Patent Publication Number: US-9904991-B2

Title: Image pickup apparatus that corrects contrast of image, control method for the image pickup apparatus, and storage medium

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an image pickup apparatus which corrects the contrast of an image, a control method for the image pickup apparatus, and a storage medium. 
     Description of the Related Art 
     Conventionally, there is known a correction method to correct the contrast of an image obtained by shooting (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. 2006-98614). According to the correction method described in Japanese Laid-Open Patent Publication (Kokai) No. 2006-98614, a tone curve for use in correcting the contrast of an image is generated from a luminance histogram of the image. Specifically, a low-tone area of the tone curve is calculated from a frequency distribution (frequency percentage) of low luminance in the luminance histogram, and a high-tone area of the tone curve is calculated from a frequency distribution of high luminance in the luminance histogram, and a mid-tone area of the tone curve is calculated from the low-tone area and the high-tone area. The contrast of an image is then corrected using a calculated tone curve across the overall tone range. As a result, an image with smooth tone continuity across the overall image is obtained. 
     Some images whose contrast is to be corrected are at least partially blurred (hereafter referred to as a “blurred image”). A blurred image may include not only a subject whose subject distance, which is a distance from an image pickup apparatus to a subject, is short (hereafter referred to as a “short-distance subject”) but also a subject whose subject distance is long (hereafter referred to as a “long-distance subject”). Due to the subject distance being long, shooting of a long-distance subject tends to be more affected by dust floating in the air and sunlight diffusely reflected by moisture in the air than shooting of a short-distance subject. For this reason, in a blurred image, an edge of a long-distance subject tends to be more difficult to recognize than an edge of a short-distance subject. It should be noted that an edge of a subject is made easier to recognize by correcting the contrast of an image of the subject, and the processing intensity with which contrast is corrected depends on a tone curve, and more particularly, its slope. 
     However, according to the correction method described in Japanese Laid-Open Patent Publication (Kokai) No. 2006-98614, a tone curve is calculated from a histogram across the board regardless of the distance to a subject, and this presents a problem that it is impossible to appropriately correct the contrasts of images of respective subjects. For example, the processing intensity of contrast correction using a calculated tone curve may be too low to correct the contrast of an image of a long-distance subject and may not satisfactorily enhance an edge of the long-distance subject. Also, the processing intensity of contrast correction using a calculated tone curve may be too high to correct the contrast of an image of a short-distance subject and may excessively enhance an edge of the short-distance subject. 
     SUMMARY OF THE INVENTION 
     The present invention provides an image pickup apparatus that is capable of appropriately correcting the contrast of an image, a control method for the image pickup apparatus, and a storage medium. 
     Accordingly, the present invention provides an image pickup apparatus that picks up an image of a subject, comprising a detecting unit configured to detect a luminance histogram from an image including the subject, a distance measurement unit configured to measure a subject distance to the subject, and a deciding unit configured to, according to the subject distance, decide a frequency percentage that defines a luminance range in the luminance histogram for use in calculating a control point for a tone curve used to correct contrast of the image, wherein the frequency percentage is higher when the subject distance is long than when the subject distance is short. 
     According to the present invention, the contrast of an image is appropriately corrected. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram schematically showing an internal arrangement of an image pickup apparatus according to a first embodiment of the present invention. 
         FIGS. 2A and 2B  are views useful in explaining a luminance histogram that is detected by a histogram detecting unit in  FIG. 1 ,  FIG. 2A  showing the luminance histogram, and  FIG. 2B  showing a tone curve obtained from the luminance histogram. 
         FIGS. 3A and 3B  are views useful in explaining the relation between slopes of tone curves generated by a tone curve generating unit in  FIG. 1  and a low-luminance side frequency percentage and a high-luminance side frequency percentage,  FIG. 3A  showing a slope of a tone curve in a case where the low-luminance side frequency percentage and the high-luminance side frequency percentage are high, and  FIG. 3B  showing a slope of a tone curve in a case where the low-luminance side frequency percentage and the high-luminance side frequency percentage are low. 
