Abstract:
An image correction circuit capable of preventing a loss of gray levels in a luminance region at the time of the direct current level conversion of a luminance signal to improve the quality of a displayed image is provided. An image correction circuit includes: a luminance detection means for detecting an average peak level of input image data in each image frame; and an image correction means for correcting the input image data so as to lower the luminance of input image data in an intermediate luminance region according to the average peak level while reducing the luminance of input image data at a predetermined rate in at least of a low luminance region.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    The present invention contains subject matter related to Japanese Patent Application JP 2006-136461 filed in the Japanese Patent Office on May 16, 2006, the entire contents of which being incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an image correction circuit having a function of performing a correction process on an image signal, an image correction method and an image display. 
         [0004]    2. Description of the Related Art 
         [0005]    Apparatuses such as television receivers, VCRs (Video Cassette Recorders), digital cameras, television cameras or printers typically have an image processing function which makes image quality correction to an input image, and then outputs the input image (for example, functions such as luminance or contrast control, and contour correction). Such a function is effectively applied mainly to a totally dark and low-contrast image or a blurred image. 
         [0006]    Among image processing circuits having such a function, a DC transmission rate circuit has a function of lowering a DC component (a direct current level) according to the magnitude of an average peak level (APL) in each image frame of input image data (for example, refer to Japanese Examined Patent Application Publication No. H2-47155). More specifically, in the case where the average peak level is high (bright), an average displayed image is obtained by shifting the whole image to darker side. Such a DC shift (a direct current level conversion) corresponds to human eye sensibility (which obtains a certain average amount of light by closing an eye&#39;s iris when it is bright, and opening the eye&#39;s iris when it is dark), so the DC transmission rate circuit achieves a high effect on improvement in the image quality of a displayed image. 
       SUMMARY OF THE INVENTION 
       [0007]    However, in such a DC transmission rate circuit in a related art, for example, as shown by input/output characteristics in  FIG. 15 , an original characteristic line L 110  is subjected to DC fluctuation according to an average peak level, as shown by a characteristic line L 111 , a part P 101  in which the gray level does not change is generated in a low luminance region (a black level region) L 101  in principle. In other words, a loss of gray levels occurs in a part of the black level region, so it is difficult to display gray levels in the part, thereby the quality of a displayed image declines. 
         [0008]    Moreover, in such a DC transmission rate circuit in the related art, for example, as shown by a characteristic line L 111  in  FIG. 15 , the luminance level in a high luminance region (a white level region) declines (by a part shown by P 102 ) in principle. Therefore, also from this viewpoint, the quality of a displayed image declines. 
         [0009]    In view of the foregoing, it is desirable to provide an image correction circuit capable of preventing a loss of gray levels in a low luminance region at the time of the direct current level conversion of a luminance signal to improve the quality of a displayed image, an image correction method and an image display. 
         [0010]    Moreover, it is desirable to provide an image correction circuit capable of preventing a decline in the luminance level on a high luminance side at the time of direct current level conversion of a luminance signal to improve the quality of a displayed image, an image correction method and an image display. 
         [0011]    According to an embodiment of the invention, there is provided a first image correction circuit including: a luminance detection means for detecting an average peak level of input image data in each image frame; and an image correction means for correcting the input image data so as to lower the luminance of input image data in an intermediate luminance region according to the average peak level while reducing the luminance of input image data at a predetermined rate in at least of a low luminance region. 
         [0012]    According to an embodiment of the invention, there is provided a first image correction method including the steps of: detecting an average peak level of input image data in each image frame; and correcting input image data so as to lower the luminance of input image data in an intermediate luminance region according to the average peak level while reducing the luminance of input image data at a predetermined rate in at least of a low luminance region. 
         [0013]    According to an embodiment of the invention, there is provided a first image display including: a luminance detection means for detecting an average peak level of input image data in each image frame; an image correction means for correcting input image data so as to lower the luminance of input image data in a low luminance region according to the average peak level while reducing the luminance of input image data at a predetermined rate in at least of a low luminance region; and a display means for displaying an image on the basis of the corrected input image data. 
