Patent Publication Number: US-7907135-B2

Title: Image display and image displaying method for gradation correction

Description:
TECHNICAL FIELD 
     The present invention relates to an image display device and an image display method, and especially relates to a technology for correcting a gradation so as to improve visibility by eliminating an effect of outside light. 
     BACKGROUND ART 
     A digital signal processing technology is conventionally applied to a wide range of fields such as an image display device like a projector. For example, a histogram conversion method is a technology for correcting an image gradation, and is also useful for improving a contrast of an image projected by a projector. In view of a projector field characteristic, further improvements of the histogram conversion method have been made. 
     A patent document 1 discloses an example of such technology improvements. Originally, a histogram conversion has been performed with regard to all luminance ranges. However, in the patent document 1, a histogram conversion is performed with regard to a range from a minimum value to a maximum value in an effective video period of a video luminance signal, using a gradation correction coefficient according to a difference between the maximum value and the minimum value. 
       FIG. 1  is a functional block diagram showing a construction of an image processor of the conventional technology. As shown in  FIG. 1 , an image processor  20  includes a minimum value detecting circuit  2001 , a maximum value detecting circuit  2002 , a differential device  2003 , a correction coefficient calculating circuit  2004 , and a correcting circuit  2005 . When receiving an input video luminance signal, the image processor  20  detects a minimum value in an effective video period of the input video luminance signal in the minimum value detecting circuit  2001 , and a maximum value in the maximum value detecting circuit  2002 . Then, a difference between the maximum value and the minimum value is obtained in the differential device  2003 , and a correction coefficient is determined according to the difference in the correction coefficient calculating circuit  2004 . The input video luminance signal is corrected according to the correction coefficient in the correcting circuit  2005 , and the corrected input video luminance signal is outputted as an output video luminance signal. This further improves an image contrast.
     Patent Document 1: Japanese Published Patent Application No. 3549356   

