradation values of a plurality of the second peripheral pixels existing in the predetermined ranges Up 2 , Down 2 , Left 2 , and Right 2  adjacent to the second content area CA 2  and obtains weighted averages of the gradation values of the second peripheral pixels by using the calculated second average values. 
     By doing so, it is possible to further smoothly connect the color tone of the second content area CA 2  after the correction of the second content area pixels and the color tone of the second background area BA 2 . 
     Operation Example 6: FIG.  17   
     Operation Example 6 is an example of a case where the second content area CA 2  is surrounded by both of the first background area BA 1  and the second background area BA 2 , and namely, an operation example of a case where the second peripheral pixels existing in the vicinity of the second content area CA 2  are included in both of the first background area BA 1  and the second background area BA 2 . 
     As illustrated in  FIG. 17 , Operation Example 6 is different from Operation Example 5 in that the entire predetermined range Up 2  in the upward direction is included in the first background area BA 1 . The generation unit  12  corrects the gradation value of the second target pixel D st   2 ( x, y ) according to Mathematical Formula (6). In Mathematical Formula (6), the second distance a 2  in the upward direction and the second average value Src Up   2 ( x, y ) in the upward direction do not exist. Therefore, the generation unit  12  calculates the second distances b 2 , c 2 , and d 2  and the second average value Src Down   2 ( x, y ), Src Left   2 ( x, y ), and Src Right   2 ( x, y ), but it does not calculates the second distance a 2  and the second average value Src Up   2 ( x, y ). Namely, Operation Example 6 is the same as Operation Example 2 in that the generation unit  12  corrects the gradation value of the second target pixel D st   2 ( x, y ) by using the second distances b 2 , c 2 , and d 2  and the second average values Src Down   2 ( x, y ), Src Left   2 ( x, y ), and Src Right   2 ( x, y ), but it is different from Operation Example 5 in that the generation unit  12  corrects the gradation value of the second target pixel D st   2 ( x, y ) not by using the second distance a 2  and the second average value Src Up   2 ( x, y ). 
     
       
         
           
             
               
                 
                   
                     Graduation 
                     ⁢ 
                     
                       
                           
                       
                       ⁢ 
                       
                           
                       
                     
                     ⁢ 
                     value 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     of 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       D 
                       st 
                     
                     ⁢ 
                     2 
                     ⁢ 
                     
                       ( 
                       
                         x 
                         , 
                         y 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           a 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           2 
                         
                         + 
                         
                           c 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           2 
                         
                         + 
                         
                           d 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           2 
                         
                       
                       
                         
                           ( 
                           
                             
                               1 
                               / 
                               a 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                           
                           ) 
                         
                         + 
                         
                           ( 
                           
                             
                               1 
                               / 
                               c 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                           
                           ) 
                         
                         + 
                         
                           ( 
                           
                             
                               1 
                               / 
                               d 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                           
                           ) 
                         
                       
                     
                     × 
                     
                       [ 
                       
                         
                           { 
                           
                             
                               1 
                               / 
                               b 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                             × 
                             
                               Src 
                               Down 
                             
                             ⁢ 
                             2 
                             ⁢ 
                             
                               ( 
                               
                                 x 
                                 , 
                                 y 
                               
                               ) 
                             
                           
                           } 
                         
                         + 
                         
                           { 
                           
                             
                               1 
                               / 
                               c 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                             × 
                             
                               Src 
                               Left 
                             
                             ⁢ 
                             2 
                             ⁢ 
                             
                               ( 
                               
                                 x 
                                 , 
                                 y 
                               
                               ) 
                             
                           
                           } 
                         
                         + 
                         
                           { 
                           
                             
                               1 
                               / 
                               d 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                             × 
                             
                               Src 
                               Right 
                             
                             ⁢ 
                             2 
                             ⁢ 
                             
                               ( 
                               
                                 x 
                                 , 
                                 y 
                               
                               ) 
                             
