Patent Publication Number: US-2012026208-A1

Title: 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. 2010-172735, filed on Jul. 30, 2010; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to an apparatus for displaying an image. 
     BACKGROUND 
     As to an apparatus for displaying an image having gradation (gray-scale levels), in order to save a power of the apparatus, processing to reduce a peak luminance of the apparatus is executed. However, when the peak luminance is reduced, a dynamic range of a display of the apparatus also drops. Because, by reducing the peak luminance, a luminance-difference of gray-scale level between a dark part (low gray-scale level part) and a bright part (high gray-scale level part) in the image is shortened. 
     A technique to solve above-mentioned problem is disclosed in JP-A 2009-17200 (Kokai). As to this technique, the total of luminance histogram (representing a frequency of appearance of gray-scale level) of the dark part and the bright part in the image is calculated respectively, and γ-curve to raise the luminance and γ-curve to lower the luminance are generated from the total of luminance histogram. Then, new γ-curve to blend above-mentioned two γ-curves is generated. 
     However, in this technique, a gain of the new γ-curve is uniformly set in the image, based on the frequency of appearance of gray-scale level in the dark part and the bright part. Accordingly, at the dark part or the bright part having low frequency of appearance, the luminance-difference of gray-scale level is shortened, and a loss of gradation occurs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an image display apparatus according to a first embodiment. 
         FIG. 2  is a block diagram of a conversion unit in  FIG. 1 . 
         FIG. 3  is a flow chart of processing of the image display apparatus in  FIG. 1 . 
         FIG. 4  is a graph showing an operation of a global-gamma calculation unit in  FIG. 1 . 
         FIG. 5  is a graph showing an operation of a local-gamma calculation unit in  FIG. 1 . 
         FIG. 6  is a graph showing an operation of a conversion unit in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     According to one embodiment, an image display apparatus includes a display unit, a set unit, a first calculation unit, a second calculation unit, and a conversion unit. The set unit is configured to set a display luminance signal indicating a luminance of the display unit. The first calculation unit is configured to calculate a first gamma characteristic signal of an image to be displayed, based on the display luminance signal and gray-scale levels of pixels of the image. The second calculation unit is configured to calculate a second gamma characteristic signal of each area of the image, based on gray-scale levels of pixels of each area. The conversion unit is configured to convert a grayscale level of each pixel of the image, based on the first gamma characteristic signal and the second gamma characteristic signal of an area including a pixel to be converted in the image. The display unit displays the image in which the gray-scale level of each pixel was converted, based on the display luminance signal. 
     Various embodiments will be described hereinafter with reference to the accompanying drawings. Moreover, as to components and processes having the same operation, common sign is assigned to each component and process, and its overlapped explanation is omitted. 
     The First Embodiment 
       FIG. 1  is a block diagram of an image display apparatus  100  according to the first embodiment. As shown in  FIG. 1 , the image display apparatus  100  includes a display luminance set unit  10 , a global-gamma calculation unit  11 , a local-gamma calculation unit  12 , a conversion unit  15 , and a display unit  16  to display an image. 
     A format of an input video signal can be variously supposed. As to the first embodiment, an example based on YCbCr transmission standards of “International Telecommunication Union” is explained. As to the YCbCr transmission standards, each pixel has three-channel signals of a luminance element, a red-green element and a blue-yellow element as a pixel value. Among a video signal at a position (u,v) in the image, the luminance element is mentioned as a gray-scale level x(u,v). The input video signal may be an input image from all devices or all media. For example, the video signal may be input from a recording medium such as HDD, an external device connected via a network, or a broadcast wave such as TV. 
     The display luminance set unit  10  sets a display luminance signal indicating a peak luminance for the display unit  16  to display the image. Based on a display-parameter (related to the video signal) inputted by a user via an interface (not shown in Fig.), the display luminance set unit  10  sets the display luminance signal. Alternatively, by equipping an illumination sensor (not shown in Fig.), based on an illuminance of an environment (where the image display apparatus  10  is located) measured by the illumination sensor, the display luminance set unit  10  may set the display luminance signal. For example, the display luminance signal is set so that the peal luminance is higher when the illuminance is high and so that the peal luminance is lower when the illuminace is low. 