     DISCLOSURE OF THE INVENTION 
     Problems the Invention is Going to Solve 
     However, when considering a usage environment of a projector, it is not necessarily the case that light is completely shielded as in a movie theater. Therefore, viewability of projected video is not always optimum by merely performing a gradation correction only in view of a video luminance signal characteristic as in the above-mentioned conventional technology. For example, it is well known that room lighting, sunlight, and the like have an effect on a visual environment, causing poor viewability. 
     To solve the above-mentioned problem, the present invention aims to provide an image display device and an image display method which display an easily viewable image regardless of a visual environment change. 
     Means of Solving the Problems 
     The above problem is solved by an image display device for displaying a multiple gradation image, comprising: a light measuring unit (such as an outside light detector  304  in  FIG. 4 ) operable to measure an outside light intensity; and a correcting unit (such as a luminance level adjusting circuit  303  in  FIG. 4 ) operable to perform a gradation correction to cause (i) luminance values to be distributed in a smaller luminance value range, the larger the outside light intensity is, and (ii) the luminance value range to include a highest luminance value displayable by the image display device (such as an offset level adjusting circuit  305  in  FIG. 4 ). 
     EFFECTS OF THE INVENTION 
     With the above-stated construction, a luminance can be increased according to an outside light intensity. Therefore, an easily viewable image can be displayed regardless of a visual environment change. 
     In the image display device of the present invention, the gradation correction is performed to cause a correction coefficient C i  to be larger, the larger a frequency of the luminance values in a portion of the luminance value range is (such as a coefficient calculating circuit  302  in  FIG. 4 ). With the above-stated construction, visibility can be improved by reducing contrast deterioration caused by an increase of a luminance lower limit. Note that a portion in which more luminances are distributed means luminances having high frequencies. 
     The image display device of the present invention further comprises a detecting unit (such as a skin color detecting circuit  806  in  FIG. 9 ) operable to detect a skin color pixel having a skin color luminance and a skin color hue, wherein the performance of the gradation correction on the skin color pixel is suppressed. With the above-stated construction, a luminance of a skin color portion is not corrected. Therefore, a natural image can be displayed by avoiding a color tone change. 
     In this case, it is preferable that the gradation correction is performed on a pixel other than the skin color pixel to cause a luminance change to be smaller, the closer to a skin color a luminance value and a hue of the pixel are. Also, the performance of the gradation correction on the skin color pixel is suppressed when other skin color pixels exist around the skin color pixel. 
     The image display device of the present invention further comprises a first calculating unit (such as an average luminance calculating circuit  1106  in  FIG. 11 ) operable to calculate an average luminance value M 0  of all pixels composing the image; and a changing unit (such as a luminance level adjusting circuit  1103  in  FIG. 11 ) operable to, if a difference between an average luminance value M 1  and the average luminance value M 0  is larger than a predetermined value, change a correction coefficient C i ′ to cause the difference to be equal to or smaller than the predetermined value, the average luminance value M 1  being obtained by the correcting unit performing the gradation correction on the average luminance value M 0 . With the above-stated construction, a big change of an average luminance can be avoided. Therefore, a natural image can be displayed by reducing an excessive change of a gradation. 
     The image display device of the present invention further comprises a lowering unit (such as a black level correcting circuit  1706  in  FIG. 17 ) operable to lower luminance values in a predetermined luminance value range including a lowest luminance value, if the luminance values are higher than a predetermined luminance value after the gradation correction is performed by the correcting unit. With the above-stated construction, a sharp image can be displayed by preventing a low luminance portion from being brighter than other portions. 
     The image display device of the present invention further comprises a second calculating unit (such as a maximum value detecting circuit  1406  and a minimum value detecting circuit  1407  in  FIG. 14 ) operable to calculate a maximum value and a minimum value of luminance values of all pixels composing the image; and a distributing unit (such as a luminance distribution detecting circuit  1401 ) operable to, as a luminance distribution, proportionally distribute frequencies of the luminance values distributed in a range of the minimum value to the maximum value, to a range of 0 to a maximum gradation number, wherein the gradation correction is performed on the proportionally distributed luminance distribution. With the above-stated construction, visibility can be improved by enhancing an image contrast. 
     The above problem is solved by an image display method performed by an image display device for displaying a multiple gradation image, the image display method comprising: a light measuring step (such as S 604  in  FIG. 7 ) of measuring an outside light intensity; and a correcting step (such as S 606  in  FIG. 7 ) of performing a gradation correction to cause (i) luminance values to be distributed in a smaller luminance value range, the larger the outside light intensity is, and (ii) the luminance value range to include a highest luminance value displayable by the image display device (such as S 607  in  FIG. 7 ). With the above-stated construction, an easily viewable image can be displayed regardless of a visual environment change because a luminance is improved according to an outside light intensity. 