         [0014]    In the first image correction circuit, the first image correction method and the first image display according to the embodiment of the invention, while the luminance of the input image data in the intermediate luminance region is lowered according to the average peak level, the luminance of the input image data in at least of the low luminance region is reduced at a predetermined rate. In this case, “reduced at a predetermined rate” means that an output is increased or decreased with an increase or a decrease in input. Therefore, while direct current level conversion is performed in the intermediate luminance region, a loss of gray levels in the low luminance region of the input image data after the conversion can be prevented. 
         [0015]    In the first image correction circuit according to the embodiment of the invention, the image correction means can include a function determination means for determining an image correction function, the image correction function including an intermediate luminance function part and a low luminance function part, the intermediate luminance function part converting the direct current level of the input image data in the intermediate luminance region according to the detected average peak level, the low luminance function part continuously connecting between a minimum luminance point of the input image data and the intermediate luminance function part; and a correction execution means for executing image correction on the input image data on the basis of the determined image correction function. In this case, “an image correction function” means a function defining a relationship between the luminance signal of input image data and the luminance signal of corrected image data. 
         [0016]    In the first image correction circuit according to the embodiment of the invention, it is preferable that the image correction means corrects the input image data so as to continuously connect between a maximum luminance point of the input image data and an intermediate luminance region while maintaining a maximum luminance point of the input image data as it is. In such a case, the maximum luminance point of the input image data is not lowered and is maintained, so a decline in the luminance level in the high luminance region can be prevented. 
         [0017]    According to an embodiment of the invention, there is provided a second image correction circuit including: a luminance detection means for detecting an average peak level of input image data in each image frame; and an image correction means for correcting input image data so as to lower the luminance of input image data in an intermediate luminance region according to the average peak level while maintaining a maximum luminance point of input image data as it is. 
         [0018]    According to an embodiment of the invention, there is provided a second image correction method including the steps of: detecting an average peak level of input image data in each image frame; and correcting input image data so as to lower the luminance of input image data in an intermediate luminance region according to the average peak level while maintaining a maximum luminance point of input image data as it is. 
         [0019]    According to an embodiment of the invention, there is provided a second image display including: a luminance detection means for detecting an average peak level of input image data in each image frame; an image correction means for correcting input image data so as to lower the luminance of input image data in an intermediate luminance region according to the average peak level while maintaining a maximum luminance point of input image data as it is; and a display means for displaying an image on the basis of the corrected input image data. 
         [0020]    In the second image correction circuit, the second image correction method and the second image display according to the embodiment of the invention, while the luminance of the input image data in the intermediate luminance region is lowered according to the average peak level, the maximum luminance point of the input image data is maintained as it is. Therefore, while direct current level conversion is performed in the intermediate luminance region, a decline in the luminance level in the high luminance region of input image data after the conversion can be prevented. 
         [0021]    In the first image correction circuit, the first image correction method or the first image display according to the embodiment of the invention, while the luminance of the input image data in at least of the low luminance region is reduced at a predetermined rate, the luminance of the input image data in the intermediate luminance region is lowered according to the average peak level, so even if direct current level conversion is performed in the intermediate luminance region, a loss of gray levels in the low luminance region after the conversion can be prevented. Therefore, gray levels in the low luminance region can be reliably displayed, and the quality of a displayed image can be improved. 
         [0022]    Moreover, in the second image correction circuit, the second image correction method or the second image display according to the embodiment of the invention, while the maximum luminance point of the input image data is maintained as it is, the luminance of the input image data in the intermediate luminance region is lowered according to the average peak level, so even if direct current level conversion is performed in the intermediate luminance region, a decline in the luminance level in the high luminance region after the conversion can be prevented. Therefore, gray levels in the high luminance region can be reliably displayed, and the quality of a displayed image can be improved. 