                           
                           } 
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     In this manner, in Operation Example 6, in a case where the second peripheral pixels are included in both of the first background area BA 1  and the second background area BA 2 , the generation unit  12  corrects the second content area pixels by using the second peripheral pixels included in the second background area BA 2  but not by using the second peripheral pixels included in the first background area BA 1 . 
     By doing so, even in a case where the second content area CA 2  is surrounded by both of the first background area BA 1  and the second background area BA 2 , it is possible to smoothly connect the color tone of the second content area CA 2  after the correction of the second content area pixels and the color tone of the second background area BA 2 . 
     Operation Example 7: FIG.  18   
     Similarly to Operation Example 6, Operation Example 7 is an example of a case where the second content area CA 2  is surrounded by both of the first background area BA 1  and the second background area BA 2 , and namely, an operation example of a case where the second peripheral pixels existing in the vicinity of the second content area CA 2  are included in both of the first background area BA 1  and the second background area BA 2 . 
     As illustrated in  FIG. 18 , Operation Example 7 is different from Operation Example 5 in that the entire predetermined range Up 2  in the upward direction is included in the first background area BA 1  and a portion of the predetermined range Left 2  in the leftward direction and a portion of the predetermined range Right 2  in the rightward direction are included in the first background area BA 1 . Similarly to Operation Example 6, the generation unit  12  corrects the gradation value of the second target pixel D st   2 ( x, y ) according to the above-described Mathematical Formula (6). Operation Example 7 is different from Operation Example 6 in that, when the generation unit  12  calculates the second average value Src Left   2 ( x, y ) in the leftward direction and the second average value Src Right   2 ( x, y ) in the rightward direction, the generation unit uses the second peripheral pixels included in the second background area BA 2 , but it does not use the second peripheral pixels included in the first background area BA 1 . 
     In this manner, in Operation Example 7, similarly to Operation Example 6, in a case where the second peripheral pixels are included in both of the first background area BA 1  and the second background area BA 2 , the generation unit  12  corrects the second content area pixels by using the second peripheral pixels included in the second background area BA 2  but not by using the second peripheral pixels included in the first background area BA 1 . 
     Operation Example 8: FIG.  17  and FIG.  18   
     Similarly to Operation Example 6, Operation Example 8 is an example of a case where the second content area CA 2  is surrounded by both of the first background area BA 1  and the second background area BA 2 , and namely, an operation example of a case where the second peripheral pixels existing in the vicinity of the second content area CA 2  are included in both of the first background area BA 1  and the second background area BA 2 . 
     Operation Example 8 is different from Operation Example 5 in that, the generation unit  12  corrects the gradation value of the second target pixel D st   2 ( x, y ) according to Mathematical Formula (7). In Mathematical Formula (7), W Up   2  is a weighted value with respect to the second average value Src Up   2 ( x, y ) in the upward direction, W Down   2  is a weighted value with respect to the second average value Src Down   2 ( x, y ) in the downward direction, W Left   2  is a weighted value with respect to the second average value Src Left   2 ( x, y ) in the leftward direction, and W Right   2  is a weighted value with respect to the second average value Src Right   2 ( x, y ) in the rightward direction. 
     
       
         
           
             
               
                 