     In the image display method of the present invention, the gradation correction is performed to cause a correction coefficient C i  to be larger, the larger a frequency of the luminance values in a portion of the luminance value range is (such as S 603  in  FIG. 7 ). With the above-stated construction, visibility can be improved by reducing contrast deterioration caused by an increase of a luminance lower limit. 
     The image display method of the present invention further comprises a detecting step (such as S 902  in  FIG. 10 ) of detecting a luminance or a hue of a skin color pixel, wherein the performance of the gradation correction on the skin color pixel is suppressed. With the above-stated construction, a luminance of a skin color portion is not corrected. Therefore, a natural image can be displayed by avoiding a color tone change. 
     In this case, it is preferable that the gradation correction is performed on a pixel other than the skin color pixel to cause a luminance change to be smaller, the closer to a skin color a luminance value and a hue of the pixel are. Also, the performance of the graduation correction on the skin color pixel is suppressed when other skin color pixels exist around the skin color pixel. 
     The image display method of the present invention further comprises a first calculating step (such as S 1202  in  FIG. 12 ) of calculating an average luminance value M 0  of all pixels composing the image; and a changing step (such as S 1206  in  FIG. 12 ) of, if a difference between an average luminance value M 1  and the average luminance value M 0  is larger than a predetermined value, changing a correction coefficient C i ′ to cause the difference to be equal to or smaller than the predetermined value, the average luminance value M 1  being obtained by the correcting unit performing the gradation correction on the average luminance value M 0 . With the above-stated construction, a big change of an average luminance can be avoided. Therefore, a natural image can be displayed by reducing an excessive change of a gradation. 
     The image display method of the present invention further comprises a lowering step (such as S 1807  in  FIG. 18 ) of lowering luminance values in a predetermined luminance value range including a lowest luminance value, if the luminance values are higher than a predetermined luminance value after the gradation correction is performed by the correcting step. With the above-stated construction, a sharp image can be displayed by preventing a low luminance portion from being brighter than other portions. 
     The image display method of the present invention further comprises a second calculating step (such as S 1502  and S 1503  in  FIG. 15 ) of calculating a maximum value and a minimum value of luminance values of all pixels composing the image; and a distributing step (such as S 1504  in  FIG. 15 ) of, as a luminance distribution, proportionally distributing frequencies of the luminance values distributed in a range of the minimum value to the maximum value, to a range of 0 to a maximum gradation number, wherein the gradation correction is performed on the proportionally distributed luminance distribution. With the above-stated construction, visibility can be improved by enhancing an image contrast. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a functional block diagram showing a construction of an image processor of a conventional technology. 
         FIG. 2  shows a usage pattern of a projector of a first embodiment of the present invention. 
         FIG. 3  is an appearance diagram of a projector  1  of the first embodiment of the present invention. 
         FIG. 4  is a block diagram showing a main functional construction of the projector  1  of the first embodiment of the present invention. 
         FIG. 5  is a graph showing a luminance distribution in an effective video period of a video luminance signal. 
         FIG. 6  is a graph showing that the projector  1  of the first embodiment of the present invention adjusts a correction coefficient. 
         FIG. 7  is a flowchart showing a gradation correction process performed by the projector  1  of the first embodiment of the present invention. 
         FIG. 8  is a graph showing a correction coefficient of a gradation correction method of a conventional technology, and a correction coefficient of a gradation correction method of the first embodiment. 
         FIG. 9  is a block diagram showing a main construction of a projector of a second embodiment of the present invention. 
         FIG. 10  is a flowchart showing an operation of a projector  8  of the second embodiment of the present invention. 
         FIG. 11  is a block diagram showing a main construction of a projector of a third embodiment of the present invention. 
         FIG. 12  is a flowchart showing an operation of a projector  11  of the third embodiment of the present invention. 
         FIG. 13  is a graph showing that the projector  11  of the third embodiment of the present invention adjusts a correction coefficient. 
         FIG. 14  is a block diagram showing a main construction of a projector of a fourth embodiment of the present invention. 
         FIG. 15  is a flowchart showing an operation of a projector  14  of the fourth embodiment of the present invention. 
         FIG. 16  is a graph showing that the projector  14  of the fourth embodiment of the present invention adjusts a correction coefficient. 
         FIG. 17  is a block diagram showing a main construction of a projector of a fifth embodiment of the present invention. 
         FIG. 18  is a flowchart showing an operation of a projector  17  of the fifth embodiment of the present invention. 
         FIG. 19  is a graph showing that a block level correcting circuit  1706  of the fifth embodiment of the present invention lowers a luminance in a low luminance portion. 
     