         [0023]    Other and further objects, features and advantages of the invention will appear more fully from the following description. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is a circuit block diagram showing the whole configuration of an image display according to a first embodiment of the invention; 
           [0025]      FIG. 2  is a circuit block diagram of a detailed configuration of a DC transmission rate correction section shown in  FIG. 1 ; 
           [0026]      FIG. 3  is a plot showing an example of an image correction function according to the first embodiment; 
           [0027]      FIGS. 4A and 4B  are plots for describing examples of a function generating operation by a black level correction function generating circuit; 
           [0028]      FIG. 5  is a plot for describing an operation of a black level adjusting section; 
           [0029]      FIG. 6  is a plot showing a histogram distribution of a luminance level detected in a γ correction section; 
           [0030]      FIG. 7  is a plot for describing an operation of the γ correction section; 
           [0031]      FIG. 8  is a plot showing an example of an image correction function according to a modification of the first embodiment; 
           [0032]      FIG. 9  is a plot for describing characteristics of the image correction function shown in  FIG. 8 ; 
           [0033]      FIGS. 10A and 10B  are circuit block diagrams showing structural examples of a luminance signal correction section according to the modification of the first embodiment; 
           [0034]      FIG. 11  is a circuit block diagram showing the configuration of a DC transmission rate correction section according to a second embodiment of the invention; 
           [0035]      FIG. 12  is a plot showing an example of an image correction function according to the second embodiment; 
           [0036]      FIG. 13  is a plot showing an example of an image correction function according to a modification of the second embodiment; 
           [0037]      FIG. 14  is a plot showing an example of an image correction function according to a modification of the invention; and 
           [0038]      FIG. 15  is a plot for describing DC transmission rate correction in an image display in a related art. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0039]    Preferred embodiments will be described in detail below referring to the accompanying drawings. 
       First Embodiment 
       [0040]      FIG. 1  shows the whole configuration of an image display according to a first embodiment of the invention. The image display includes a image processing function section including a tuner  11 , a Y/C separation circuit  12 , a chroma decoder  13 , a switch  14 , a delay circuit  15 , a luminance signal correction section  2  and an image processing circuit  3  and an image display function section including a matrix circuit  41 , a driver  42  and a display  5 . An image correction circuit and an image correction method according to a first embodiment of the invention are embodied by the image display according to the embodiment, so they will be also described below. 
         [0041]    Image signals inputted into the image display may be outputs from a VCR (Video Cassette Recorder), a DVD (Digital Versatile Disc) or the like in addition to a TV signal from a TV (television). It has become common practice for recent televisions and personal computers (PCs) to obtain image information from a plurality of kinds of media and display an image corresponding to each of the media. 
         [0042]    The tuner  11  receives and demodulates the TV signal from the TV, and outputs the TV signal as a composite video burst signal (CVBS). 
         [0043]    The Y/C separation circuit  12  separates the composite video burst signal from the tuner  11  or a composite video burst signal from a VCR or a DVD 1  into a luminance signal Y 1  and a chrominance signal C 1  to output them. 
         [0044]    The chroma decoder  13  outputs the luminance signal Y 1  and the chrominance signal C 1  separated by the Y/C separation circuit  12  as YUV signals (Y 1 , U 1 , V 1 ) including the luminance signal Y 1  and color-difference signals U 1  and V 1 . 
         [0045]    The YUV signals are image data of a digital image, and a set of pixel values corresponding to a position on a two-dimensional image. A luminance signal Y represents a luminance level, and takes an amplitude value between a white level which is 100% white and a black level. Moreover, a 100% white image signal is 100 (IRE) in a unit called IRE (Institute of Radio Engineers) representing a relative ratio of an image signal. The black level is 0 IRE. On the other hand, the color-difference signals U and V correspond to a signal B-Y produced by subtracting the luminance signal Y from blue (B), and a signal R-Y produced by subtracting the luminance signal Y from red (R), respectively, and when the signals U and V are combined with the luminance signal Y, colors (color phases, chroma saturation, luminance) can be shown. 
         [0046]    The switch  14  switches YUV signals from a plurality of kinds of media (in this case, the YUV signals (Y 1 , U 1 , V 1 ) and YUV signals (Y 2 , U 2 , V 2 ) from a DVD 2 ) so as to output selected signals as YUV signals (Yin, Uin, Vin). 
         [0047]    The luminance signal correction section  2  corrects the luminance signal Yin of the YUV signals (Yin, Uin, Vin) outputted from the switch  14 , and includes a DC transmission rate correction section  21 , a black level adjusting section  22  and a γ correction section  23 . 
         [0048]      FIG. 2  shows the circuit configuration of the DC transmission rate correction section  21 . The DC transmission rate correction section  21  includes an APL detection circuit  211 , a DC shift circuit  212 , a black level correction function generating circuit  213 , a correction function determining circuit  214  and a correction execution circuit  215 . Moreover,  FIG. 3  shows input/output characteristics in the DC transmission rate correction section  21 , and shows an image correction function defining a relationship between an inputted luminance signal Yin and an outputted luminance signal Yout 11 . A line L 10  in  FIG. 3  shows a reference image correction function (before DC transmission rate correction) in which the inputted luminance Yin=outputted luminance Yout 11 . 
