Patent Publication Number: US-9406113-B2

Title: Image processing apparatus and image display apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2012-284867, filed on Dec. 27, 2012; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to an image processing apparatus and an image display apparatus. 
     BACKGROUND 
     For example, for the purpose of power saving, technique to reduce a peak luminance of a display device is well known. However, if the peak luminance is reduced, a dynamic range of the display device falls. Because, by reducing the peak luminance, a difference of luminance between dark area and bright area in the image is decreased. 
     In order to remove this defect, by calculating a gamma signal of entire image and a gamma signal of each local region therein, technique to convert the image using the gamma signals is disclosed. By this technique, even if the peak luminance of entire image is lowly set, the image in which gradation collapse and luminance irregularity of each local region are suppressed can be displayed. On the other hand, in a display device on which bright black phenomenon is increased by external light reflection, in addition to fall of the peak luminance, dynamic range is further fallen by bright black phenomenon of dark part. In this case, even if the conventional technique is applied, fall of contrast of dark part in the image cannot be avoided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an image processing apparatus  100  according to the first embodiment. 
         FIG. 2  is a flow chart of operation of the image processing apparatus  100 . 
         FIG. 3  is a schematic diagram to explain concepts of first and second gamma characteristics. 
         FIG. 4  is a schematic diagram to show one example of a gamma conversion function according to the first embodiment. 
         FIG. 5  is a block diagram of an image processing apparatus  200  according to the second embodiment. 
         FIG. 6  is a flow chart of operation of the image processing apparatus  200 . 
         FIG. 7  is a block diagram of an image processing apparatus  300  according to the third embodiment. 
         FIG. 8  is a flow chart of operation of the image processing apparatus  300 . 
         FIG. 9  is a block diagram of an image display apparatus  400  according to the fourth embodiment. 
         FIG. 10  is a block diagram of an image display apparatus  500  according to the fifth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     According to one embodiment, an image processing apparatus includes a weight calculation unit, a gamma calculation unit, and a conversion unit. The weight calculation unit is configured to calculate a weight coefficient to calculate a second gamma characteristic based on a light information of a target image. The second gamma characteristic has a narrower range of lightness than a first gamma characteristic. The first gamma characteristic represents a characteristic of a display connected to the apparatus in the dark. The gamma calculation unit is configured to calculate the second gamma characteristic based on the weight coefficient, and to calculate a gamma conversion function based on the second gamma characteristic. The second gamma characteristic more approximates a lightness of the first gamma characteristic if the light information is darker, and more approximates a gradation of the lightness of the first gamma characteristic if the light information is brighter. The conversion unit is configured to convert pixel values of the target image, based on the gamma conversion function. 
     Various embodiments will be described hereinafter with reference to the accompanying drawings. 
     (The First Embodiment) 
       FIG. 1  is a block diagram of an image processing apparatus  100  according to the first embodiment. The image processing apparatus  100  includes a weight calculation unit  10 , a gamma calculation unit  11 , and a conversion unit. In the image processing apparatus  100  of the first embodiment, an input image signal is converted to an output image signal, and outputted. At post stage of the image processing apparatus  100 , a display device (not shown in  FIG. 19 ) is connected. The display device displays the output image signal received. The display device may be any of a display and a projector. In case of the display, a CRT display, a liquid crystal display, a plasma display, an organic EL display, or other displays, may be used. 
     In the first embodiment, an example that a format of the input image signal of a processing target is based on YCbCr transmission standard of International Telecommunication Union will be explained. In the YCbCr transmission standard, each pixel includes three channel signals (luminance component, red-green component, blue-yellow component) as pixel values. In following explanation, among pixel values at a position (u,v) in the image, a luminance component is described as x(u,v). Moreover, format of the input image signal may be represented by another color system except for YCbCr system such as YUV, RGB, HSV. Furthermore, in the first embodiment, a dynamic range of color of each pixel is represented as eight bits (0˜255). However, the dynamic range is not limited to eight bits. The input image signal may be an image inputted from all devices or media. For example, the input image signal is inputted from a recording medium such as HDD, an external device connected via a network, or a broadcasting wave such as TV. 
     The weight calculation unit  10  calculates lightness information of the image. By using the lightness information, the weight calculation unit  10  calculates a weight coefficient λ representing that a second gamma characteristic of the display device (connected to the image processing apparatus  100 ) is approximated (close) to which of a lightness and a gradient of the lightness of a first gamma characteristic. The first gamma characteristic represents a display characteristic when an image displayed by the display device is viewed in an ideal environment. The ideal environment represents an environment not influenced by external environmental light, such as inside a dark room. The second gamma characteristic represents a display characteristic when an image displayed by the display device is viewed in a viewing environment actually displayed. The viewing environment actually imagined may be a viewing environment displayed by actually observing an external environmental light in real time with an illuminance sensor. Furthermore, it may be a viewing environment previously imagined without the sensor. Based on these viewing environments, the second gamma characteristic is determined. Processing by the weight coefficient λ will be explained afterwards. 