    
    
     DESCRIPTION OF REFERENCE NUMERALS 
       1 ,  8 ,  11 ,  14 ,  17 : projector 
       20 : image processor 
       101 : personal computer 
       102 : screen 
       201 : video input terminal 
       202 : S-video input terminal 
       203 : RGB/YPbPr input terminal 
       204 : remote control light receiving unit 
       205 : lens 
       206 : direction key 
       207 : determination button 
       208 : outside light sensor 
       301 ,  801 ,  1101 ,  1401 ,  1701 : luminance distribution detecting circuit 
       302 ,  802 ,  1102 ,  1402 ,  1702 : coefficient calculating circuit 
       303 ,  803 ,  1103 ,  1403 ,  1703 : luminance level adjusting circuit 
       304 ,  804 ,  1104 ,  1404 ,  1704 : outside light detector 
       305 ,  805 ,  1105 ,  1405 ,  1705 : offset level adjusting circuit 
       806 : skin color detecting circuit 
       1106 : average luminance calculating circuit 
       1406 ,  2002 : maximum value detecting circuit 
       1407 ,  2001 : minimum value detecting circuit 
       1706 : black level correcting circuit 
       2003 : differential device 
       2004 : correction coefficient calculating circuit 
       2005 : correcting circuit 
     BEST MODE FOR CARRYING OUT THE INVENTION 
     The following describes an image display device and an image display method according to embodiments of the present invention by taking a projector as an example, with reference to the attached drawings. 
     [1] First Embodiment 
     Firstly, a projector of a first embodiment of the present invention will be described. 
     (1) Construction of Projector 
       FIG. 2  shows a usage pattern of the projector of the first embodiment. As shown in  FIG. 2 , a projector  1  receives a video luminance signal from a personal computer  101 , and processes the video luminance signal. Then, the projector  1  outputs the processed video luminance signal to a built-in liquid crystal panel to project the processed video luminance signal onto a screen  102 . 
       FIG. 3  is an appearance diagram of the projector  1 . As shown in  FIG. 3 , the projector  1  includes a video input terminal  201 , an S-video input terminal  202 , an RGB/YPbPr input terminal  203 , a remote control light receiving unit  204 , a lens  205 , direction keys  206 , a determination button  207 , and an outside light sensor  208 . The video input terminal  201 , the S-video input terminal  202 , and the RGB/YPbPr input terminal  203  are arranged on a side of an enclosure. The remote control light receiving unit  204  and the lens  205  are arranged on a front of the enclosure. The direction keys  206 , the determination button  207 , and the outside light sensor  208  are arranged on an upper surface of the enclosure. The outside light sensor  208  detects brightness around the projector  1 . 
       FIG. 4  is a block diagram showing a main functional construction of the projector  1 . As shown in  FIG. 4 , the projector  1  includes a luminance distribution detecting circuit  301 , a coefficient calculating circuit  302 , a luminance level adjusting circuit  303 , an outside light detector  304 , and an offset level adjusting circuit  305 . 
     The luminance distribution detecting circuit  301  detects a luminance distribution in an effective video period of an inputted video luminance signal.  FIG. 5  is a graph showing a luminance distribution in an effective video period of a video luminance signal.  FIG. 5A  shows the luminance distribution in the effective video period of the video luminance signal, and  FIG. 5B  shows a luminance distribution detected by the luminance distribution detecting circuit  301  when the video luminance signal having the luminance distribution is inputted. 
     In detail, as shown in  FIG. 5A , the video luminance signal having the luminance distribution of 8 bits: 256 gradations is inputted to the luminance distribution detecting circuit  301 . The luminance distribution detecting circuit  301  detects four luminance distributions which are made by dividing the 256 gradations into four levels, i.e. from 0 to 63, from 64 to 127, from 128 to 191, and from 192 to 255 ( FIG. 5B ). 
     When receiving the four-level luminance distributions from the luminance distribution detecting circuit  301 , the coefficient calculating circuit  302  calculates a correction coefficient C i  (i=1 to 4) for each of the four-level luminance distributions in order to correct the gradations, using the following formula ( FIG. 6A ). 
     
       
         
           
             
               
                 
                   
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     Here, P is the number of all pixels in an image, and F i  (i=1 to 4) indicates a frequency for each of the four-level luminance distributions: 
     
       
         
           
             
               
                 
                   
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     As mentioned above, the correction coefficient C i  is calculated using a luminance distribution. Note that the correction coefficient C i  indicates a slope of each part of a graph shown in  FIG. 6A . 
     The outside light detector  304  detects brightness around the projector  1  using the outside light sensor  208 , and inputs a brightness signal indicating the detected brightness to the luminance level adjusting circuit  303 . 
     The luminance level adjusting circuit  303  adjusts the correction coefficient C i  received from the coefficient calculating circuit  302 , based on the brightness signal received from the outside light detector  304 . In other words, the luminance level adjusting circuit  303  adjusts the correction coefficient C i  using a correction coefficient Q indicated by the brightness signal, according to the following formula. 
     
       
         
           
             
               
                 
                   
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       FIG. 6B  is a graph showing a correction coefficient C i ′ (i=1 to 4) in the process of being adjusted. The correction coefficient is in a range of 0 to 255 before being adjusted. However, as shown in  FIG. 6B , the correction coefficient is proportionally distributed in a range of 0 to 255-Q because the luminance level adjusting circuit  303  adjusts the correction coefficient. 
     Next, luminances of all pixels are increased by the correction coefficient Q. In other words, a luminance after the correction is obtained by the following formula. 
     