         [0049]    The APL detection circuit  211  detects an average peak level (APL) in each image frame on the basis of the luminance signal Yin. The detected average peak level is outputted to the DC shift circuit  212 . 
         [0050]    For example, as shown in  FIG. 3 , the DC shift circuit  212  generates an intermediate•high luminance function part L 12 , which lowers a luminance level in an intermediate•high luminance region to lower than an original luminance level (the luminance level of the reference image correction function L 10 ) according to the average peak level in each image frame detected by the APL detection circuit  211 , in an image correction function (for example, an image correction function L 14  in  FIG. 3 ) in the DC transmission rate correction circuit  21 . More specifically, the amount of DC shift is fixed irrespective of average luminance, and the whole image is shifted to darker side, thereby the intermediate•high luminance function part L 12  is generated. 
         [0051]    For example, as shown in  FIG. 3 , the black level correction function generating circuit  213  generates a low luminance function part (a black level correction function) L 11 , which is a part continuously connecting between a minimum luminance point P 0  and the intermediate•high luminance function part L 12  at a connection point P 1  so as to reduce a luminance signal in a low luminance region from an original luminance level though at a predetermined rate, in the image correction function (for example, the image correction function L 14  in  FIG. 3 ) in the DC transmission rate correction circuit  21 . For example, in  FIG. 3 , the low luminance function part L 11  connects between the minimum luminance point P 0  and the intermediate•high luminance function part L 12  with a line with a predetermined change rate (a gradient K 1 ). As a method of generating such a low luminance function part L 11 , the following two methods are cited. 
         [0052]    At first, as one of the methods, for example, as shown in  FIG. 4A , a line L 110  having a given gradient except for 0 and passing through the minimum luminance point P 0  is predetermined, and the points of intersection of the line L 110  and intermediate•high luminance function parts (for example, intermediate•high luminance function parts L 12 A and L 12 B) are set as connection points (for example, connection points P 1 A and P 1 B) to the intermediate•high luminance function parts. In other words, irrespective of the DC fluctuation amounts of the intermediate•high luminance function parts, the low luminance function part L 11  with a fixed gradient continuously connects to the intermediate•high luminance parts at the connection points along the line L 110 . 
         [0053]    As another method, for example, as shown in  FIG. 4B , a given output luminance Yt 12  is predetermined, and points at which intermediate•high luminance function parts (for example, intermediate•high luminance function parts L 12 C and L 12 D) meet the output luminance Yt 12  are set as connection points (for example, connection points P 1 C and P 1 D) to the intermediate•high luminance function parts. In other words, in the low luminance function parts L 11 C and L 11 D, the gradients of lines are changed according to the DC fluctuation amounts of the intermediate•high luminance function parts (for example, gradients K 1 C and K 1 D). A low luminance function part is represented by Formula 1, and an intermediate•high luminance function part is represented by Formula 2, and the output luminance at the point (connection point) of intersection of these function parts is Yt 12 , so the gradient K of the line in this case is represented by Formula 3. In other words, as the value of α in Formula 3 is increased with an increase in the DC fluctuation amount of the intermediate•high luminance function part, the gradient K of the line is decreased. 
         [0000]        Y out11= Y in−α  (1) 
         [0000]        Y out11= K×Y in  (2) 
         [0000]        K=Yt 12/( Yt 12+α)  (3) 
         [0054]    The low luminance function part L 11  generated by the black level correction function generating circuit  213  in such a manner is outputted to the correction function determining circuit  214 . 
         [0055]    Referring back to  FIG. 2 , the correction function determining circuit  214  determines an image correction function (for example, an image correction function L 14  in  FIG. 3 ) including these function parts on the basis of the intermediate•high luminance function part L 12  generated by the DC shift circuit  212  and the low luminance function part L 11  generated by the black level correction function generating circuit  213 . 
         [0056]    Moreover, the correction execution circuit  215  actually corrects the luminance signal Yin from the switch  14  on the basis of the image correction function determined by the correction function determining circuit  214 . The luminance signal corrected in such a manner is outputted to the black level adjusting section  22  as a luminance signal Yout 11 . 