         FIG. 4  is a flowchart showing the procedure of a correction process that is carried out by the image pickup apparatus in  FIG. 1 . 
         FIG. 5  is a flowchart showing the procedure of a correction process that is carried out by an image pickup apparatus according to a second embodiment of the present invention. 
         FIG. 6  is a flowchart showing the procedure of a correction process that is carried out by an image pickup apparatus according to a third embodiment of the present invention. 
         FIG. 7  is a view useful in explaining image data divided into a plurality of areas in step S 601  in  FIG. 6 . 
         FIG. 8  is a view useful in explaining distance measurement results obtained by measuring subject distances to subjects included in the respective areas of the image data in  FIG. 7 . 
         FIG. 9  is a view useful in explaining a low-luminance side frequency percentage and a high-luminance side frequency percentage in each of areas obtained in step S 602  in  FIG. 6 . 
         FIG. 10  is a flowchart showing the procedure of a correction process that is carried out by an image pickup apparatus according to a fourth embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereafter, a first embodiment of the present invention will be described in detail with reference to the drawings. 
       FIG. 1  is a block diagram schematically showing an internal arrangement of an image pickup apparatus  100  according to the first embodiment of the present invention. 
     The image pickup apparatus  100  in  FIG. 1  has an image pickup unit  101 , an image data generating unit  102 , a histogram detecting unit  103 , a tone curve generating unit  104 , a subject distance detecting unit  105 , and an image display unit  106 . The image pickup unit  101  has a lens group  107 , which is comprised of a plurality of lenses, an IRCF (infrared ray cut filter)  108 , and an image pickup device  109  such as a CCD sensor or a CMOS sensor. The image pickup unit  101  also has a CDS (correlated double sampling) circuit  110 , an AGC (automatic gain control) amplifier  111 , and an A/D converter  112 . The image data generating unit  102  has a gamma correction unit  113  and an image signal processing unit  114 . 
     When the image pickup apparatus  100  shoots a subject, an optical image is formed on the image pickup device  109  through the lens group  107  and the IRCF  108 . The image pickup device  109  performs photoelectric conversion of the formed optical image and transmits analog image signal. The CDS circuit  110  receives an analog image signal from the image pickup device  109  and performs, for example, correlated double sampling on the received analog image signal. The AGC amplifier  111  receives an analog image signal from the CDS circuit  110  and carries out, for example, an amplification process on the received analog image signal. The A/D converter  112  receives an analog image signal from the AGC amplifier  111  and converts the received analog image signal into a digital image signal. The image data generating unit  102  receives the digital image signal and generates image data based on the received digital image signal. 
     The histogram detecting unit  103  detects a luminance histogram ( FIG. 2A ) showing the distribution of luminance levels in generated image data and outputs the image data to the image display unit  106 . The tone curve generating unit  104  generates a tone curve, which is for use in correcting the contrast of an image, from a luminance histogram detected by the histogram detecting unit  103  and sends the tone curve to the gamma correction unit  113 . The subject distance detecting unit  105  measures subject distances to respective subjects in an optical image based on, for example, a zoom position and a focus positions of each lens and sends the measured subject distances to the tone curve generating unit  104 . The gamma correction unit  113  carries out a gamma correction process on image data using a tone curve received from the tone curve generating unit  104 . The image signal processing unit  114  subjects image data to processes other than the gamma correction process, for example, a color conversion process, an AE process, and a WB process. 
     A description will now be given of a generating process in which the tone curve generating unit  104  generates a tone curve based on a luminance histogram detected by the histogram detecting unit  103 . 
