Patent Publication Number: US-9838658-B2

Title: Image processing apparatus that performs tone correction, image processing method, and storage medium

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to image processing on photographed data taken by an image pickup apparatus, and in particular to an image processing apparatus and an image processing method which are capable of adaptively reflecting a dynamic range, which is stored as photographed data, to a developing result according to the photographed data, as well as a storage medium. 
     Description of the Related Art 
     In image pickup apparatuses such as digital cameras, a brightness adjustment is made to automatically correct the tone of photographed data to a target tone. For example, there is known a technique to analyze the tone and others of a dark part and a bright part in photographed data and automatically correct tone so that the overall photographed data can have a suitable tone. Specifically, there has been proposed a method in which, when the brightness of an object to be taken has changed, tone is corrected so that the object to be taken can be displayed with a predetermined brightness (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. 2002-359773). There has also been proposed a method in which for photographed data, tone is corrected based on a reference value of exposure for shooting (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. 2005-268952). 
     However, the techniques described in Japanese Laid-Open Patent Publication (Kokai) No. 2002-359773 and Japanese Laid-Open Patent Publication (Kokai) No. 2005-268952 present a problem that the atmosphere of an image in shooting is damaged because the overall brightness is corrected to a large extent in some scenes. 
     SUMMARY OF THE INVENTION 
     The present invention provides an image processing apparatus and an image processing method which are capable of properly performing tone correction without damaging the atmosphere of an image in shooting, as well as a storage medium. 
     Accordingly, a first aspect of the present invention provides an image processing apparatus which develops photographed data, comprising a dynamic range adjustment unit configured to, based on a luminance histogram for the photographed data, determine an input lower limit and an input upper limit of luminance values for use in developing the photographed data, a correction amount adjustment unit configured to, based on at least one of the input lower limit and the input upper limit determined by the dynamic range adjustment unit, calculate a reference level of a gamma curve for use in carrying out gamma correction on the photographed data, and a gamma correction unit configured to perform gamma correction on the photographed data using the input lower limit and the input upper limit and the gamma curve corresponding to the reference level calculated by the correction amount adjustment unit. 
     Accordingly, a second aspect of the present invention provides an image processing apparatus which develops photographed data, comprising a dynamic range adjustment unit configured to, based on a luminance histogram for the photographed data, determine an input lower limit and an input upper limit of luminance values for use in developing the photographed data, a correction amount adjustment unit configured to, based on a ratio of the input upper limit after adjustment by the dynamic range adjustment unit to a predetermined threshold value, calculate a reference level of a gamma curve for use in carrying out gamma correction on the photographed data, and a gamma correction unit configured to perform gamma correction on the photographed data using the input lower limit and the input upper limit and the gamma curve corresponding to the reference level calculated by the correction amount adjustment unit. 
     Accordingly, a third aspect of the present invention provides An image processing method for an image processing apparatus which develops photographed data, comprising a dynamic range adjustment step of, based on a luminance histogram for the photographed data, determining an input lower limit and an input upper limit of luminance values for use in developing the photographed data, a correction amount adjustment step of, based on at least one of the input lower limit and the input upper limit determined in the dynamic range adjustment step, calculating a reference level of a gamma curve for use in carrying out gamma correction on the photographed data, and a gamma correction step of performing gamma correction on the photographed data using the input lower limit and the input upper limit and the gamma curve corresponding to the reference level calculated in the correction amount adjustment step. 
     Accordingly, a fourth aspect of the present invention provides an image processing method for an image processing apparatus which develops photographed data, comprising a dynamic range adjustment step of, based on a luminance histogram for the photographed data, determining an input lower limit and an input upper limit of luminance values for use in developing the photographed data, a correction amount adjustment step of, based on a ratio of the input upper limit after adjustment in the dynamic range adjustment step to a predetermined threshold value, calculating a reference level of a gamma curve for use in carrying out gamma correction on the photographed data, and a gamma correction step of performing gamma correction on the photographed data using the input lower limit and the input upper limit and the gamma curve corresponding to the reference level calculated in the correction amount adjustment step. 