         [0057]    Referring back to  FIG. 1 , the black level adjusting section  22  detects a low luminance level region (a blackest level region) in an image frame on the basis of the inputted luminance signal Yout 11 , and in the case where the blackest level region exists in a certain area range, for example, as shown in  FIG. 5 , the black level adjusting section  22  shifts the luminance signal in the blackest level region to black side (the luminance level is lowered) so as to correct the luminance signal Yout 11  to a luminance signal Yout 12 . Thus, in the black level adjusting section  22 , the luminance signal is corrected so that a black level in a displayed image is enhanced, and the corrected luminance signal is outputted to the γ correction section  23  as the luminance signal Yout 12 . 
         [0058]    For example, as shown in  FIG. 6 , the γ correction section  23  detects the histogram distribution of the luminance signal in each image frame on the basis of the inputted luminance signal Yout 12 , and, for example, as shown in  FIG. 7 , the γ correction section  23  adaptively changes an input/output characteristic (a γ characteristic) (for example, changes a γ characteristic L 30  into a γ characteristic L 31  or L 32 ) on the basis of the luminance histogram distribution, and corrects the luminance signal Yout 12  into a luminance signal Yout 13  on the basis of the γ characteristic. Thus, in the γ correction section  23 , the luminance signal is corrected on the basis of the detected luminance histogram distribution so that the contrast is improved, and the corrected luminance signal is outputted to the image processing circuit  3  as the luminance signal Yout 13 . 
         [0059]    The delay circuit  15  delays the color-difference signals Um and Vin outputted from the switch  14 , and synchronizes the color-difference signals Um and Vin and the corrected luminance signal Yout 13  outputted from the luminance signal correction section  2  to output them to the image processing circuit  3 . 
         [0060]    The image processing circuit  3  performs predetermined image processing such as, for example, sharpness processing on the corrected luminance signal Yout 13  outputted from the luminance signal correction section  2  and UV signals (Uout 1 , Vout 1 ) which are outputted from the switch  14  and pass through the delay circuit  15 . The YUV signals (Yout 2 , Uout 2 , Vout 2 ) after image processing in such a manner are outputted to the matrix circuit  41 . 
         [0061]    The matrix circuit  41  reproduces RGB signals from the YUV signals (Yout 2 , Uout 2 , Vout 2 ) after image processing by the image processing circuit  3 , and outputs the reproduced RGB signals (Rout, Gout, Bout) to the driver  42 . 
         [0062]    The driver  42  produces a driving signal for the display  5  on the basis of the RGB signals (Rout, Gout, Bout) outputted from the matrix circuit  41 , and outputs the driving signal to the display  5 . 
         [0063]    The display  5  displays an image on the basis of the YUV signals (Yout 2 , Uout 2 , Vout 2 ) after the luminance signal is corrected by the luminance signal correction section  2 , and image processing is performed by the image processing circuit  3  according to the driving signal outputted from the driver  42 . The display  5  may be any kind of display device. For example, a CRT (Cathode-Ray Tube)  51 , a LCD (Liquid Crystal Display)  52 , a PDP (Plasma Display Panel; not shown) or the like is used. 
         [0064]    The YUV signals (Yin, Uin, Vin) correspond to specific examples of “input image data” in the invention. The DC transmission rate correction section  21  corresponds to a specific example of “an image correction circuit” in the invention, and the APL detection circuit  211  corresponds to a specific example of “a luminance detection means” in the invention, and the DC shift circuit  212 , the black level correction function generating circuit  213 , the correction function determining circuit  214  and the correction execution circuit  215  correspond to specific examples of “an image correction means” in the invention. The DC shift circuit  212 , the black level correction function generating circuit  213  and the correction function determining circuit  214  correspond to specific examples of “a function determination means” in the invention, and the correction execution circuit  215  corresponds to a specific example of “a correction execution means” in the invention. 
         [0065]    Next, the operation of the image display according to the embodiment will be described below. 
         [0066]    At first, an image signal to be inputted into the image display is demodulated into the YUV signals. More specifically, a TV signal from the TV is demodulated into a composite video burst signal by the tuner  11 , and a composite video burst signal is directly inputted into the image display from the VCR or the DVD 1 . Then, the composite video burst signals are separated into the luminance signal Y 1  and the chrominance signal C 1  in the Y/C separation circuit  12 , and then the luminance signal Y 1  and the chrominance signal C 1  are decoded into the YUV signals (Y 1 , U 1 , V 1 ) in the chroma decoder  13 . On the other hand, YUV signals (Y 2 , U 2 , V 2 ) are directly inputted into the image display from the DVD 2 . 