     The tone curve generating unit  104  receives a subject distance from the subject distance detecting unit  105  and obtains (determines) a low-luminance side frequency percentage  201  and a high-luminance side frequency percentage  202  determined in advance according to the subject distance. The low-luminance side frequency percentage  201  is a proportion of an integral value of luminance levels within a predetermined luminance range on a low-luminance side to an integral value of all luminance levels in the luminance histogram and is used to calculate a low-luminance side control point  203  which is a threshold between a low-tone area and a mid-tone area of a tone curve. Specifically, the low-luminance side frequency percentage  201  defines a luminance range for use in calculating the low-luminance side control point  203 . The high-luminance side frequency percentage  202  is a proportion of an integral value of luminance levels within a predetermined luminance range on a high-luminance side to an integral value of all luminance levels in the luminance histogram and is used to calculate a high-luminance side control point  204  which is a threshold between a mid-tone area and a high-tone area of a tone curve. Specifically, the high-luminance side frequency percentage  202  defines a luminance range for use in calculating the high-luminance side control point  204 . 
     The tone curve generating unit  104  calculates the low-luminance side control point  203  and the high-luminance side control point  204  from the obtained low-luminance side frequency percentage  201  and high-luminance side frequency percentage  202  and generates a tone curve ( FIG. 2B ) using an equation 1 below. It should be noted that in the equation 1 below, a point A corresponds to the low-luminance side control point  203 , and a point B corresponds to the high-luminance side control point  204 . 
     
       
         
           
             
               
                 
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     t(x): a value (luminance frequency) in a y-axis direction of a tone curve in a luminance histogram 
     Ax, Ay: x, y coordinates at a point A on the tone curve in the luminance histogram 
     Bx, By: x, y coordinates at a point B on the tone curve in the luminance histogram 
     Ix, Iy: x, y coordinates at a point I on the tone curve in the luminance histogram 
     Mx, My: x, y coordinates at a point M on the tone curve in the luminance histogram 
     In general, a low-tone area of a tone curve is used for correction performed on a low-luminance area of image data and enhances the darkness of the low-luminance area. A high-tone area of a tone curve is used for correction performed on a high-luminance area of image data and enhances the brightness of the low-luminance area. A mid-tone area of a tone curve is used for correction performed on a mid-luminance area of image data and makes a difference between light and dark areas smooth. The greater a low-tone area and a high-tone area of a tone curve, the greater an area where brightness or darkness is enhanced in image data, and hence the more enhanced the contrast of the entire image data. Namely, the greater a low-tone area and a high-tone area of a tone curve, the higher the processing intensity with which the contrast of image data is corrected. It should be noted that the greater a low-tone area and a high-tone area of a tone curve, the shorter the distance between the low-luminance side control point  203  and the high-luminance side control point  204 , and hence the steeper the slopes of tone curves. Therefore, using a tone curve with a steeper slope leads to a higher processing strength of contrast correction. 
     As described above, the longer the subject distance, the harder it is to recognize a subject&#39;s edge. Accordingly, in the present embodiment, as the subject distance increases, the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  are set to increase, and the slope of a generated tone curve becomes steeper. For example, when the subject distance is equal to or longer than 200 m and shorter than 300 m, the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  are set to 15%. When the subject distance is equal to or longer than 100 m and shorter than 200 m, the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  are set to 10%. When the subject distance is equal to or longer than 0 m and shorter than 100 m, the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  are set to 5%. A tone curve is generated individually in each case, and image data is subjected to a gamma correction process using the generated tone curve. Here, when the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  are 15%, a low-tone area and a high-tone area of a generated tone curve are relatively large, and hence the slope of the tone curve ( FIG. 3A ) is steep. When the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  are 5%, a low-tone area and a high-tone area of a generated tone curve are relatively small, and hence the slope of the tone curve ( FIG. 3B ) is gentle. Namely, in gamma correction performed on an image including a subject with a long subject distance, a tone curve with a steep slope is used to correct contrast with high processing intensity. In gamma correction preformed on an image including a subject with a short subject distance, a tone curve with a gentle slope is used to correct contrast with low processing intensity. 
       FIG. 4  is a flowchart showing the procedure of a correction process that is carried out by the image pickup apparatus in  FIG. 1 . 