       
         
           
             
               
                 
                   
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     Here, D I  is the luminance before the correction, and belongs to an n-th level (n=1 to 4) of the four-level luminance distributions. Also, D 0  is the luminance after the correction. This enables a pixel having a lowest input luminance to be increased to a luminance Q, according to a brightness signal. Thus, the offset level adjusting circuit  305  adjusts a luminance of each pixel which belongs to an effective video period of a video luminance signal, using the correction coefficient C i ′ obtained by the luminance level adjusting circuit  303 . 
     (2) Operation of Projector 
     Then, an operation of the projector  1 , especially a process for correcting a gradation will be described.  FIG. 7  is a flowchart showing the gradation correction process performed by the projector  1 . As shown in  FIG. 7 , when receiving a video luminance signal (S 601 ), the projector  1  detects a luminance distribution in an effective video period of the video luminance signal, in the luminance distribution detecting circuit  301  (S 602 ), and calculates a correction coefficient C i  for each of the four-level luminance distributions in the coefficient calculating circuit  302  (S 603 ). 
     Next, the projector  1  generates a brightness signal indicating surrounding brightness in the outside light detector  304  (S 604 ), and obtains a correction coefficient C i ′ from the correction coefficient C i , using the brightness signal in the luminance level adjusting circuit  303  (S 605 ) to correct an image gradation (S 606 ). Finally, the projector  1  adjusts an offset level in the offset level adjusting circuit  305  (S 607 ). 
     (3) Comparison with Conventional Gradation Correction Method 
     Then, a gradation correction method of a conventional technology is compared with a gradation correction method of the first embodiment.  FIG. 8  is a graph showing a correction coefficient of the gradation correction method of the conventional technology, and a correction coefficient of the gradation correction method of the first embodiment. 
     As shown in  FIG. 8A , if simply using a correction coefficient obtained from a histogram, a highest luminance in an effective video period of a video luminance signal is corrected to 255, and a lowest luminance is corrected to 0. Therefore, a contrast of a whole image is improved, but visibility in a low luminance portion is reduced. 
     As shown in  FIG. 8B , if simply increasing a luminance to maintain visibility, the luminance is saturated in a high luminance portion, and a collision of a black level with a white level occurs because the luminance cannot be larger than 255. As a result, visibility is reduced. 
     On the other hand, in the first embodiment, a minimum value of a luminance is increased to Q according to a visual environment, and a correction coefficient is proportionally distributed in a range of Q to 255 as shown in  FIG. 8C . Therefore, visibilities in both a low luminance portion and a high luminance portion can be improved at the same time. 
     [2] Second Embodiment 
     Next, a projector of a second embodiment of the present invention will be described. The projector of the second embodiment has a similar construction to the projector of the first embodiment, but differs in that a skin color pixel in an image is detected and a correction of the skin color pixel is prohibited. This difference will be mainly described below. 
     (1) Construction of Projector 
       FIG. 9  is a block diagram showing a main construction of the projector of the second embodiment. As shown in  FIG. 9 , a projector  8  includes a luminance distribution detecting circuit  801 , a coefficient calculating circuit  802 , a luminance level adjusting circuit  803 , an outside light detector  804 , an offset level adjusting circuit  805 , and a skin color detecting circuit  806 . 
     The skin color detecting circuit  806  detects a skin color pixel in an effective video period of a video luminance signal, and notifies the luminance level adjusting circuit  803  and the offset level adjusting circuit  805  of the skin color pixel. The luminance level adjusting circuit  803  does not perform a gradation correction on the skin color pixel, and the offset level adjusting circuit  805  does not adjust an offset level of the skin color pixel. 
     (2) Operation of Projector 
       FIG. 10  is a flowchart showing an operation of the projector  8 . As shown in  FIG. 10 , when receiving a video luminance signal (S 901 ), the projector  8  firstly detects a skin color pixel in the skin color detecting circuit  806  (S 902 ), detects a luminance distribution in the luminance distribution detecting circuit  801  (S 903 ), and calculates a correction coefficient C i  in the coefficient calculating circuit  802  (S 904 ). 
     Next, the projector  8  generates a brightness signal in the outside light detector  804  (S 905 ), and obtains a correction coefficient C i ′ in the luminance level adjusting circuit  803 , using the brightness signal (S 906 ) to correct an image gradation (S 907 ). Finally, the projector  8  adjusts an offset level in the offset level adjusting circuit  805  (S 908 ). 