         [0067]    Next, in the switch  14 , either the YUV signals (Y 1 , U 1 , V 1 ) or the YUV signals (Y 2 , U 2 , V 2 ) are selected to be outputted as the YUV signals (Yin, Uin, Vin). Then, the luminance signal Yin of the YUV signals (Yin, Uin, Vin) is outputted into the luminance signal correction section  2 , and the color-difference signals Uin and Vin are outputted to the delay circuit  15 . 
         [0068]    In the luminance signal correction section  2 , the following operation of correcting the luminance signal is performed on the basis of the inputted luminance signal Yin. 
         [0069]    At first, in the DC transmission rate correction section  21 , the APL detection circuit  211  detects the average peak level in each image frame on the basis of the inputted luminance signal Yin, and the DC shift circuit  212  generates the intermediate•high luminance function part L 12  which lowers the luminance level in the intermediate•high luminance region according to the detected average peak level. On the other hand, the black level correction function generating circuit  213  generates the low luminance function part L 11  which is a part continuously connecting between the minimum luminance point P 0  and the intermediate•high luminance function part L 12  at the connection point P 1  so as to reduce the luminance signal in the low luminance region though at a predetermined rate. Then, the correction function determining circuit  214  determines the image correction function including the intermediate•high luminance function part L 12  and the low luminance function part L 11 , and the correction execution circuit  215  corrects the luminance signal Yin from the switch  14  on the basis of the determined image correction function. 
         [0070]    As described above, the determined image correction function is generated so that the input luminance signal is lowered according to the average peak level in the intermediate•high luminance region, and the input luminance signal is maintained at a level lower than an original level in the low luminance region (refer to the intermediate•high luminance function part L 12  and the low luminance function part L 11  in  FIG. 3 ), so while the DC level conversion is performed in the intermediate•high luminance region, a loss of gray levels in the low luminance region of the converted luminance signal can be prevented. 
         [0071]    Next, the black level adjusting section  22  detects the blackest level region in the image frame on the basis of the luminance signal Yout 11 , and in the case where the blackest level region exists in a certain area range, the black level adjusting section  22  shifts the luminance signal in the blackest level region to black side (the luminance level is lowered) so as to correct the luminance signal so that the black level in the displayed image is enhanced. Then, the γ correction section  23  detects the luminance histogram distribution in each image frame on the basis of the luminance signal Yout 12 , and corrects the luminance signal on the basis of the characteristic adaptively changed according to the luminance histogram distribution so that the contrast is improved. The luminance signal corrected in such a manner is outputted to the image processing circuit  3  as the luminance signal Yout 13 . 
         [0072]    On the other hand, the delay circuit  15  delays the color-difference signals Uin and Vin, and as a result, the color-difference signals Um and Vin and the luminance signal Yout 13  outputted from the luminance signal adjusting section  2  are synchronized. 
         [0073]    Next, the image processing circuit  3  performs predetermined image processing such as, for example, sharpness processing on the corrected luminance signal Yout 13  outputted from the luminance signal correction section  2  and the UV signals (Uout 1 , Vout 1 ) which are outputted from the switch  14  and pass through the delay circuit  15 . 
         [0074]    Then, the matrix circuit  41  reproduces RGB signals (Rout, Gout, Bout) from the YUV signals (Yout 2 , Uout 2 , Vout 2 ) after image processing, and the driver  42  produces a driving signal on the basis of the RGB signals (Rout, Gout, Bout), and an image is displayed on the display  5  on the basis of the driving signal. 
         [0075]    As described above, in the embodiment, in the DC transmission rate correction section  21 , while the luminance signal level in the low luminance region is reduced from the original luminance signal level though at a predetermined rate, the luminance signal level in the intermediate•high luminance region is lowered according to the average peak level detected by the APL detection circuit  211 , so even if the DC level conversion is performed in the intermediate•high luminance region as in the case of a related art, a loss of gray levels in the low luminance region after the conversion can be prevented. Therefore, gray levels in the low luminance region can be reliably displayed, and the quality of a displayed image can be improved. 