     Referring to  FIG. 4 , first, the subject distance detecting unit  105  measures a subject distance (step S 401 ), and the hygrogram detecting unit  103  detects a luminance histogram based on image data generated by the image data generating unit  102  (step S 402 ). Next, the tone curve generating unit  104  obtains the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  according to the measured subject distance (step S 403 ). Then, the tone curve generating unit  104  calculates the low-luminance side control point  203  and the high-luminance side control point  204  from the luminance histogram as well as the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  (step S 404 ). After that, the tone curve generating unit  104  generates a tone curve using the equation 1 above (step S 405 ). 
     Then, the gamma correction unit  113  reflects the generated tone curve on a gamma curve using an equation 2 below to generate a contrast enhancement gamma curve (step S 406 ). 
     [Mathematical Expression 2]
 
 h ( x )= g ( t ( x ))  (Equation 2)
 
     h(x): gamma curve for contrast enhancement 
     g(x): normal gamma curve 
     t(x): tone curve 
     Then, the gamma correction unit  113  subjects the image data to a gamma correction process based on the contrast enhancement gamma curve (step S 407 ), followed by the present process being terminated. 
     According to the present embodiment, the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  for calculating the low-luminance side control point  203  and the high-luminance side control point  204  are higher when the subject distance is long than when the subject distance is short (step S 403 ). As the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  increase, the slope of a tone curve becomes steeper, and the processing intensity of contrast correction increases. Therefore, when the subject distance is long, image data on a subject is subjected to contrast correction with high processing intensity, and hence a subject whose edge is hard to recognize due to its long subject distance is more easily recognized. Namely, the contrast of an image is appropriately corrected. 
     A detailed description will now be given of a second embodiment of the present invention with reference to the drawings. The second embodiment of the present invention is basically the same as the first embodiment described above in terms of constructions and operations, differing from the first embodiment in that the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  are maintained at a predetermined value when the subject distance is shorter than a threshold value. Features of constructions and operations that are the same as those in the first embodiment will thus not be described, only constructions and operations different from those of the first embodiment being described below. 
       FIG. 5  is a flowchart showing the procedure of a correction process that is carried out by an image pickup apparatus  100  according to the second embodiment of the present invention. Steps S 401  to S 407  in  FIG. 5  are the same as the steps S 401  to S 407  in  FIG. 4 , and therefore, only differences from  FIG. 4  will be described below. 
     Referring to  FIG. 5 , when the histogram detecting unit  103  detects a luminance histogram (step S 402 ), the tone curve generating unit  104  determines whether or not a threshold distance is set in advance in the image pickup apparatus  100  (step S 501 ). As a result in the determination in the step S 501 , when no threshold distance is set in advance in the image pickup apparatus  100 , the process proceeds to the step S 403 . 
     On the other hand, when a threshold distance is set in advance in the image pickup apparatus  100 , the tone curve generating unit  104  obtains the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  according to a subject distance with consideration given to the threshold distance (step S 502 ). For example, when the threshold distance is set to 250 m, and the subject distance is shorter than 250 m, the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  are maintained at 5% (a predetermined value). On the other hand, when the subject distance is equal to or longer than 250 m, and the subject distance is equal to or longer than 250 m and shorter than 350 m, the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  are set to 10%. Further, when the subject distance is equal to or longer than 350 m and shorter than 450 m, the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  are maintained at 15%. After that, the process proceeds to the step S 404 . 
     According to the process in  FIG. 5 , when the subject distance is shorter than a threshold value, for example, 250 m, the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  are maintained at a predetermined value, for example, 5% (step S 502 ). When subject distance is shorter than 250 m, this means that the distance to a subject is short, and when contrast correction with high processing intensity is performed on the image of the subject, the subject may become harder to recognize. At this time, the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  are maintained at 5%, and hence the slope of a tone curve generated from the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  is unchanged, and the processing intensity of contrast correction on the subject is constant. This prevents an image of a subject at a short distance from being subjected to contrast correction with higher processing strength than necessary and making the subject hard to recognize. 
     A detailed description will now be given of a third embodiment of the present invention with reference to the drawings. The third embodiment of the present invention is basically the same as the first embodiment described above in terms of constructions and operations, differing from the first embodiment in that a plurality of subjects is spread out over image data. Features of constructions and operations that are the same as those in the first embodiment will thus not be described, only constructions and operations different from those of the first embodiment being described below. 