     The weight calculation unit  10  sends the weight coefficient λ to the gamma calculation unit  11 . By using the weight coefficient λ and pixel values of the input image signal, the gamma calculation unit  11  calculates a gamma conversion function to approximate the second gamma characteristic of the display device to any of a lightness and a gradient of the lightness of the first gamma characteristic, and sends this function to the conversion unit  12 . The conversion unit  12  converts pixel values of the input image signal by using the gamma conversion function, and obtains an output image signal. 
     Next, operation of each unit will be explained.  FIG. 2  is a flow chart of operation of the image display apparatus  100 . The weight calculation unit  10  accepts the input image signal (S 101 ), and calculates lightness information of the image (S 102 ). By using the lightness information, the weight calculation unit  10  calculates a weight coefficient λ representing that the second gamma characteristic of the display device (to be connected) is approximated to which of a lightness and a gradient of the lightness of the first gamma characteristic, and sends the weight coefficient λ to the gamma calculation unit  11  (S 103 ). 
     At S 102 , the lightness information represents at least one information correlative to a lightness obtained from the input image signal. For example, the lightness information had better be an average, a maximum, a minimum, a median, or a percentile among pixel values of a plurality of pixels included in a region of the image signal. The lightness information may be calculated by another statistical operation. Furthermore, the lightness information may be calculated from not only one image but also a plurality of images temporarily continued such as sequential dynamic image. Furthermore, different lightness information may be calculated from regions separated in space. 
     At S 103 , by using the lightness information, a weight coefficient representing that the second gamma coefficient of the display device is approximated to which of a lightness and a gradation of the lightness is calculated. 
       FIG. 3  is a schematic diagram to show relationship between the first gamma characteristic and the second gamma characteristic of the display device connected. In  FIG. 3 , G 1  is the first gamma characteristic of the display device connected. As to G 1 , the lightness is continuously changed among all pixel values, and a ratio of the lightness is high, i.e., contrast thereof is high. A range of lightness able to be represented by the display device is DR 1 . 
     On the other hand, in a general display device, bright black phenomenon is occurred by reflection of surrounding light, or a peak luminance is lower than an ideal value by characteristic of the display device. As a result, a range of lightness able to be represented is narrower. Here, the peak luminance is a screen luminance of the display to video signal having 100% level. A range of this narrowed lightness is DR 2 , and a lightness of an image displayed on the display device connected is within DR 2 . 
     In this case, G 1  (solid line) is the first gamma when the input image signal is displayed as it is on the display device connected. On the other hand, G 2  (double line) is the second gamma when the input image signal is displayed on condition that a lightness of the second gamma is approximated to a lightness of the first gamma (emphasis on lightness). Furthermore, G 3  (broken line) is the second gamma when the input image signal is displayed on condition that a gradation of lightness of the second gamma is approximated to a gradation of lightness of the first gamma (emphasis on gradation of lightness). Now, quality of low gradation area (dark area of the image) of G 2  and G 3  are noticed. As to G 2  in comparison with G 1 , an image having high contrast (bright black phenomenon of dark area is reduced) is outputted. As to G 3  in comparison with G 1 , an image having fine tone is outputted. 
     In the image processing apparatus  100 , in order for the gamma calculation unit  11  to calculate a gamma conversion function achieving the second gamma (such as G 2  and G 3  as mentioned-above), the weight calculation unit  10  calculates a weight coefficient λ by using the lightness information. Specifically, the weight coefficient λ is set so that the darker the lightness information is, the more the lightness is emphasized. Furthermore, the weight coefficient λ is set so that the brighter the lightness information is, the more the gradation of lightness is emphasized. As a result, the image having higher contrast is outputted if the lightness information is darker, and the image having higher tone is outputted if the lightness information is brighter. 
     As mentioned-above, at S 101 ˜S 103 , the weight coefficient λ is outputted based on lightness information of the input image signal. Accordingly, a control signal to control quality of contrast and tone in the output image can be sent to the gamma calculation unit  11 . 
     The gamma calculation unit  11  calculates a pixel value histogram H from pixel values of pixels included in the image (S 104 ). At S 104  in the first embodiment, the histogram H had better be prepared with bins of x max  units as maximum pixel values of each pixel. 
     By using the weight coefficient λ and the histogram H, a gamma conversion function L(x) to approximate the second gamma characteristic of the display device to a lightness and a gradation of the lightness of the first gamma characteristic is calculated, and sent to the conversion unit  12  (S 105 ). The gamma conversion function L(x) is calculated by following equation. 
     