     Generally, an intermediate hue such as a skin color is easy to remain in a human memory, and a subtle color tone difference of a skin color is easy to be recognized. On the other hand, in the second embodiment, a gradation correction of a skin color portion is prohibited for each pixel. In other words, a skin color pixel is outputted without correcting a luminance thereof. Therefore, a gradation can be corrected without deteriorating a color tone in a skin color portion in an image. 
     Note that instead of the above-mentioned construction, a gradation in a skin color portion may be corrected to the extent that a color tone is not deteriorated. In other words, when a surrounding pixel color is other than a skin color, and a correction amount of a luminance is large, a skin color portion does not fit in the surrounding pixel if a luminance of the skin color pixel is not varied at all. In this case, a luminance of a skin color pixel may be corrected. 
     However, even in this case, it is preferable that the luminance of the skin color pixel is corrected so that a correction amount of the skin color pixel is smaller than the surrounding pixel. With the above-mentioned construction, a natural image display can be realized because a skin color portion fits in a surrounding pixel. 
     If a surrounding pixel color is a skin color, a luminance of a skin color pixel may not be corrected. A change of a skin color is sensitively recognized in a human skin portion. Therefore, whether a pixel is a human skin portion is judged by whether a surrounding pixel is a skin color, and if the surrounding pixel is a skin color, a luminance of a skin color pixel is not corrected. With this construction, a natural figure can be displayed by avoiding a color modulation in a human skin portion. 
     Note that regarding a yellow race, a skin color is an intermediate color between a red color and a yellow color, and a luminance of a skin color is in a range of 50% to 60% of a maximum luminance which is a brightest luminance. If a skin color is expressed by mixing three primary colors RGB in 256 gradations from 0 to 255, a skin color of a yellow race is expressed by a red color  255 , a green color  232 , and a blue color  192 , for example. A skin color of a white race is expressed by a red color  255 , a green color  208 , and a blue color  192 , for example. A skin color of a black race is expressed by a red color  105 , a green color  52 , and a blue color  44 , for example. Note that a luminance of 0 is darkest, and a luminance of 255 is brightest. 
     [3] Third Embodiment 
     Next, a projector of a third embodiment of the present invention will be described. The projector of the third embodiment has a similar construction to the projector of the first embodiment, but differs in that a correction coefficient is adjusted by detecting an average luminance of an image. This difference will be mainly described below. 
     (1) Construction of Projector 
       FIG. 11  is a block diagram showing a main construction of the projector of the third embodiment. As shown in  FIG. 11 , a projector  11  includes a luminance distribution detecting circuit  1101 , a coefficient calculating circuit  1102 , a luminance level adjusting circuit  1103 , an outside light detector  1104 , an offset level adjusting circuit  1105 , and an average luminance calculating circuit  1106 . 
     The average luminance calculating circuit  1106  calculates an average luminance in an effective video period of a video luminance signal, and notifies the luminance level adjusting circuit  1103  of the average luminance. The luminance level adjusting circuit  1103  adjusts a correction coefficient obtained in the coefficient calculating circuit  1102  based on a result in the average luminance calculating circuit  1106 , so that a correction value is equal to or smaller than a predetermined value by preventing the correction coefficient from being excessively corrected in a dark gradation level and a bright gradation level. 
     (2) Operation of Projector  11   
       FIG. 12  is a flowchart showing an operation of the projector  11 . As shown in  FIG. 12 , when receiving a video luminance signal (S 1201 ), the projector  11  firstly calculates an average luminance in the average luminance calculating circuit  1106  (S 1202 ), detects a luminance distribution in the luminance distribution detecting circuit  1101  (S 1203 ), and calculates a correction coefficient C i  in the coefficient calculating circuit  1102  (S 1204 ). 
     Next, the projector  11  generates a brightness signal in the outside light detector  1104  (S 1205 ), and obtains a correction coefficient C i ′ in the luminance level adjusting circuit  1103 , using the brightness signal. In this case, the correction coefficient is obtained by preventing an image having a biased frequency from being excessively corrected (S 1206 ). Then, an image gradation is corrected (S 1207 ). Finally, the projector  11  adjusts an offset level in the offset level adjusting circuit  1105  (S 1208 ). 
     Suppose, for instance, there is an image having a black background, and a foreground in which a black object such as a crow is expressed. In this case, if a gradation is corrected simply based on a histogram, a luminance of the craw in the foreground may be too high because the correction is excessive. On the other hand, in the third embodiment, an average luminance of an image has been detected in advance, and a gradation is corrected without largely deviating from the average luminance. 