     Accordingly, a fifth aspect of the present invention provides a non-transitory computer-readable storage medium storing a control program for causing a computer to execute an image processing method for an image processing apparatus which develops photographed data, the image processing method comprising a dynamic range adjustment step of, based on a luminance histogram for the photographed data, determining an input lower limit and an input upper limit of luminance values for use in developing the photographed data, a correction amount adjustment step of, based on at least one of the input lower limit and the input upper limit determined in the dynamic range adjustment step, calculating a reference level of a gamma curve for use in carrying out gamma correction on the photographed data, and a gamma correction step of performing gamma correction on the photographed data using the input lower limit and the input upper limit and the gamma curve corresponding to the reference level calculated in the correction amount adjustment step. 
     Accordingly, a sixth aspect of the present invention provides a non-transitory computer-readable storage medium storing a control program for causing a computer to execute an image processing method for an image processing apparatus which develops photographed data, the image processing method comprising a dynamic range adjustment step of, based on a luminance histogram for the photographed data, determining an input lower limit and an input upper limit of luminance values for use in developing the photographed data, a correction amount adjustment step of, based on a ratio of the input upper limit after adjustment in the dynamic range adjustment step to a predetermined threshold value, calculating a reference level of a gamma curve for use in carrying out gamma correction on the photographed data, and a gamma correction step of performing gamma correction on the photographed data using the input lower limit and the input upper limit and the gamma curve corresponding to the reference level calculated in the correction amount adjustment step. 
     According to the present invention, a suitable level of a gamma curve is shifted toward a high-luminance side when a dynamic range is extended. The suitable level of the gamma curve is shifted toward a low-luminance side when the dynamic range is compressed to a predetermined value or greater. As a result, tone is properly corrected without damaging the atmosphere of an image in shooting. 
     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 arrangement of an image pickup apparatus according to an embodiment of the present invention. 
         FIG. 2  is a diagram showing an exemplary WB adjustment process which is carried out by a WB adjustment unit which a developing unit has. 
         FIG. 3  is a flowchart of a first determination method for the amount of tone correction, which is carried out by the developing unit. 
         FIG. 4  is a diagram schematically showing a process in which suitable levels for gamma curves are calculated by a correction amount adjustment unit, which the developing unit has, in step S 307  in  FIG. 3 . 
         FIG. 5  is a view showing a table of the relationship between a correction coefficient K 1  and the degree of bias (var1) on a low luminance side of a luminance histogram, the table being used in step S 307  in  FIG. 3 . 
         FIGS. 6A and 6B  are views showing examples of a dark part correction degree table and a bright part correction degree table which are used in step S 308  in  FIG. 3 . 
         FIGS. 7A and 7B  are views showing examples of a dark part correction amount table and a bright part correction amount table which are used in the step S 308  in  FIG. 3 . 
         FIG. 8  is a flowchart showing a second determination method for the amount of tone correction, which is carried out by the developing unit. 
         FIG. 9  is a diagram schematically showing a process in which suitable levels for gamma curves are calculated by the correction amount adjustment unit, which the developing unit has, in step S 807  in  FIG. 8 . 
         FIG. 10  is a view showing a table of the relationship between a correction coefficient K 1  and the degree of bias (var2) on a high luminance side of a luminance histogram, the table being used in step S 807  in  FIG. 8 . 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The present invention will now be described in detail with reference to the drawings showing an embodiment thereof. 
       FIG. 1  is a block diagram schematically showing an arrangement of an image pickup apparatus  100  according to an embodiment of the present invention. The image pickup apparatus  100  is typically a digital still camera or a digital video camera having a movie shooting function, but the image pickup apparatus  100  is not limited to this and may be any of various electronic apparatuses having a camera function (a shooting function of obtaining an image (video) using an image pickup device). For example, the image pickup apparatus  100  may be a mobile communication terminal (a cellular phone, a smartphone, or the like) with a camera function, a mobile computer (a tablet computer, a laptop computer, or the like) with a camera function, or a mobile game machine with a camera function. 