       FIG. 6  is a flowchart showing the procedure of a correction process that is carried out by an image pickup apparatus  100  according to the third embodiment of the present invention. Steps S 401  to S 407  in  FIG. 6  are the same as the steps S 401  to S 407  in  FIG. 4 , and therefore, only differences from  FIG. 4  will be described below. 
     Referring to  FIG. 6 , first, the subject distance detecting unit  105  divides image data into a plurality of areas correspondingly to a plurality of subjects and measures subject distances to the subjects included in the respective areas (step S 601 ). In the present embodiment, the subject distance detecting unit  105  divides image data into, for example, nine areas including an area  701  and an area  702  adjacent to the area  701  as shown in  FIG. 7  and measures subject distances to subjects included in the respective areas to obtain measurements results for the respective areas as shown in  FIG. 8 , for example. 
     Next, the histogram detecting unit  103  detects a luminance histogram from the image data generated by the image data generating unit  102  (step S 402 ). Then, the tone curve generating unit  104  obtains the low-luminance side frequency percentages  201  and the high-luminance side frequency percentages  202 , for example, as shown in  FIG. 9  according to the measured subject distances to the subjects included in the respective areas (step S 602 ). In the present embodiment, for example, in an area where the subject distance is equal to or longer than 0 m and shorter than 100 m, the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  are set to 5%. In an area where the subject distance is equal to or longer than 100 m and shorter than 200 m, the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  are set to 10%. In an area where the subject distance is equal to or longer than 200 m and shorter than 300 m, the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  are set to 15%. In an area where the subject distance is equal to or longer than 300 m, the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  are set to 20%. 
     The tone curve generating unit  104  obtains a weighted average of the obtained low-luminance side frequency percentages  201  and high-luminance side frequency percentages  202  in the respective areas to calculate the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  that are to be applied to the entire image data (step S 603 ). In the step S 603 , to obtain a weighted average of the low-luminance side frequency percentages  201  and the high-luminance side frequency percentages  202  in the respective areas, the low-luminance side frequency percentages  201  and the high-luminance side frequency percentages  202  in the respective areas are multiplied by weight coefficients. The weight coefficients are determined for the respective areas, and for example, a weight coefficient for an area including a subject that draws a lot of attention from a user although it is hard to recognize due to its long subject distance is set to a greater value than a weight coefficient for an area including a subject at a short subject distance or an area including a subject that draws little attention from a user. After that, the process proceeds to the step S 405 . 
     According to the process in  FIG. 6 , image data is divided into a plurality of areas, and subject distances to subjects included in the respective areas are measured (step S 601 ). Further, the low-luminance side frequency percentages  201  and the high-luminance side frequency percentages  202  in the respective areas are obtained according to the subject distances to the subjects included in the respective areas (step S 602 ). After that, a weighted average of the obtained low-luminance side frequency percentages  201  and high-luminance side frequency percentages  202  in the respective areas is obtained to calculate the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  for entire image data (step S 603 ). Weight coefficients for use in obtaining the weighted average are determined for the respective areas. At this time, a weight coefficient for an area including a subject that draws a lot of attention from a user although it is hard to recognize due to its long subject distance is set to a greater value than a weight coefficient for an area including a subject at a short subject distance or an area including a subject that draws little attention from a user. As a result, even when only a tone curve to be applied to entire image data is generated and used in a correction process without generating tone curves for respective areas, the contrast of an edge of a subject that draws a lot of attention from a user is reliably enhanced. 
     It should be noted that although in the process in  FIG. 6 , the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  that are to be applied to entire image data are calculated by obtaining a weighted average of the obtained low-luminance side frequency percentages  201  and high-luminance side frequency percentages  202  in the respective areas, they may be calculated by obtaining a simple average of the obtained low-luminance side frequency percentages  201  and high-luminance side frequency percentages  202  in the respective areas. 