       FIG. 13  is a graph showing that the projector  11  adjusts a correction coefficient. Firstly, the coefficient calculating circuit  1102  calculates a correction coefficient ( FIG. 13A ). In  FIG. 13A , M 0  is an average luminance obtained from a luminance distribution before a correction. Even if a correction coefficient is adjusted according to the first embodiment when there is outside light, an image is brightly displayed when a signal having a slightly-high input luminance level is inputted because of the following reason. The reason is that a correction coefficient in a low input luminance level portion is too large, though the average luminance M 0  is small (a dark image scene). On the other hand, when the average luminance M 0  is small (a dark image scene), a correction is performed so as to prevent a correction coefficient from being larger than a predetermined correction coefficient. This can prevent an image from being unnatural ( FIG. 13B ). 
     [4] Fourth Embodiment 
     Next, a projector of a fourth embodiment of the present invention will be described. The projector of the fourth embodiment has a similar construction to the projector of the first embodiment, but differs in that a luminance distribution is corrected by detecting a minimum value and a maximum value of a luminance in an effective video period of a video luminance signal. This difference will be mainly described below. 
     (1) Construction of Projector 
       FIG. 14  is a block diagram showing a main construction of the projector of the fourth embodiment. As shown in  FIG. 14 , a projector  14  includes a luminance distribution detecting circuit  1401 , a coefficient calculating circuit  1402 , a luminance level adjusting circuit  1403 , an outside light detector  1404 , an offset level adjusting circuit  1405 , a maximum value detecting circuit  1406 , and a minimum value detecting circuit  1407 . 
     The maximum value detecting circuit  1406  detects a maximum value of a luminance in an effective video period of a video luminance signal. The minimum value detecting circuit  1407  detects a minimum value in the effective video period. Then, the luminance distribution detecting circuit  1401  is notified of the maximum value and the minimum value. The luminance distribution detecting circuit  1401  proportionally distributes frequencies of luminances distributed in a range of the minimum value to the maximum value, to a range of 0 to 255 as a luminance distribution. 
     (2) Operation of Projector  14   
       FIG. 15  is a flowchart showing an operation of the projector  14 . As shown in  FIG. 15 , when receiving a video luminance signal (S 1501 ), the projector  14  firstly calculates a maximum value of a luminance in the maximum value detecting circuit  1406  (S 1502 ), and calculates a minimum value of the luminance in the minimum value detecting circuit  1407  (S 1503 ). Then, the projector  14  detects a luminance distribution in the luminance distribution detecting circuit  1401  (S 1504 ), and calculates a correction coefficient C i  in the coefficient calculating circuit  1402  (S 1505 ). 
     Next, the projector  14  generates a brightness signal in the outside light detector  1404  (S 1506 ), and adjusts a correction coefficient in the luminance level adjusting circuit  1403 , using the brightness signal. In this case, the projector  14  obtains a correction coefficient C i ′ so as to proportionally distribute frequencies of luminances distributed in a range of the minimum value to the maximum value, to a range of 0 to 255 (S 1507 ). Then, an image gradation is corrected (S 1508 ). Finally, the projector  14  adjusts an offset level in the offset level adjusting circuit  1405  (S 1509 ). 
       FIG. 16  is a graph showing that the projector  14  adjusts a correction coefficient. As shown in  FIG. 16 , firstly, a correction coefficient is adjusted so as to correct an input luminance in a range of a minimum value to a maximum value of a luminance in an effective video period of a video luminance signal, to an output luminance in a range of a luminance Q which is higher than the minimum value to the maximum value. Then, the correction coefficient is adjusted so that the maximum value of the input luminance is converted into a luminance of 255. This can improve visibility by further enhancing an image contrast. 
     [5] Fifth Embodiment 
     Next, a projector of a fifth embodiment of the present invention will be described. The projector of the fifth embodiment has a similar construction to the projector of the first embodiment, but differs in that a black level of an image is corrected after a gradation is corrected. This difference will be mainly described below. 
     (1) Construction of Projector 
       FIG. 17  is a block diagram showing a main construction of the projector of the fifth embodiment. As shown in  FIG. 17 , a projector  17  includes a luminance distribution detecting circuit  1701 , a coefficient calculating circuit  1702 , a luminance level adjusting circuit  1703 , an outside light detector  1704 , an offset level adjusting circuit  1705 , and a black level correcting circuit  1706 . The black level correcting circuit  1706  lowers a luminance in a low luminance portion. 