         [0076]    Moreover, for example, as shown in  FIGS. 3 ,  4 A and  4 B, the low luminance function part is represented by a line continuously connecting between the minimum luminance point P 0  and the intermediate•high luminance function part, so the image correction function including these function parts can be easily generated. Therefore, the configurations of the black level correction function generating circuit  213  and the correction function determining section  214  can be simplified, and the circuit sizes can be reduced. 
         [0077]    Further, in the correction function determining circuit  214 , in the low luminance region, a smaller value (the low luminance function part) is selected and determined between the intermediate•high luminance function part and the low luminance function part, so also in this viewpoint, the image correction function can be easily generated. 
       [Modification] 
       [0078]    Next, a modification of the first embodiment will be described below. In the modification, for example, as shown by an image correction function L 14 A in  FIG. 8 , in the DC transmission rate correction section  21 , a low luminance function part which includes a part L 11 A passing through the minimum luminance point P 0  and having the same shape as the image correction function L 10  before DC level conversion is generated, and the luminance signal is corrected by the image correction function including such a low luminance function part. 
         [0079]    More specifically, for example, in  FIG. 8 , the low luminance function part L 11  includes a line part L 11 A connecting between the minimum luminance point P 0  and a connection point P 20  on the image correction function L 10  and a line part L 11 B connecting between the connection point P 20  and the intermediate•high luminance function part L 12  at a connection point P 21 . In other words, in the image correction function in the first embodiment, the minimum luminance point P 0  and the intermediate•high luminance function part are directly connected by a line with a fixed gradient, but in the modification, they are connected by the part L 11 A having the same shape as the image correction function L 10  before DC level conversion. 
         [0080]    Therefore, in the modification, for example, as shown by the image correction function L 14  in  FIG. 9 , even if the luminance level of the corrected luminance signal Yout 11  is further lowered by the DC transmission rate correction section  21  in the low luminance region, for example, as in the case of an image correction function L 24 , a loss of gray levels can be prevented. 
         [0081]    Therefore, even if the black level adjusting section  22  is arranged before the DC transmission rate correction section  21 , or the DC transmission rate correction section  21 , the black level adjusting section  22  and the γ correction section  23  are arranged in parallel, for example, as shown in luminance signal correction sections  2 A and  2 B in  FIGS. 10A and 10B , respectively, as described above, a loss of gray levels can be reliably prevented. 
         [0082]    As described above, in the modification, in the DC transmission rate correction section  21 , the low luminance function part which includes the part L 11 A passing through the minimum luminance point P 0  and having the same shape as the image correction function L 10  before DC level conversion is generated, and the luminance signal is corrected by the image correction function including such a low luminance function part, so, for example, even if the luminance level in the low luminance region is further lowered after the DC transmission rate correction, a loss of gray levels can be reliably prevented. Therefore, in addition to the effects in the first embodiment, irrespective of the arrangement of the luminance signal correction section  2  or the like, gray levels in the low luminance region can be displayed more reliably, and the quality of a displayed image can be further improved. 
         [0083]    Moreover, as the line part L 11 A, the image correction function L 10  before DC level conversion may be used as it is, so the image correction function can be easily generated and achieved. 
         [0084]    Further, irrespective of the arrangement of the luminance signal correction section  2  or the like, the modification can be applied to luminance signal correction sections with various configurations, so flexibility in device design can be improved. 
       Second Embodiment 
       [0085]    Next, a second embodiment of the invention will be described below. An image display according to the embodiment adjusts an image correction function in a high luminance region (a white level region) in addition to the adjustment of an image correction function in a low luminance region (a black level region) described in the first embodiment. 
         [0086]      FIG. 11  shows the circuit configuration of a DC transmission rate correction section  21 A according to the embodiment. The DC transmission rate correction section  21 A further includes a white level correction function generating circuit  216  in the DC transmission rate correction section  21  described in the first embodiment. Like components are denoted by like numerals as of the first embodiment and will not be further described. 
         [0087]    For example, as shown in  FIG. 12 , the white level correction function generating circuit  216  generates a high luminance function part (a white level correction function) L 13 , which is a part continuously connecting between a maximum luminance point P 4  and an intermediate luminance function part L 14  while maintaining the maximum luminance point P 4  at the maximum luminance point in the image correction function L 10 , in an image correction function (for example, an image correction function L 15  in the drawing) in the DC transmission rate correction section  21 A. For example, in  FIG. 12 , the high luminance function part L 13  connects between the maximum luminance point P 4  and the intermediate luminance function part L 12  with a line with a predetermined change rate (a gradient K 2 ). 