       
         
           
             
               
                 
                   
                       
                   
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     In the equation (1), x represents a pixel value of the input image signal. E(x) represents an evaluation value of the gamma conversion function L(x) in case that the pixel value is x. R(x) represents a lightness of the pixel value x at the first gamma characteristic of the display device connected. r(x) represents a lightness of the pixel value x at the second gamma characteristic of the display device connected. λ is the weight coefficient. w α (x) and w β (x) represent weights of gradation x in the histogram H. A first term in the equation (1) represents a squared error between a lightness of the first gamma characteristic and a lightness of the second gamma characteristic. A second term in the equation (1) represents a squared error between a gradation of lightness of the first gamma characteristic and a gradation of lightness of the second gamma characteristic. In the first embodiment, based on the weight coefficient λ and the histogram H, a conversion function to minimize E(x) for the pixel value x is calculated as the gamma conversion function L(x). 
     A method for calculating the gamma conversion function L(x) will be explained. Assume that x max =256. First, as to 128 gradation as an internally dividing point between 0 gradation and 256 gradation, an output gradation L(x 0,1 ) thereof is calculated. Next, as to 64 gradation as an internally dividing point between 0 gradation and 128 gradation, an output gradation L(x 1,1 ) thereof is calculated. Furthermore, as to 192 gradation as an internally dividing point between 129 gradation and 256 gradation, an output gradation L(x 1,3 ) thereof is calculated. Hereafter, until all output gradations (all pixel values) are determined, above processing is repeated. Here, a hierarchical number of repeat processing is l, a number of position of input/output gradation is p, and an input gradation of each hierarchy is x l,p . 
     Now, as to an internally dividing point x l,p  between input gradations x i,p−1  and x i,p+1 , calculation of an output gradation L(x l,p ) is thought about. Here, by representing the histogram H as two partial histograms having frequencies of gradations x i,p−1 ˜x i,p  and frequencies of gradations x i,p ˜x i,p+1 , two weights w α (x) and w β (x) for gradations x i,p−1 ˜x i,p+1  are calculated by following equation. 
     
       
         
           
             
               
                 
                   
                     
                       
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     In the equation (2), H(i) represents appearance frequency of the pixel value i. 
       FIG. 4  shows one example of the gamma conversion function L(x) calculated by equations (1) and (2) in case that the weight coefficient λ emphasizes a gradation of lightness (λ=0.0). In graph of  FIG. 4 , a horizontal axis represents pixel values, a solid line along a vertical axis represents an appearance frequency, and a broken line along the vertical axis represents output characteristic of the gamma conversion function L(x). As shown in the gamma conversion function L(x) of  FIG. 4 , a gradation of lightness is larger at gradations having high appearance frequency. This reason is, in case that the weight coefficient λ emphasizes a gradation of lightness (λ=0.0), the gamma conversion function L(x) is calculated so as to minimize an error of gradation of lightness of the second term in the equation (1). Furthermore, in case that the weight coefficient λ emphasizes a lightness (λ=1.0), as to the gamma conversion function L(x) not shown in  FIG. 4 , a luminance is higher at gradations having high appearance frequency. This reason is, in case that the weight coefficient λ emphasizes a lightness (λ=1.0), the gamma conversion function L(x) is calculated so as to minimize an error of lightness of the first term in the equation (1). 
     As mentioned-above, at S 104 ˜S 105 , by using the weight coefficient λ and pixel values of the input image signal, the gamma conversion function to approximate the second gamma characteristic of the display device to a lightness and a gradation of the lightness of the first gamma characteristic is calculated. Here, by S 101 ˜S 103 , the weight coefficient λ is outputted based on lightness information of the input image signal. Accordingly, at S 104 ˜S 105 , the gamma conversion function to suitably improve quality (contrast and tone) based on lightness information of the input image signal is outputted. 
     The conversion unit  12  converts pixel values of the input image signal by using the gamma conversion function L(x), and obtains an output image signal. In the first embodiment, as to a horizontal pixel position u and a vertical pixel position v in the input image signal, and a pixel value x(u, v) as a luminance component corresponding thereto, a video signal f(u,v) is calculated by converting a gradation of the pixel value x(u,v) with following equation.
 
 f ( u,v )= L ( x ( u,v ))  (3)
 