     (2) Operation of Projector  17   
       FIG. 18  is a flowchart showing an operation of the projector  17 . As shown in  FIG. 18 , when receiving a video luminance signal (S 1801 ), the projector  17  detects a luminance distribution in the luminance distribution detecting circuit  1701  (S 1802 ), and calculates a correction coefficient C i  in the coefficient calculating circuit  1702  (S 1803 ). 
     Next, the projector  17  generates a brightness signal in the outside light detector  1704  (S 1804 ), obtains a correction coefficient C i ′ in the luminance level adjusting circuit  1703  (S 1805 ), and corrects an image gradation (S 1806 ). Then, the projector  17  adjusts an offset level in the offset level adjusting circuit  1705  (S 1807 ). Finally, the projector  17  lowers a luminance in a low luminance portion in the black level correcting circuit  1706  (S 1808 ). 
     According to the fifth embodiment, when a correction coefficient Q is large because of high-intensity outside light, a luminance in a lowest luminance portion may be too high after a correction. On the other hand, the black level correcting circuit  1706  lowers a luminance in a low luminance portion. Therefore, an image can be clarified because blackness is emphasized.  FIG. 19  is a graph showing that the block level correction circuit  1706  lowers a luminance in a low luminance portion. As shown in  FIG. 19 , the block level correcting circuit  1706  lowers an output luminance in a low luminance portion by adjusting an output luminance Q 0  corresponding to an input luminance of 0 to an output luminance Q 1 . 
     [6] Modification 
     Up to now, the image display device and the image display method of the present invention have been described specifically through the embodiments. However, the technical scope of the present invention is not limited to the above-described embodiments. For example, the following are modifications.
     (1) In the above-described embodiments, the present invention is described by mainly taking a projector as an example. However, the present invention is not limited to a projector, and may be applied to an image display device other than a projector. Especially, it is preferable that the present invention is applied to an image display device having a problem that a screen is not easily viewable because of an effect of outside light and the like.   (2) In the above-described embodiments, a case where an outside light sensor is fitted on a main body of a projector as a unit is mainly described. However, the present invention is not limited to this case, and an effect of the present invention can be obtained without fitting an outside light sensor on a main body of a projector as a unit. For example, an outside light sensor may be fitted on a front of a screen to measure an intensity of outside light entering into a screen. Also, an outside light sensor may be detached from a projector so as to measure brightness at a proper position according to an environment in which the projector is used.   (3) In the above-described embodiments, a case where a correction coefficient is determined by dividing luminances of 256 gradations into 4 levels is mainly described. However, the present invention is not limited to this, and a correction coefficient may be determined by dividing luminances of 256 gradations into a plurality of levels other than 4. Also, an effect of the present invention can be obtained by using the number of gradations such as 10 bits:1024 gradations and the like, which is other than 8 bits:256 gradations.   (4) Although it is not especially mentioned in the above-described embodiments, the image processor of the present invention may be a program for performing the above-mentioned gradation correction by receiving a signal indicating an outside light intensity, or may be exclusive LSI (Large Scale Integration).   (5) In the above-described embodiments, a case where a correction coefficient C i ′ is obtained by adjusting all correction coefficients C i  (i=1 to 4) is described. However, the present invention is not limited to this, and a correction coefficient C i  may be adjusted regarding only a low luminance portion. In this case, an adjusted coefficient may be only a correction coefficient C i  in a lowest luminance portion, or C 3  may be adjusted from C 1  and C 2  or from C 1 .   (6) Although it is not especially mentioned in the above-described embodiments, a value of a correction coefficient Q in the first embodiment may be no more than 80 in case of 256 gradations. Also, a value of a correction coefficient Q may be no more than 160 in case of 512 gradations. This can minimize a problem of a contrast degradation and the like, which is caused because an output luminance range is restricted.   (7) In the above-described embodiments, all pixels of an image are sampled when obtaining an average luminance level of the image. However, the present invention is not limited to this, and thinning-out sampling may be performed such as every two pixels or four pixels. This can reduce a circuit size.   

     INDUSTRIAL APPLICABILITY 
     The image display device and the image display method of the present invention are useful as a technology for correcting a gradation so as to improve visibility by eliminating an influence of outside light in an image display device.