     The image pickup apparatus  100  is comprised mainly of an image pickup unit  1 , a developing unit  2 , and a storage-reproduction unit  3 . It should be noted that the image pickup apparatus  100  has a central control unit (CPU), not shown, which controls the overall operation of the image pickup apparatus  100 . The image pickup unit  1 , the developing unit  2 , and the storage-reproduction unit  3  carry out predetermined operations and processes under the control of the central control unit (CPU). 
     The image pickup unit  1  has various lenses such as a zoom lens and a focus lens, a diaphragm, an image pickup device such as a CMOS image sensor, an A/D converter, a photometric measurement sensor, and so on and generates photographed data (hereafter referred to as “RAW data”). It should be noted that Raw data includes undeveloped photographed data and photographed data to which a part of a developing process is subjected. The developing unit  2  develops RAW data generated by the image pickup unit  1  to generate developed data. 
     The storage-reproduction unit  3  stores RAW data generated by the image pickup unit  1  and developed data generated by the developing unit  2  in a storage medium and also reads RAW data and developed data stored in the storage medium. It should be noted that the image pickup apparatus  100  is also able to read RAW data stored in the storage-reproduction unit  3  with arbitrary timing and develop the same. 
     The developing unit  2  has a white balance adjustment unit  10  (hereafter referred to as “the WB adjustment unit  10 ”). The WB adjustment unit  10  multiplies signal values of respective color signals by white balance coefficients (hereafter referred to as “WB coefficients”) to adjust levels of the respective color signals so that a gray subject can be output in gray with levels of the respective color signals being uniform. WB coefficients are gains varying with the color signals. The image pickup unit  1  stores, as RAW data, WB coefficients as well as signal values from the image pickup device and multiplies the signal values of the respective color signals by WB coefficients, so that the color signals have the same signal value in a gray subject. 
     It should be noted that WB coefficients may be stored with consideration with the assumption that shooting is performed under a standard light source. Also, WB coefficients may be calculated based on color temperatures input by a user or may be calculated by extracting signal values of the respective color signals from a part of RAW data or developed data. The WB adjustment unit  10  may not use WB coefficients stored as RAW data but may calculate WB coefficients using a method designated by the user at the time of development and use the same. 
       FIG. 2  is a diagram showing an exemplary WB adjustment process which is carried out by the WB adjustment unit  10 . In  FIG. 2 , the axis of ordinate indicates the magnitudes of signal values. Signal values  20  in  FIG. 2  are signal values of the respective color signals stored as RAW data. A sensor saturation value  21  is an upper limit to the signal values  20  and determined by a spectral sensitivity characteristic of an image pickup device, the processing accuracy of the image pickup unit  1 , and a predetermined threshold value. In the example shown in  FIG. 2 , the sensor saturation value  21  is the same for the color signals, but the sensor saturation value  21  may vary with the color signals. 
     Signal values  22  in  FIG. 2  are signal values after WB adjustment obtained by the WB adjustment unit  10  multiplying the signal values  20  of the respective color signals by WB coefficients. Multiplying the signal values  20  by WB coefficients changes upper limits to the respective color signals. In the example shown in  FIG. 2 , it is assumed that a WB coefficient for the color signal R is “2”, a WB coefficient for the color signal B is “1.5”, and a WB coefficient for the color signal G is “1”. Thus, a saturation level  23  for the color signal R is twice as large as the sensor saturation value  21 , a saturation level  24  for the color signal B is 1.5 times as large as the sensor saturation value  21 , and a saturation level  25  for the color signal G is equal to the sensor saturation value  21 . 