     Next, it is decided whether a previous pixel in the image is already converted (S 107 ). If the previous pixel is not converted yet (No at S 107 ), processing is forwarded to S 108 . The pixel (not converted yet) is selected, and processing is returned to S 106  (S 108 ). If the pixel is already converted (Yes at S 107 ), processing is forwarded to S 109 . Last, the output image signal is outputted (S 109 ). 
     As mentioned-above, according to the image processing apparatus  100  of the first embodiment, the gamma conversion function L(x) to approximate the second gamma characteristic of the display device to a lightness and a gradation of the lightness of the first gamma characteristic is calculated by using the weight coefficient λ and the histogram H. As a result, if light information of the image is dark and the weight coefficient λ is set to emphasize the lightness, contrast of the image is improved. On the other hand, if light information of the image is bright and the weight coefficient λ is set to emphasize the gradation of lightness, tone of the image is improved. 
     The first gamma characteristic represents a display characteristic in case that the display device (connected) is not influenced by an external light. The second gamma characteristic represents a display characteristic in case that the display device includes fall of peak luminance and bright black phenomenon of dark area in actual viewing environment. By using the first gamma characteristic and the second gamma characteristic, at both a bright area having insufficient peak luminance and a dark area having bright black phenomenon in the image, suitable local area gamma characteristic can be respectively calculated. As a result, even if the second gamma characteristic of the display device is narrower than the first gamma characteristic, effect that contrast and tone are suitably heightened based on lightness information of the image can be acquired. 
     (The Second Embodiment) 
       FIG. 5  is a block diagram of an image processing apparatus  200  of the second embodiment. The image processing apparatus  200  includes a peak luminance calculation unit  20 , a bright black phenomenon amount calculation unit  21 , a weight calculation unit  22 , a gamma calculation unit  23 , and a conversion unit  24 . 
     The peak luminance calculation unit  20  calculates a first peak luminance as ideal peak luminance of the display device connected, and a second peak luminance as a peak luminance actually settable. The peak luminance calculation unit  20  sends the first peak luminance and the second peak luminance to the weight calculation unit  22  and the gamma calculation unit  23 . 
     The bright black phenomenon amount calculation unit  21  calculates a bright black phenomenon amount of a screen of the display device, and sends the bright black phenomenon amount to the weight calculation unit  22 . 
     The weight calculation unit  22  calculates lightness information of the image. By using the lightness information, the first peak luminance, the second peak luminance, and the bright black phenomenon amount, the weight calculation unit  22  calculates a weight coefficient λ representing that the second gamma characteristic of the display device is approximated to which of a lightness and a gradation of the lightness of the first gamma characteristic, and sends the weight coefficient λ to the gamma calculation unit  23 . 
     By using the weight coefficient λ, the first peak luminance, the second peak luminance, and pixel values of the input image signal, the gamma calculation unit  23  calculates a gamma conversion function to approximate the second gamma coefficient of the display device to a lightness and a gradation of the lightness of the first gamma characteristic, and sends the gamma conversion function to the conversion unit  24 . The conversion unit  24  converts pixel values of the input image signal by using the gamma conversion function, and outputs the image signal. 
     Next, operation of each unit  20 - 24  will be explained.  FIG. 6  is a flow chart of operation of the image display apparatus  200 . Steps of S 201 ˜S 202  and S 206 ˜S 209  are same as steps of S 101 ˜S 102  and S 106 ˜S 109 . Accordingly, explanation thereof is omitted. 
     The peak luminance calculation unit  20  calculates a first peak luminance PL as ideal peak luminance of the display device, and a second peak luminance pl as a peak luminance actually settable (S 210 ). The peak luminance calculation unit  20  sends the first peak luminance PL and the second peak luminance pl to the weight calculation unit  22 . 
     PL is ideal peak luminance of the display device connected to the image processing apparatus  200 . If the display device is direct viewing type such as LCD, PL had better be a maximum luminance displayable on the display when a light source such as LED is set to a maximum lightness settable. Furthermore, if the display device is projection type such as a projector, PL had better be a maximum luminance displayable on the projector when a light source such as lamp is set to a maximum lightness settable. 