         [0088]    The DC shift circuit  212 , the black level correction function generating circuit  213 , the white level correction function generating circuit  216 , the correction function determining circuit  214  and the correction execution circuit  215  in the embodiment correspond to specific examples of “an image correction means” in the invention. The DC shift circuit  212 , the black level correction function generating circuit  213 , the white level correction function generating circuit  216  and the correction function determining circuit  214  correspond to specific examples of “a function determination means” in the invention. 
         [0089]    By the structure of such an image correction function, in the DC transmission rate correction section  21 A in the embodiment, while the luminance level of the luminance signal Yin in the intermediate luminance region is lowered according to the detected average peak level by a predetermined level, the maximum luminance point P 4  is maintained in the high luminance region. Therefore, while DC level conversion is performed in the intermediate luminance region, a decline in the luminance level can be prevented in the high luminance region of the luminance signal Yout 11  after the conversion. 
         [0090]    As described above, in the embodiment, while the maximum luminance point P 4  is maintained in the high luminance region, the luminance level of the luminance signal Yin in the intermediate luminance region is lowered according to the detected average peak level, so even if the DC level conversion is performed in the intermediate luminance region, a decline in the luminance level in the high luminance region after the conversion can be prevented. Therefore, in addition to the effect of preventing a loss of gray levels in the low luminance region in the first embodiment, gray levels in the high luminance region can be reliably displayed, and the quality of a displayed image can be improved. 
         [0091]    Moreover, for example, as shown in  FIG. 12 , the high luminance function part is also represented by a line continuously connecting between the maximum luminance point P 4  and the intermediate luminance function part, so an image correction function including these function parts can be easily generated. Therefore, the configuration of the white level correction function generating circuit  216  can be simplified, and the circuit size can be reduced. 
         [0092]    Further, the black level correction function generating circuit  213  and the white level correction function generating circuit  216  are separately arranged, so the operation of adjusting an image correction function in the low luminance region and the operation of adjusting an image correction in the high luminance region can be individually performed. 
         [0093]    Also in the image correction function described in the embodiment, for example, as shown by an image correction function L 15 A in  FIG. 13 , like the modification of the first embodiment (refer to  FIG. 8 ), the low luminance function part L 11  may include a part L 11 A which passes through the minimum luminance point P 0  and has the same shape as the image correction function L 10  before DC level conversion. In such a configuration, in addition to the effects in the embodiment, irrespective of the arrangement of the luminance signal correction section or the like, gray levels in the low luminance region can be displayed more reliably, and the quality of a displayed image can be further improved. 
         [0094]    Although the invention is described referring to the first embodiment and the second embodiment; however, the invention is not limited to them, and can be variously modified. 
         [0095]    For example, in the above embodiments, the case where the image correction function in the low luminance region (the black level region) is adjusted (the case of the first embodiment) and the case where in addition to the image correction function in the low luminance region, the image correction function in the high luminance region (the white level region) is adjusted (the case of the second embodiment) are described; however, for example, as shown by an image correction function L 15 B in  FIG. 14 , only the image correction function in the high luminance region (the white level region) may be adjusted. In such a configuration, while the DC level conversion is performed in the intermediate luminance region, a decline in the luminance level in the high luminance region after the conversion can be prevented. Therefore, gray levels in the high luminance region can be reliably displayed, and the quality of a displayed image can be further improved. 
         [0096]    Moreover, in the above-described embodiments, the case where the low luminance function part or the high luminance function part is represented by a line with a fixed gradient is described; however, as long as a loss of gray levels in the low luminance region or a decline in the luminance level in the high luminance region can be prevented, and the low luminance function part or the high luminance function part can continuously connect to the intermediate luminance function part, the function part may be represented by a curve instead of a line. 
         [0097]    Further, in the above-described embodiments, the luminance signal correction section  2  includes the black level adjusting section  22  and the γ correction section  23  in addition to the DC transmission rate correction section  21 ; however, the luminance signal correction section  2  may include only the DC transmission rate correction section  21 , or the luminance signal correction section  2  may further include another luminance signal correction circuit in addition to them. 
         [0098]    It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.