     The developing unit  2  also has an optical correction unit  11 , a color interpolation unit  12 , a dynamic range adjustment unit  13  (hereafter referred to as “the D range adjustment unit  13 ”), a replacement processing unit  14 , and a noise removal unit  15 . The developing unit  2  also has a correction amount advisement unit  16 , a gamma correction unit  17 , a tone correction unit  18 , a sharpness processing unit  19 , and a color processing unit  110 . 
     It should be noted that the image pickup unit  1  may be equipped with a part of these component elements (including the WB adjustment unit  10 ) which the developing unit  2  has, and also, the developing unit  2  may be equipped with other processing units. Moreover, the component elements which the developing unit  2  has may be functional blocks of the central control unit itself or may be functional blocks which perform predetermined functions under the control of the central control unit. 
     The optical correction unit  11 , for example, reduces peripheral illumination arising from various lenses of the image pickup unit  1 , corrects for chromatic aberration of magnification, eliminates axial chromatic aberration, and corrects for distortion. The color interpolation unit  12  demosaics pixels comprised of monochrome signals. The D range adjustment unit  13  determines an input lower limit Bk and an input upper limit Wt of luminance values (that is, an input lower limit Bk and an input upper limit Wt of gamma curves) for use in development. The replacement processing unit  14  replaces a color signal nearly saturated with another color signal for each pixel. The noise removal unit  15  removes luminance noise and color noise by a filtering process, a hierarchical process, or the like. 
     The correction amount adjustment unit  16  calculates a reference level of a gamma curve for use in gamma correction performed on an input signal (photographed data to be processed in steps S 301  to S 303  and S 307 , to be described later) and generates tone correction tables. It should be noted that the reference level means a level of an input signal targeted for a signal after gamma correction. 
     In the description of the present embodiment, it is assumed that a level of an input signal targeted for a signal after gamma correction is set at a luminance value (luminance level) enabling correct exposure, and more specifically, a luminance value (luminance level) in a case where gray with a reflectivity of 18% is shot. Accordingly, in the following description, the reference level is expressed as a suitable level. It should be noted that a targeted level should not always set in the way described above but may be set according to a shooting scene or may be set by a user. 
     The gamma correction unit  17  carries out a gamma correction process in which it adjusts the contrast and dynamic range of the entire taken image using the input lower limit Bk and the input upper limit Wt determined by the D range adjustment unit  13  and a gamma curve determined by the correction amount adjustment unit  16 . 
     The tone correction unit  18  corrects tone of local luminance of only a dark part or a bright part according to tone correction tables generated by the correction amount adjustment unit  16 . The sharpness processing unit  19  enhances an edge of an image to adjust the sharpness of the entire image. The color processing unit  110  adjusts the hue of an image and suppresses color curving in a high-luminance region. Although in  FIG. 1 , the processing units of the developing unit  2  are shown in the order of preferable processes, and such effects as noise reduction and reduction of coloring on an edge are obtained, but this order is not limitative. 
     Referring to  FIGS. 3 to 7 , a description will be given of a first determination method for the amount of tone correction.  FIG. 3  is a flowchart showing the first determination method for the amount of tone correction, which is carried out by the developing unit  2 . It should be noted that processes in  FIG. 3  are implemented by the central control unit (CPU), not shown, which controls the overall operation of the image pickup apparatus  100 , executing predetermined programs and controlling operation of the component elements constituting the developing unit  2 . 
     In step S 301 , the WB adjustment unit  10  makes a WB adjustment by multiplying signal values of respective color signals in RAW data by WB coefficients. In step S 302 , the replacement processing unit  14  replaces a color signal nearly saturated with another color signal. For this replacement process, a well-known technique (for example, the technique described in Japanese Laid-Open Patent Publication (Kokai) No. 2004-328564, Japanese Laid-Open Patent Publication (Kokai) No. 2012-85360, or the like) may be used, but detailed description thereof is omitted here. 