     pl is a peak luminance actually settable to the display device. As to a general display device, from viewpoint of heat generation or power saving, a peak luminance lower than the first peak luminance is often set to the display device while actually being displayed. Accordingly, the peak luminance actually set to the display device had better be set as pl. pl may be set by an external device connected to the image processing apparatus  200  and by a user. Furthermore, for example, pl may be set by input values detected from a photo sensor. 
     As mentioned-above, at S 210 , the peak luminance of the display device actually connected is suitably set. Accordingly, accuracy of a gamma conversion function (calculated by the gamma calculation unit  23 ) based on the first gamma characteristic and the second gamma characteristic is improved. As a result, contrast and tone of an output image realized by the gamma conversion function is improved. 
     The bright black phenomenon amount calculation unit  21  calculates a bright black phenomenon amount RF of a screen of the display device (S 211 ), and sends the bright black phenomenon RF to the weight calculation unit  22 . The bright black phenomenon amount represents a difference between black actually viewed and ideal black when black is displayed on the screen. In this case, the bright black phenomenon amount RF is imagined to be previously set based on the display device. For example, if the display device is direct viewing type such as LCD, the bright black phenomenon amount had better be set based on contrast of liquid crystal panel. Here, lightness of backlight thereof should be taken into consideration. 
     Furthermore, the bright black phenomenon amount RF may be set by using information of a viewing environment of the display device. In general, if the display device is direct viewing type such as LCD, while an external light incident on a display screen is brightened, the bright black phenomenon is increased. Furthermore, if the display device is projection type such as a projector, while the external light incident onto a projection screen is brightened, the bright black phenomenon is increased. In these situations, by acquiring intensity of external light incident onto the display screen or the projection screen using a photo sensor or a camera, the bright black phenomenon amount RF had better be increased/decreased based on the intensity. 
     As mentioned-above, by processing of S 211 , the bright black phenomenon amount can be set based on device characteristic of the display device or lightness of external light in the viewing environment. 
     The weight calculation unit  22  calculates lightness information of the image (S 202 ). By using the lightness information, the first peak luminance, the second peak luminance, and the bright black phenomenon amount, the weight calculation unit  22  calculates a weight coefficient λ representing that the second gamma characteristic of the display device is approximated to which of a lightness and a gradation of the lightness of the first gamma characteristic, and sends the weight coefficient λ to the gamma calculation unit  23  (S 203 ). 
     In this case, if a difference between the first peak luminance PL and the second peak luminance pl is large and if the lightness information is brighter, the weight coefficient λ is set so as to more emphasize the gradation of lightness. This reason is, if a difference between the first peak luminance PL and the second peak luminance pl is large, in order to raise tone of the bright image, the second gamma characteristic of the display device needs to be approximated to the gradation of lightness of the first gamma characteristic. 
     Furthermore, if the bright black phenomenon amount is large and if the lightness information is darker, the weight coefficient λ is set so as to more emphasize the lightness. This reason is, if the bright black phenomenon information amount is large, in order to suppress bright black phenomenon of dark image, the second gamma characteristic of the display device needs to be approximated to the lightness of the first gamma characteristic. 
     The gamma calculation unit  23  calculates a pixel value histogram H from pixel values of pixels included in the image (S 204 ). At S 204  in the second embodiment, the histogram H had better be prepared with bins of x max  units as maximum pixel values of each pixel. 
     By using the first peak luminance PL, the second peak luminance pl, the weight coefficient λ and the histogram H, a gamma conversion function L(x) to approximate the second gamma characteristic of the display device to a lightness and a gradation of the lightness of the first gamma characteristic is calculated, and sent to the conversion unit  24  (S 205 ). 
     The gamma conversion function L(x) is calculated by following equation. 
     