     A detailed description will now be given of a fourth embodiment of the present invention with reference to the drawings. The fourth embodiment of the present invention is basically the same as the third embodiment described above in terms of constructions and operations, differing from the third embodiment in that a tone curve is generated on an area-by-area basis. Features of constructions and operations that are the same as those in the third embodiment will thus not be described, only constructions and operations different from those of the third embodiment being described below. 
       FIG. 10  is a flowchart showing the procedure of a correction process that is carried out by an image pickup apparatus  100  according to the fourth embodiment of the present invention. Steps S 402  and S 601  to S 602  in  FIG. 10  are the same as the steps S 402  and S 601  to S 602  in  FIG. 6 , and therefore, only differences from  FIG. 6  will be described below. 
     Referring to  FIG. 10 , first, the low-luminance side frequency percentages  201  and the high-luminance side frequency percentages  202  are obtained according to measured subject distances to subjects included in respective areas (steps S 602 , S 602 ). Next, the tone curve generating unit  104  calculates the low-luminance side control points  203  and the high-luminance side control points  204  from the low-luminance side frequency percentages  201  and the high-luminance side frequency percentages  202  in the respective areas (step S 1001 ). Then, the tone curve generating unit  104  generates tone curves in the respective areas using the equation 1 above (step S 1002 ). 
     After that, the gamma correction unit  113  reflects the generated tone curves in the respective areas on gamma curves in the respective areas using the equation 2 above to generate contrast enhancement gamma curves for the respective areas (step S 1003 ). Then, the gamma correction unit  113  subjects image data in the respective areas to gamma correction processes based on the contrast enhancement gamma curves for the respective areas (step S 1004 ), followed by the present process being terminated. 
     According to the process in  FIG. 10 , the low-luminance side control points  203  and the high-luminance side control points  204  are calculated from the low-luminance side frequency percentages  201  and the high-luminance side frequency percentages  202  obtained according to measured subject distances to subjects included in respective areas (step  1001 ). Based on the low-luminance side control points  203  and the high-luminance side control points  204  for the respective areas thus calculated, tone curves in the respective areas are generated (step S 1002 ). As a result, on an area-by-area basis, contrast is corrected according to a subject distance, and hence, for example, even when a plurality of subjects at different subject distances is scattered across image data, contrasts of the respective subjects are corrected with consideration given to the subject distances to the respective subjects. 
     In the process in  FIG. 10 , for example, it may be further determined whether or not a difference between the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  in the area  701  and the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  in the area  702  (hereafter referred to as “the frequency percentage difference”) is equal to or greater than a predetermined value. When the frequency percentage difference is equal to or greater than the predetermined value, and tone curves generated from the area  701  and the area  702  are used, there is a great difference in slope between the tone curves generated from the area  701  and the area  702 . Therefore, processing intensities of contrast correction performed on the area  701  and the area  702  may greatly differ from each other, and tone continuity may be lost in the area  701  and the area  702  after they are subjected to contrast correction. To address this, in order that the frequency percentage difference can be smaller than the predetermined value, the tone curve generating unit  104  changes the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  in each area. For example, assume that when the predetermined value for the frequency percentage difference is 3%, a value of 15% is obtained as the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  in the area  701 , and a value of 10% is obtained as the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  in the area  702 . In this case, the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  in each area are changed so that the frequency percentage difference can be smaller than 3%. Specifically, the tone curve generating unit  104  changes the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  in the area  701  to 13%, or changes the low-luminance side frequency percentage  201  and the high-luminance side frequency percentage  202  in the area  702  to 12%. This prevents slopes of tone curves generated from the areas  701  and  702  from greatly differing from each other. As a result, the continuity of tone is obtained in the area  701  and the area  702  after they are subjected to contrast correction. 
     Other Embodiments 
     Embodiment(s) of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a ‘non-transitory computer-readable storage medium’) to perform the functions of one or more of the above-described embodiment(s) and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s) and/or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, a memory card, and the like. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2015-157132, filed Aug. 7, 2015, which is hereby incorporated by reference herein in its entirety.