     In step S 303 , the gamma correction unit  17  carries out a gamma correction process on photographed data which has been subjected to the process in the step S 302 . Here, the gamma correction unit  17  performs tone conversion using gamma curves calculated based on the input lower limit Bk and the input upper limit Wt determined by the D range adjustment unit  13 . The input lower limit Bk and the input upper limit Wt can be, for example, luminance values in the bottom 1% and top 1% of photographed data. 
     In step S 304 , the correction amount adjustment unit  16  calculates a luminance histogram for photographed data which has been subjected to the gamma correction in the step S 303 . Specifically, an RGB signal after the gamma correction in the step S 303  is converted into a luminance value (Y) to calculate a luminance histogram for the overall photographed data. It should be noted that for example, an equation 1 below may be used for conversion from an RGB signal into a luminance value (Y). 
     In step S 305 , the correction amount adjustment unit  16  which acts as bias calculation means for calculating the degree of bias in a luminance histogram calculates the degrees of bias on a lower luminance side and a higher luminance side relative to a central luminance. 
     For example, equations 2 and 3 below may be used to calculate the degree of bias in a luminance histogram. The equations 2 and 3 are for calculating the degree of bias (var1) on a lower luminance side relative to a central luminance in a luminance histogram of which the number of tones  256  and the degree of bias (var2) on a higher luminance side relative to the central luminance. It should be noted that in the equations 2 and 3, “Y[i]” is a histogram frequency for a luminance value i. 
     
       
         
           
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     In step S 306 , the correction amount adjustment unit  16  determines whether or not a dynamic range after the adjustment using the input lower limit Bk and the input upper limit Wt determined by the D range adjustment unit  13  is wider than a dynamic range before the adjustment (whether or not the dynamic range has extended due to adjustment). When the dynamic range has become wider (YES in the step S 306 ), the process proceeds to step S 307 , and when the dynamic range has not become wider (NO in the step S 306 ), the process proceeds to step S 308 . 
     In the step S 307 , the correction amount adjustment unit  16  calculates suitable levels of gamma curves.  FIG. 4  is a diagram schematically showing a process in which suitable levels of gamma curves are calculated in the step S 307 .  FIG. 4  show three gamma curves, i.e. a first gamma curve  40 , a second gamma curve  41 , and a third gamma curve  42 . 
     The first gamma curve  40  is a gamma curve in a case where no dynamic range adjustment is made. The second gamma curve  41  is a gamma curve of which a suitable level is the same as that before dynamic range adjustment and for which an input lower limit and an input upper limit have been adjusted. The third gamma curve  42  is a gamma curve obtained by adjusting the suitable level of the second gamma curve  41 . 
     Referring to  FIG. 4 , an input lower limit  43  is an input lower limit to the first gamma curve  40 , and an input lower limit  44  is an input lower limit to the second gamma curve  41  and the third gamma curve  42 . A suitable level  45  is a suitable level of the first gamma curve  40  and the second gamma curve  41 , and a suitable level  46  is a suitable level of the third gamma curve  42 . 
     An input upper limit  47  is an input upper limit to the first gamma curve  40 . An input upper limit  48  is an input upper limit to the second gamma curve  41  and the third gamma curve  42 . It should be noted that these values ( 43  to  48 ) are expressed on a log scale. An output upper limit  49  is an output upper limit after gamma correction. 
     The gradient of the second gamma curve  41  which has been adjusted so as to extend the dynamic range without changing the suitable level of the first gamma curve  40  is smaller on a higher luminance side relative to the suitable level, and hence the tone is poor in this luminance range. On the other hand, the third gamma curve  42  is adjusted by shifting the suitable level toward a high luminance side, and hence the tone on the high luminance side is made close to the first gamma curve  40  (the gradient is kept constant) in the case where no dynamic range adjustment is carried out. 