                   
                     Graduation 
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     For example, as illustrated in  FIG. 17 , in a case where the entire predetermined range Down 2  in the downward direction, the entire predetermined range Left 2  in the leftward direction, and the entire predetermined range Right 2  in the rightward direction are included in the second background area BA 2  and the entire predetermined range Up 2  in the upward direction is included in the first background area BA 1 , the generation unit  12  sets the values of W Down   2 , W Left   2 , and W Right   2  to be larger than the value of W Up   2  (W Down   2 , W Left   2 , W Right   2 &gt;W Up   2 ) in Mathematical Formula (7). 
     In addition, for example, as illustrated in  FIG. 18 , in a case where the entire predetermined range Down 2  in the downward direction is included in the second background area BA 2 , the entire predetermined range Up 2  in the upward direction is included in the first background area BA 1 , and a portion of the predetermined range Left 2  in the leftward direction and a portion of the predetermined range Right 2  in the rightward direction are included in the first background area BA 1 , the generation unit  12  sets the values of W Down   2 , W Left   2 , and W Right   2  to be larger than the value of W Up   2  and sets the value of W Down   2  to be larger than the values of W Left   2  and W Right   2  (W Down   2 &gt;W Left   2 , W Right   2 &gt;W Up   2 ) in Mathematical Formula (7). 
     In this manner, in Operation Example 8, in a case where the second peripheral pixels are included in both of the first background area BA 1  and the second background area BA 2 , the generation unit  12  sets a weighting factor of the gradation values of the second peripheral pixels included in the second background area BA 2  to be larger than a weighting factor of the gradation values of the second peripheral pixels included in the first background area BA 1  and corrects the second content area pixels by replacing the gradation values of the second content area pixels with weighted average values of the gradation values of the second peripheral pixels according to the second distances a 2 , b 2 , c 2 , and d 2  between the second content area pixels and the second peripheral pixels. 
     By doing so, in a case where the second content area CA 2  is surrounded by both of the first background area BA 1  and the second background area BA 2 , by the correction of the second content area pixels, it is possible to allow a degree of influence of the second peripheral pixels included in the second background area BA 2  to be larger than a degree of influence of the second peripheral pixels included in the first background area BA 1 . Therefore, it is possible to suppress a change in color tone in the boundary between the second content area CA 2  after the correction of the second content area pixels and the second background area BA 2 . 
     Operation of Compensation Unit 
       FIG. 19  is a diagram illustrating an operation example of the compensation unit according to the first embodiment. In  FIG. 19 , V(x 0 , y 0 ) are gradation values of the first corrected content area pixel CP 1 , CP 2 , and CP 3 , and V(x m , y n ) are gradation values of the second corrected content area pixel CP 4  and CP 5 . 
     The compensation unit  15  calculates shadow values S(x 0 , y 0 ) of the first corrected content area pixel CP 1 , CP 2 , and CP 3  by subtracting the gradation values V(x 0 , y 0 ) from the maximum gradation value of 255. Next, the compensation unit  15  generates a tone curve TC 1  illustrated in  FIG. 19  on the basis of the calculated shadow values S(x 0 , y 0 ). Highlight H in  FIG. 19  is set, for example, as  242  that is the value of 95% of the maximum gradation value of 255. The compensation unit  15  calculates after-compensation gradation values of first content pixels by applying the tone curve TC 1  to the gradation values of the first content pixels forming a first content image (namely, a content image of Characters  1  and  2  and Pictures  1  and  2  illustrated in  FIG. 10 ) included in a first removed image obtained by the removal unit  13  removing the first background image from the input image IM. With respect to all the first content pixels, the compensation unit  15  calculates the after-compensation gradation value for every first content pixel by using the tone curve TC 1 . Namely, the compensation unit  15  compensates for the gradation values of the first content pixels forming a first content image included in the first removed image obtained by the removal unit  13  removing the first background image from the input image IM by using the gradation values of first corrected content area pixels. 
     In addition, in a case where the edge area EA exists in the input image IM, the compensation unit  15  calculates shadow values S(x m , y n ) of the second corrected content area pixels CP 4  and CP 5  by subtracting the gradation values V(x m , y n ) from the maximum gradation value of 255. Next, the compensation unit  15  generates a tone curve TC 2  illustrated in  FIG. 19  on the basis of the calculated shadow values S(x m , y n ). In general, since the gradation value V(x 0 , y 0 ) is larger than the gradation value V(x m , y n ), the slop of the tone curve TC 2  is larger than the slope of the tone curve TC 1 . The compensation unit  15  calculates after-compensation gradation values of the second content pixels by applying the tone curve TC 2  to the gradation values of the second content pixels forming a second content image (namely, a content image of Characters  3  and  4  and Picture  3  illustrated in  FIG. 10 ) included in a second removed image obtained by the removal unit  13  removing the second background image from the input image IM. With respect to all the second content pixels, the compensation unit  15  calculates the after-compensation gradation value for every second content pixel by using the tone curve TC 2 . Namely, the compensation unit  15  compensates for the gradation values of the second content pixels forming a second content image included in the second removed image obtained by the removal unit  13  removing the second background image from the input image IM by using the gradation values of the second corrected content area pixels. 