       
         
           
             
               
                 
                   
                       
                   
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     In the equation (4), x represents a pixel value of the input image signal. PL represents the first peak luminance as mentioned-above. pl represents the second peak luminance actually set to the display device connected. E(x) represents an evaluation value of the gamma conversion function L(x) in case that the pixel value is x, ideal peak luminance is PL, and the second peak luminance is pl. R(x,PL) represents a lightness of the pixel value x at the first gamma characteristic of the display device connected. r(x,pl) represents a lightness of the pixel value x at the second gamma characteristic of the display device connected. λ is the weight coefficient. w α (x) and w β (x) represent weights of gradation x in the histogram H shown in the first embodiment. A first term in the equation (4) represents a squared error between a lightness of the first gamma characteristic and a lightness of the second gamma characteristic. A second term in the equation (4) represents a squared error between a gradation of lightness of the first gamma characteristic and a gradation of lightness of the second gamma characteristic. In the second embodiment, a function L(x) to minimize E(x) is outputted as the gamma conversion function (S 205 ). A concrete method for calculating the gamma conversion function L(x) is same as that of the first embodiment. Accordingly, explanation thereof is omitted. 
     As mentioned-above, by steps S 202 ˜S 205  and S 210 ˜S 211  of the second embodiment, the gamma conversion function L(x) to approximate the second gamma characteristic of the display device to the lightness and the gradation thereof of the first gamma characteristic is outputted by using the first peak luminance and the second peak luminance. Accordingly, even if the display device (to be connected) or the viewing environment is variously changed, the image of which contrast and tone are suitably heightened can be generated. 
     (The Third Embodiment) 
       FIG. 7  is a block diagram of an image processing apparatus  300  of the third embodiment. The image processing apparatus  300  includes a region division unit  30 , a peak luminance calculation unit  31 , a bright black phenomenon amount calculation unit  32 , a weight calculation unit  33 , a gamma calculation unit  34 , and a conversion unit  35 . The peak luminance calculation unit  31  and the bright black phenomenon amount calculation unit  32  are same as the peak luminance calculation unit  20  and the bright black phenomenon amount calculation unit  21  of the image processing apparatus  200  of the second embodiment. Accordingly, explanation thereof is omitted. 
     The region division unit  30  divides the input image into a plurality of (at least two) regions each including at least one pixel, and outputs the regions to the weight calculation unit  33  and the gamma calculation unit  34 . 
     By using pixel values of pixels in the input image signal of each region (outputted from the region division unit  30 ) and the weight coefficient λ thereof (outputted from the weight calculation unit  33 ), the gamma calculation unit  34  calculates a plurality of gamma conversion functions to approximate the second gamma characteristic of the display device to a lightness and a gradation of the lightness of the first gamma characteristic, and outputs the gamma conversion functions to the conversion unit  35 . 
     By using the gamma conversion functions outputted from the gamma calculation unit  34 , the conversion unit  35  converts pixel values of the input image signal, and outputs the image signal. 
     Next, operation of each unit will be explained.  FIG. 8  is a flow chart of operation of the image display apparatus  300 . In  FIG. 8 , steps of S 307 ˜S 312  are same as steps of S 206 ˜S 211  of the image processing apparatus  200  of the second embodiment. Accordingly, explanation thereof is omitted. 
     The region division unit  30  accepts the input image (S 301 ), divides the input image into a plurality of (at least two) regions each including at least one pixel, and outputs the regions to the weight calculation unit  33  and the gamma calculation unit  34  (S 302 ). 
     In the third embodiment, the input image signal is divided into regions of m×n units. A shape of region can be variously imaged. In following explanation, the shape of region is imagined by dividing into m units along a horizontal direction and by dividing into n units along a vertical direction. Moreover, this region may be non-linear shape by clustering each area of human or each object. 
     The weight calculation unit  33  calculates light information of each of m×n regions (outputted from the region division unit  30 ) (S 303 ). By using the lightness information of each region, the weight calculation unit  33  calculates weights coefficients λ mn  representing that the second gamma characteristic of each region of the display device is approximated to which of a lightness and a gradation of the lightness of the first gamma characteristic, and outputs the weight coefficients to the gamma calculation unit  34  (S 304 ). 
     At S 303 , the lightness information represents at least one information correlative to a lightness obtained from each region of the input image signal. By using pixel values of pixels in a part (over one pixel) of each region, the lightness information had better be calculated by statistical operation, such as an average, a maximum, a minimum, a median, or a percentile. Furthermore, the lightness information may be calculated from not only one image but also a plurality of images temporarily continued such as sequential dynamic image. Furthermore, different lightness information may be calculated from regions separated in space. 
     As mentioned-above, at S 301 ˜S 304 , the weight coefficient λ mn  is outputted based on lightness information of each region of the input image signal. Accordingly, in comparison with the image processing apparatus  100  of the first embodiment, local contrast and tone of each region can be more finely controlled. 
     The gamma calculation unit  34  calculates a pixel value histogram H mn  from pixel values of pixels included in m×n regions of the image (outputted from the region division unit  30 ) (S 305 ). At S 305  in the third embodiment, the histogram H mn  had better be prepared with bins of x max  units as maximum pixel values of each pixel. 
     By using the weight coefficient λ mn  and the histogram H mn  a gamma conversion function L mn (x) to approximate the second gamma characteristic of the display device to a lightness and a gradation of the lightness of the first gamma characteristic is calculated, and sent to the conversion unit  35  (S 306 ). The gamma conversion function L mn (x) is calculated by following equation. 
     
       
         
           
             
               
                 
                   
                       
                   
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     In the equation (5), x represents a pixel value of the input image signal. E mn (x) represents an evaluation value of the gamma conversion function L mn (x) in case that the pixel value is x. R(x) represents a lightness of the pixel value x at the first gamma characteristic of the display device connected. r(x) represents a lightness of the pixel value x at the second gamma characteristic of the display device connected. λ is the weight coefficient. w mn     α   (x) and w mn     β   (x) represent weights of gradation x in the histogram H. A first term in the equation (4) represents a squared error between a lightness of the first gamma characteristic and a lightness of the second gamma characteristic. A second term in the equation (4) represents a squared error between a gradation of lightness of the first gamma characteristic and a gradation of lightness of the second gamma characteristic. In the third embodiment, based on the weight coefficient λ mn  and the histogram H mn  a conversion function to minimize E(x) for each gradation x is calculated as the gamma conversion function L mn (x). 
     A method for calculating the gamma conversion function L mn (x) will be explained. Assume that x max =256. First, as to 128 gradation as an internally dividing point between 0 gradation and 256 gradation, an output gradation L mn (x 0,1 ) thereof is calculated. Next, as to 64 gradation as an internally dividing point between 0 gradation and 128 gradation, an output gradation L mn (x 1,1 ) thereof is calculated. Furthermore, as to 192 gradation as an internally dividing point between 129 gradation and 256 gradation, an output gradation L mn (x 1,3 ) thereof is calculated. Hereafter, until all output gradations (all pixel values) are determined, above processing is repeated. Here, a hierarchical number of repeat processing is l, a number of position of input/output gradation is p, and an input gradation of each hierarchy is x l,p . 
     Now, as to an internally dividing point x l,p  between input gradations x i,p−1  and x i,p+1 , calculation of an output gradation L mn (x l,p ) is thought about. Here, by representing the histogram H mn  as two partial histograms having frequencies of gradations x i,p−1 ˜x i,p  and frequencies of gradations x i,p ˜x i,p+1 , two weights w mn     α   (x) and w mn     β   (x) for gradations x i,p−1 ˜x i,p+1  are calculated by following equation. 
     