     Also, the suitable level of the third gamma curve  42  is shifted toward the high luminance side, and as a result, the tone on the low luminance side as well is made close to the first gamma curve  40  in the case where no dynamic range adjustment is carried out. An exemplary equation 4 for computation of a suitable level is given below. 
     
       
         
           
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     In the above equation 4, “Bk1” is a value (luminance value) of the input lower limit  43 , “Wt1” is a value (luminance value) of the input upper limit  47 , and “Wt2” is a value (luminance value) of the input upper limit  48 . Also, “Mid” is a value (luminance value) of a suitable level (suitable level  45 ) before shifting, and “MidNew” is a value (luminance value) of a suitable level (suitable level  46 ) after shifting. 
     “K 1 ” is a correction coefficient which assumes values from 0 to 1 and determines a ratio between portions of a dynamic range extension assigned to a low luminance side and a high luminance side relative to a suitable level. Thus, according to the above equation 4, a suitable level is adjusted based on at least one of the ratio between dynamic ranges on a lower luminance side relative to the suitable level before and after dynamic range adjustment and the ratio between dynamic ranges on a higher luminance side relative to the suitable level before and after dynamic range adjustment. It is preferred that a suitable level is adjusted so that the ratio between a dynamic range on a lower luminance side relative to the suitable level and the ratio between a dynamic range on a higher luminance side relative to the suitable level can be kept constant before and after gamma correction. 
     The value of the correction coefficient K 1  is determined according to the degree of bias (var1) on the low luminance side of the luminance histogram calculated in the step S 305 .  FIG. 5  is a view showing a table of the relationship between the correction coefficient K 1  and the degree of bias (var1) on the low luminance side of the luminance histogram. A first threshold value  50  and a second threshold value  51  in  FIG. 5  are for switching values of the correction coefficient K 1  and set in advance with consideration given to the amount of tone correction in the step S 308  in a later stage. 
     When the degree of bias (var1) on the low luminance side of the luminance histogram is small, the correction coefficient K 1  is set at 1, and by assigning a dynamic range extension only to the low luminance side in this manner, the tone on the high luminance side is maintained. However, as with the third gamma curve in  FIG. 4 , this leads to an adverse result in scenes where a dark part is desired to be bright because gamma output values are corrected to low luminance values as a whole. 
     Accordingly, when the degree of bias (var1) on the low luminance side of the luminance histogram is large, the amount of tone correction for the dark part calculated in the step S 308  is large, and hence setting the correction coefficient K 1  at a small value in advance makes the shift amount of the suitable level modest and prevents the image from being dark. Simply, by keeping the correction coefficient K 1  at 0.5, portions of a dynamic range extension are equally assigned to the low luminance side and the high luminance side. 
     It is preferred that a limit is placed on the shift amount of the suitable level (the amount of change in the value of the suitable level before and after calculation) so as to prevent MidNew, which is the result of calculation using the equation 4, from greatly changing from Mid. Strictly, the gamma shape is changed by changing the suitable level in the step S 307 , and hence it is preferred that the steps S 304  and S 305  are performed again to recalculate the degree of bias from the central luminance in the luminance histogram, and then the amount of tone correction for the dark part and the bright part are calculated in the step S 308 . It should be noted that when the shift amount of the suitable level is small, this process may be dispensed with because the image quality is less likely to be affected. 
     In the step S 308 , the correction amount adjustment unit  16  calculates the amounts of tone correction for the dark part and the bright part (determines tone correction tables). In the step S 308 , first, the correction amount adjustment unit  16  determines the degrees of correction for the dark part and the bright part, respectively, according to a dark part correction degree table and a bright part correction degree table. Examples of the dark part correction degree table and the bright part correction degree table are shown in  FIGS. 6A and 6B . 
     Threshold values  60 ,  61 ,  62 , and  63  in  FIGS. 6A and 6B  are for use in determining the degree of correction and set as fixed values in advance. Here, for the degree of bias in the low luminance side of the luminance (var1) and the degree of bias in the high luminance side of the luminance (var2), they are set so that when the degree of bias is small, the degree of correction can be small, and when the degree of bias is large, the degree of correction can be large. This is because leveling-out of the luminance histogram is aimed at. 