     According to the compensation of the compensation unit  15 , since the gradation values in the content area are decreased by an amount increased according to the removal of the background image, it is possible to compensate for the color tone of the content image so as to be close to the color tone of the input image IM. 
     Processes of Image Processing Device 
       FIG. 20  is a flowchart illustrating a process example of the image processing device according to the first embodiment. 
     In  FIG. 20 , in step S 11 , the division unit  11  performs an area dividing process.  FIG. 21  is a flowchart illustrating an example of the area dividing process according to the first embodiment. 
     In  FIG. 21 , in step S 31 , the division unit  11  obtains a histogram HG 1  in the entire input image IM. 
     Next, in step S 33 , the division unit  11  calculates a first background slice BS 1  on the basis of peaks of the histogram HG 1 . 
     Next, in step S 35 , the division unit  11  divides the input image IM into a first content area CA 1  and a first background area BA 1  on the basis of the first background slice BS 1 . 
     Next, in step S 37 , the division unit  11  determines whether or not a first content area CA 1  being in contact with an image edge of the input image IM exists. In a case where the first content area CA 1  being in contact with the image edge of the input image IM does not exist (No in step S 37 ), the process proceeds to step S 39 , and in a case where the first content area CA 1  being in contact with the image edge of the input image IM exists (Yes in step S 37 ), the process proceeds to step S 41 . 
     In step S 39 , the division unit  11  sets an existence flag to FALSE. By the process in step S 39 , the area dividing process is ended. 
     On the other hand, in step S 41 , the division unit  11  sets the existence flag to TRUE. 
     Next, in step S 43 , the division unit  11  obtains a histogram HG 2  in only the edge image EI in the input image IM. 
     Next, in step S 45 , the division unit  11  calculates a second background slice BS 2  on the basis of peaks of the histogram HG 2 . 
     Next, in step S 47 , the division unit  11  divides the edge image EI into a second content area CA 2  and a second background area BA 2  on the basis of the second background slice BS 2 . By the process in step S 47 , the area dividing process is ended. 
     Returning to  FIG. 20 , in step S 13 , the generation unit  12  performs a background image generating process.  FIG. 22  is a flowchart illustrating an example of the background image generating process according to the first embodiment. 
     In  FIG. 22 , in step S 51 , the generation unit  12  generates a first background image by correcting first content area pixels included in the first content area CA 1  by using first peripheral pixels existing in the vicinity of the first content area CA 1 . 
     Next, in step S 53 , the generation unit  12  determines whether or not the existence flag is TRUE. 
     In a case where the existence flag is TRUE (Yes in step S 53 ), the process proceeds to step S 55 , and in step S 55 , the generation unit  12  generates a second background image by correcting second content area pixels included in the second content area CA 2  by using second peripheral pixels existing in the vicinity of the second content area CA 2 . By the process in step S 55 , the background image generating process is ended. 
     On the other hand, in a case where the existence flag is not TRUE, namely, in a case where the existence flag is FALSE (No in step S 53 ), without performing the process in step S 55 , the background image generating process is ended. 
     Returning to  FIG. 20 , in step S 15 , the removal unit  13  removes the first background image from the input image IM. In addition, in a case where the existence flag is TRUE, the removal unit  13  further removes the second background image from the input image IM. 
     Next, in step S 17 , the restoration unit  14  restores an edge of a content image included in a removed image. 
     Next, in step S 19 , the compensation unit  15  compensates for gradation values of an after-edge-restoration content image. 
     As described above, in the first embodiment, the image processing device  1  is configured to include the division unit  11 , the generation unit  12 , and the removal unit  13 . The division unit  11  divides the input image IM into the first content area CA 1  and the first background area BA 1 . The generation unit  12  corrects the first content area pixels included in the first content area CA 1  by using the first peripheral pixels existing in the vicinity of the first content area CA 1  among the first background area pixels included in the first background area BA 1 . By the correction, the generation unit  12  generates the first background image formed with the after-correction first content area pixels and the first background area pixels. The removal unit  13  removes the first background image from the input image IM. 
     By doing so, since the background except for the content in the input image IM becomes white, the content are emphasized as contrasted with the background, and thus, it is possible to improve a degree of recognition for the content in the input image IM. Therefore, for example, in a case where the content is a character, accuracy of character recognition by OCR is improved. In addition, since the background image is generated by correcting the content area pixels by using the peripheral pixels, it is possible to generate a smooth background image. 