       
         
           
             
               
                 
                   
                     
                       
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     In the equation (2), H mn (i) represents appearance frequency of the pixel value i 
     As mentioned-above, at S 305 ˜S 306 , by using the weight coefficient λ mn  and pixel values of the input image signal, the gamma conversion function to approximate the second gamma characteristic of each region of the display device to a lightness and a gradation of the lightness of the first gamma characteristic is calculated. Here, by S 301 ˜S 304 , the weight coefficient λ mn  is outputted based on lightness information of each region of the input image signal. Accordingly, at S 305 ˜S 306 , the gamma conversion function to suitably improve quality (contrast and tone) based on lightness information of each region of the input image signal is outputted. 
     The conversion unit  35  converts a pixel value of each pixel of the input image signal by using the gamma conversion function L(x) of each region including the pixel, and outputs an image signal f. In the third embodiment, as to a horizontal pixel position u and a vertical pixel position v in the input image signal, and a pixel value x(u,v) as a luminance component corresponding thereto, a video signal f (u,v) is calculated by converting a gradation of the pixel value x(u,v) with following equation.
 
 f ( u,v )= L   mn ( x ( u,v ))  (7)
 
     Next, it is decided whether a previous pixel in the image is already converted (S 308 ). If the previous pixel is not converted yet (No at S 308 ), processing is forwarded to S 309 . The pixel (not converted yet) is selected, and processing is returned to S 307  (S 309 ). If the pixel is already converted (Yes at S 307 ), processing is forwarded to S 310 . Last, the output image signal is outputted (S 310 ). 
     As mentioned-above, according to the image processing apparatus  300  of the third embodiment, the gamma conversion function L mn (x) to approximate the second gamma characteristic of each region of the display device to a lightness and a gradation of the lightness of the first gamma characteristic is calculated by using the weight coefficient λ mn  of each region and the histogram of each region. As a result, if light information of a local region of of the image is dark and the weight coefficient λ mn  is set to emphasize the lightness, contrast of the local region of the image is improved. On the other hand, if light information of a local region of the image is bright and the weight coefficient λ mn  is set to emphasize the gradation of lightness, tone of the local region of the image is improved. As a result, even if the second gamma characteristic of the display device is narrower than the first gamma characteristic, effect that contrast and tone are suitably heightened based on lightness information of each region of the image can be acquired. 
     (The Fourth Embodiment) 
       FIG. 9  is a block diagram of an image display apparatus  400  of the fourth embodiment. The image display apparatus  400  includes a photo sensor  40 , a region division unit  41 , a peak luminance calculation unit  42 , a bright black phenomenon amount calculation unit  43 , a weight calculation unit  44 , a gamma calculation unit  45 , a conversion unit  46 , and a display unit  47 . The region division unit  41  and the weight calculation unit  44  are same as the region division unit  30  and the weight calculation unit  33  of the image processing apparatus  300  of the third embodiment. Accordingly, explanation thereof is omitted. 
     The photo sensor calculates a lightness of an external light incident onto the image display apparatus  400 , and outputs the lightness to the peak luminance calculation unit  42  and the bright black phenomenon amount calculation unit  43 . 
     The peak luminance calculation unit  42  calculates a first peak luminance as ideal peak luminance of the image display apparatus  400 , and a second peak luminance as a peak luminance actually settable. The peak luminance calculation unit  42  sends the first peak luminance and the second peak luminance to the weight calculation unit  44  and the gamma calculation unit  45 . In general, in order to maintain feeling of lightness of the display image, if a lightness of the external light is brighter, the second peak luminance needs to be higher. Accordingly, when the lightness of the external light is brighter, the peak luminance calculation unit  42  operates so as to heighten the second peak luminance. 
     The bright black phenomenon amount calculation unit  43  calculates a bright black phenomenon amount of a screen of the image display apparatus  400 , and sends the bright black phenomenon amount to the weight calculation unit  44 . In general, the brighter the lightness of the external light is, the larger the bright black phenomenon amount is, because reflection of the external light at the screen increases. Accordingly, in the fourth embodiment, when the lightness of the external light is brighter, the bright black phenomenon amount calculation unit  43  operates so as to calculate the bright black phenomenon amount highly. 
     By using pixel values of pixels of each region (outputted from the region division unit  41 ), the weight coefficients λ mn  (outputted from the weight calculation unit  44 ), the first peak luminance and the second peak luminance (outputted from the peak luminance calculation unit  42 ), the gamma calculation unit  45  calculates the gamma conversion function L mn (x) to approximate the second gamma coefficient of each region of the display unit  47  to a lightness and a gradation of the lightness of the first gamma characteristic, and outputs the gamma conversion function to the conversion unit  46 . 
     The conversion unit  46  converts pixel values of each region of the input image signal by using the gamma conversion function corresponding to the region, and outputs an image signal having converted pixel values to the display unit  47 . The display unit  47  displays the image signal. 
     As mentioned-above, according to the fourth embodiment, the peak luminance and the bright black phenomenon amount are dynamically calculated based on lightness of the external light incident onto the image display apparatus  400 , and the weight coefficient is calculated based on the situation. Accordingly, even if a viewing environment of the image display apparatus  400  is variously changed, the image of which contrast and tone are suitable heightened can be displayed. 
     (The Fifth Embodiment) 
       FIG. 10  is a block diagram of an image display (projection) apparatus  500  of the fifth embodiment. The image display apparatus  500  includes a camera  50 , a region division unit  51 , a peak luminance calculation unit  52 , a bright black phenomenon amount calculation unit  53 , a weight calculation unit  54 , a gamma calculation unit  55 , a conversion unit  56 , and a projection unit  57 . The region division unit  51 , the peak luminance calculation unit  52 , the bright black phenomenon amount calculation unit  53 , the weight calculation unit  54 , the gamma calculation unit  55 , the conversion unit  56 , and the projection unit  57 , are same as the region division unit  41 , the peak luminance calculation unit  42 , the bright black phenomenon amount calculation unit  43 , the weight calculation unit  44 , the gamma calculation unit  45 , the conversion unit  46 , and the display unit  47  of the fourth embodiment. Accordingly, explanation thereof is omitted. 
     The camera  50  photographs a scene of a projection plane projected by the projection apparatus, calculates a lightness of the external light incident onto the projection plane from the photographed scene, and outputs the lightness to the peak luminance calculation unit  52  and the bright black phenomenon amount calculation unit  53 . The lightness E of the external light is calculated by following equation.
 