     Subsequently, in the step S 308 , the correction amount adjustment unit  16  determines tone correction tables according to a dark part correction amount table and a bright part correction amount table which are held in advance. Examples of the dark part correction amount table and the bright part correction amount table are shown in  FIGS. 7A and 7B . 
     Correction tables  70 ,  71 ,  72 ,  73 , and  74  show the amounts of correction in cases where the degree of correction for the dark part is 0%, 25%, 50%, 75%, and 100%, respectively. Correction tables  75 ,  76 ,  77 ,  78 , and  79  show the amounts of correction in cases where the degree of correction for the bright part is 0%, 25%, 50%, 75%, and 100%, respectively. The correction amount adjustment unit  16  determines tone correction tables to be used by the tone correction unit  18  by selecting correction tables according to the degrees of correction determined previously. 
     It should be noted that to correct tone based on the determined amounts of correction, the tone correction unit  18  may use a tone curve correction method, or a color dodge correction method in which correction is carried out with a local constant being maintained. Moreover, an interface that allows the user to select the intensity of correction from “low”, “standard”, and “high” may be prepared in advance, and according to a setting selected by the user, the tables in  FIGS. 6A and 6B  and the tables in  FIGS. 7A and 7B  may be switched to respective other similar tables. The execution of the step S 308  is followed by terminating the present process. 
     As described above, according to the first determination method for the amount of tone correction, when a dynamic range is extended, a suitable level of a gamma curve is controlled to be shifted toward a high luminance side. This prevents the tone from being poor within a scope which does not damage the atmosphere of brightness in shooting. 
     Referring to  FIGS. 8 to 10 , a description will now be given of a second determination method for the amount of tone correction.  FIG. 8  is a flowchart showing the second determination method for the amount of tone correction, which is carried out by the developing unit  2 . It should be noted that processes in  FIG. 8  are implemented by the central control unit (CPU), not shown, which controls the overall operation of the image pickup apparatus  100 , executing predetermined programs and controlling the operation of the component elements constituting the developing unit  2 . 
     Processes in the steps S 801  to S 805  and S 808  in  FIG. 8  are the same as those in the steps S 301  to S 305  and S 308  in  FIG. 3 , and hence description thereof is omitted here. In step S 806 , the correction amount adjustment unit  16  determines whether or not a dynamic range after adjustment using the input lower limit Bk and the input upper limit Wt calculated by the D range adjustment unit  13  is narrower than a predetermined dynamic range. 
     When the adjusted dynamic range is narrower than the predetermined dynamic range (YES in the step S 806 ), the process proceeds to step S 807 , and when the adjusted dynamic range is not narrower than the predetermined dynamic range (NO in the step S 806 ), the process proceeds to the step S 808 . 
     In the step S 807 , the correction amount adjustment unit  16  calculates a suitable level of a gamma curve.  FIG. 9  is a diagram schematically showing a process in which a suitable level of a gamma curve is calculated in the step S 807 . In  FIG. 9 , a first gamma curve  90  is a gamma curve in a case where no dynamic range adjustment is made. A second gamma curve  91  is a gamma curve of which a suitable level is the same as that before dynamic range adjustment and for which an input lower limit and an input upper limit value have been adjusted. A third gamma curve  92  is a gamma curve obtained by adjusting the suitable level of the second gamma curve  91 . 
     An input lower limit  93  is an input lower limit to the first gamma curve  90 . An input lower limit  94  is an input lower limit to the second gamma curve  91  and the third gamma curve  92 . A suitable level  95  is a suitable level of the third gamma curve  92 . A suitable level  96  is a suitable level of the first gamma curve  90  and the second gamma curve  91 . An input upper limit  97  is an input upper limit to the second gamma curve  91  and the third gamma curve  92 . 