     In addition, in the first embodiment, in a case where the area EA being in contact with the image edge of the input image IM exists in the first content area CA 1 , the division unit  11 , the generation unit  12 , and the removal unit  13  further perform the following processes. Namely, the division unit  11  further divides the edge image existing in the edge area EA into the second content area CA 2  and the second background area BA 2 . The generation unit  12  further corrects the second content area pixels included in the second content area CA 2  by using second peripheral pixels existing in the vicinity of the second content area CA 2  among the second background area pixels included in the second background area BA 2 . By the correction, the generation unit  12  generates the second background image formed with the after-correction second content area pixels and the second background area pixels. The removal unit  13  further removes the second background image from the input image IM. 
     By doing so, with respect to the edge area EA corresponding to the shadow area SD, since the background (namely, shadow) except for the content becomes white, it is possible to improve a degree of recognition for the content in the shadow area SD. 
     [b] Second Embodiment 
       FIG. 23  is a diagram illustrating a configuration example of a mobile terminal according to a second embodiment. In  FIG. 23 , a mobile terminal  10  is configured to include an image processing device  1 , a storage unit  101 , a capturing unit  102 , a manipulation unit  103 , a display unit  104 , and a communication unit  105 . The storage unit  101  is implemented as hardware, for example, by a memory. As an example of the memory, there may be exemplified random access memory (RAM) such as synchronous dynamic random access memory (SDRAM), read only memory (ROM), flash memory, or the like. The capturing unit  102  is implemented as hardware, for example, by a camera. The manipulation unit  103  and the display unit  104  are implemented as hardware, for example, by a touch panel. The communication unit  105  is implemented as hardware, for example, by a wireless communication module or a network interface module. 
     In the mobile terminal  10 , for example, an image captured by the capturing unit  102  or an image stored in the storage unit  101  becomes an input image of the image processing device  1 . In addition, an output image from the image processing device  1  is stored in the storage unit  101  or displayed on the display unit  104 . 
     [c] Third Embodiment 
       FIG. 24  is a diagram illustrating a configuration example of a computer device according to a third embodiment. A computer device  20  illustrated in  FIG. 24  is, for example, a server. In  FIG. 24 , the computer device  20  is configured to include an image processing device  1 , a storage unit  201 , and a communication unit  202 . The storage unit  201  is implemented as hardware, for example, by a memory, a Hard Disk Drive (HDD), a solid state drive (SSD), or the like. As an example of the memory, there may be exemplified RAM such as SDRAM, ROM, flash memory, or the like. The communication unit  202  is implemented as hardware, for example, by a wireless communication module or a network interface module. 
     In the computer device  20 , for example, an image received from the mobile terminal  10  by the communication unit  202  or an image stored in the storage unit  201  becomes an input image of the image processing device  1 . In addition, an output image from the image processing device  1  is stored in the storage unit  201  or transmitted to the mobile terminal  10  by the communication unit  202 . 
     [d] Other Embodiments 
     [1] All or a portion of each of the above-described processes in the image processing device  1  may be implemented by causing a processor included in the image processing device  1  to execute a program corresponding to each process. For example, the program corresponding to each of the above-described processes may be stored in a memory, and the program may be read from the memory to be executed by the processor. In addition, the program may be stored in a program server connected to the mobile terminal  10  or the computer device  20  via an arbitrary network, and the program may be downloaded from the program server to the mobile terminal  10  or the computer device  20  to be executed. The program may be stored in a recording medium that can be read by the computer device  20 , and the program may be read from the recording medium to be executed. The recording medium that can be read by the computer device  20  includes a portable storage medium such as a memory card, a USB memory, an SD card, a flexible disk, a magneto optical disk, a CD-ROM, a DVD, or a Blu-ray (registered trade mark) disk. In addition, the program is a data processing method described in arbitrary language or arbitrary describing method, and any format such as a source code or a binary code may be used. In addition, the program is not necessarily limited to be configured as a single entity, but the program may be configured as a plurality of modules or a plurality of libraries in a distributed manner, or the program may achieve its functions in cooperation with a separate program typified by an OS. 
     [2] Specific distributed or integrated forms of the image processing device  1  are not limited to the illustrated forms, but all or a portion of the image processing device  1  may be functionally or physically distributed or integrated in arbitrary units according to various additions or according to functional loads to be configured. 
     As described above, according to an aspect of the embodiments, it is possible to improve a degree of recognition for content in an image. 
     All examples and conditional language recited herein are intended for pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.