 E=C ( x   max )− C ( x   min )  (8)
 
     In the equation (8), x max  is maximum pixel value able to be represented by the projection apparatus  500 , x min  is minimum pixel value able to be represented by the projection apparatus  500 , and C( ) is an output image from the camera  50  when the camera  50  photographs a projection image (having gradation x) projected by the projection apparatus  500 . Briefly, a difference of photographed image between an entire white projection image and an entire black projection image is calculated as the lightness of the external light incident onto the projection plane. 
     The peak luminance calculation unit  52  calculates a first peak luminance as ideal peak luminance of the image display apparatus  500 , and a second peak luminance as a peak luminance actually settable. The peak luminance calculation unit  52  sends the first peak luminance and the second peak luminance to the weight calculation unit  54  and the gamma calculation unit  55 . In general, in order to maintain feeling of lightness of the projection image, if a lightness of the external light is brighter, the second peak luminance needs to be higher. Accordingly, when the lightness of the external light is brighter, the peak luminance calculation unit  52  operates so as to heighten the second peak luminance. 
     The bright black phenomenon amount calculation unit  53  calculates a bright black phenomenon amount of a projection plane of the image display apparatus  500 , and sends the bright black phenomenon amount to the weight calculation unit  54 . In general, the brighter the lightness of the external light is, the larger the bright black phenomenon amount is, because reflection of the external light at the projection plane increases. Accordingly, in the fifth embodiment, when the lightness of the external light is brighter, the bright black phenomenon amount calculation unit  53  operates so as to calculate the bright black phenomenon amount highly. 
     By using pixel values of pixels of each region (outputted from the region division unit  51 ), the weight coefficients λ mn  (outputted from the weight calculation unit  54 ), the first peak luminance and the second peak luminance (outputted from the peak luminance calculation unit  52 ), the gamma calculation unit  55  calculates the gamma conversion function L mn (x) to approximate the second gamma coefficient of each region of the projection unit  57  to a lightness and a gradation of the lightness of the first gamma characteristic, and outputs the gamma conversion function to the conversion unit  56 . 
     The conversion unit  56  converts pixel values of each region of the input image signal by using the gamma conversion function corresponding to the region, and outputs an image signal having converted pixel values to the projection unit  57 . The projection unit  57  projects the image signal onto the projection plane. 
     As mentioned-above, according to the fifth embodiment, the peak luminance and the bright black phenomenon amount are dynamically calculated based on lightness of the external light incident onto the projection plane, and the weight coefficient is calculated based on the situation. Accordingly, even if a viewing environment of the projection plane is variously changed, the image of which contrast and tone are suitable heightened can be displayed. 
     While certain embodiments have been described, these embodiments have been presented by way of examples only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.