     An input upper limit  98  is an input upper limit to the first gamma curve  90 . A threshold value  910  is a threshold value for an input upper limit in shifting a suitable level and set in advance. These values ( 93  to  98 ,  910 , to be described later) are expressed on a log scale. An output upper limit  99  is an output upper limit after gamma correction. 
     As is apparent from the second gamma curve  91  in  FIG. 9 , when a dynamic range is adjusted so as to be compressed without changing a suitable level, the gradient of the gamma curve is greater on a higher luminance side relative to the suitable level. 
     Here, when the gradient of the gamma curve is too high, an image tends to be unnatural because the tone is too steep. For this reason, the threshold value  910  is set in advance, and when the input upper limit becomes smaller than the threshold value  910 , the suitable level is shifted to a low luminance side, thus making the gradient of the gamma curve low. An exemplary equation 5 for use in calculation of the suitable level in this case is given below. 
     
       
         
           
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                   MidNew 
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                     Mid 
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                   5 
                 
               
             
           
         
       
     
     In the above equation 5, “Wt2” is a value (luminance value) of the input upper limit  97 , and “Wt_th” is a value (luminance value) of the threshold value  910  in  FIG. 9 . “Mid” is a value (luminance value) of a suitable level (suitable level  96 ) before shifting, and “MidNew” is a value (luminance value) of a suitable level (suitable level  95 ) after shifting. “K 2 ” is a correction coefficient assuming values from 0 to 1 and used to control the ratio at which the suitable level is shifted with respect to an amount for which an input upper limit is set at a value equal to or smaller than the threshold value  910 . The value of the correction coefficient K 2  is determined according to the degree of bias (var2) on the high luminance side of the luminance histogram calculated in the step S 805 . 
     Namely, according to the first determination method described earlier, an image is darkened when a suitable level is shifted toward a high luminance side, and hence as for an image of which the tone is desired to be corrected so that a dark part can be bright, the correction coefficient K 1  is set so that the shift amount of a suitable level can be small. 
     Conversely, according to the second determination method, an image is lightened when a suitable level is shifted toward a low luminance side, and hence as for an image of which the tone is desired to be corrected so that a bright part can be dark, the correction coefficient K 2  is set so that the shift amount can be small. 
       FIG. 10  is a view showing a table of the relationship between the correction coefficient K 2  and the degree of bias (var2) on the high luminance side of the luminance histogram. A first threshold value  150  and a second threshold value  160  in  FIG. 10  are for switching values of the correction coefficient K 2  and set in advance with consideration given to the amount of tone correction in the step S 808  in a later stage. 
     As described above, according to the second determination method for the amount of tone correction, when a dynamic range is compressed to a predetermined value or greater, a suitable level of a gamma curve is controlled to shift toward a low luminance side. As a result, an image is prevented from being unnatural due to a too steep tone within a scope which does not damage the atmosphere of brightness in shooting. 
     It should be noted that in the present embodiment described above, the image pickup apparatus  100  comprised of the image pickup unit  1 , the developing unit  2 , and the storage-reproduction unit  3  is taken as an exemplary image processing apparatus. The present invasion, however, is not limited to this, but the image pickup unit  1 , the developing unit  2 , and the storage-reproducing unit  3  may be configured as separate apparatuses capable of communicating with one another. 
     For example, the present invention may be arranged such that RAW data taken and generated by a camera is transmitted to an external image processing apparatus (for example, a personal computer) via a network, the image processing apparatus develops the RAW data, and the developed data is stored in a storage device connected to the image processing apparatus so that they can communicate with each other. 
     The developing unit  2  does not need to process RAW data, which is taken by a camera, in real time, but may be an apparatus, such as a typical personal computer, which is capable of reading RAW data stored in a storage medium and performing the development process described above. 
     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. 2014-072496, filed Mar. 31, 2014, which is hereby incorporated by reference herein in its entirety.