     Furthermore, if the image display apparatus  100  is driven by a buttery (not shown in Fig.), based on a remained quantity of the buttery, the display luminance set unit  10  may set the display luminance signal. For example, the display luminance signal is set so that the peal luminance is higher when the remained quantity is much and the peal luminance is lower when the remained quantity is few. This display luminance signal is sent to the global-gamma calculation unit  11  and the display unit  16 . 
     Based on the display luminance signal, the global-gamma calculation unit  11  calculates one global-gamma signal (representing gamma characteristic) for the input video signal of a processing object, and sends it to the conversion unit  15 . Briefly, one global-gamma signal is calculated for one image (one frame or one field). 
     As to each area in the input video signal, the local-gamma calculation unit  12  calculates a local-gamma signal (representing gamma characteristic) based on pixel values in each area, and sends it to the conversion unit  15 . The local-gamma calculation unit  12  divides the input video signal into small areas of (M×N) units, and calculates a histogram of pixel values in (m,n)-th small area. Based on the histogram of pixel values in the (m,n)-th small area, the local-gamma calculation unit  12  calculates the local-gamma signal holding local characteristics of gradation of the (m,n)-th small area. 
     By using the global-gamma signal and the local-gamma signal (corresponding to an area to be converted), the conversion unit  15  converts a gray-scale level of each pixel of the input video signal to a video signal. Detail component of the conversion unit  15  will be explained afterwards. 
     The display unit  16  displays the image. In the first embodiment, an example that the display unit  16  is a liquid crystal display is explained. For that reason, the display unit  16  equips a backlight  161  and a liquid crystal panel  162 . The backlight  161  is lightened at a luminance based on the display luminance signal. Furthermore, by writing the video signal having gamma characteristic (converted by the conversion unit  15 ) into the liquid crystal panel  162 , the image is displayed. 
     Moreover, the display unit  16  may be any unit able to display information such as the image. For example, an organic EL panel, a plasma display panel or a CRT, may be used. In case of using a display device of self-emission type such as the organic EL panel, the display luminance signal means a peak luminance of emission of a display element, and the video signal means a signal value assigned to each display element. By controlling a gain of a driving electric current flown into each element based on the display-luminance signal, each element emits based on the video signal and the gain. As a result, the image is displayed. 
       FIG. 2  is a detail component of the conversion unit  15 . The conversion unit  15  includes a luminance quantity calculation unit  153 , a blend ratio calculation unit  154 , and a gradation conversion unit  155 . 
     By using the input video signal, the luminance quantity calculation unit  153  calculates a luminance quantity representing a distribution of spatial luminance in the image. 
     By using the luminance quantity, the blend ratio calculation unit  154  calculates a blend ratio representing a composition degree between the global-gamma signal and the local-gamma signal, and sends it to the gradation conversion unit  155 . The blend ratio may be calculated for each pixel, or calculated for a surrounding area including the pixel. 
     By using the global-gamma signal, the local-gamma signal and the blend ratio, the gradation conversion unit  155  computes a video signal, and send it to the display unit  16 . The video signal has a gray-scale level converted from the input video signal. 
       FIG. 3  is a flow chart of operation of the image display apparatus  100 . First, a video signal of a processing object is input to the image display apparatus  100  (S 31 ). The display luminance set unit  10  sets a display luminance signal “pl” indicating a display luminance of the display unit  16  in case of displaying the input video signal (S 32 ). As mentioned-above, various methods may be used as a method for setting the display luminance signal. 
     By using the display luminance signal “pl”, the global-gamma calculation unit  11  calculates a global-gamma signal “G” as gamma characteristic uniformly determined for the display (S 33 ). The global-gamma signal “G” is represented as a following equation (1). 
     
       
         
           
             
               
                 
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     In the equation (1), “x” represents a gray-scale level of the input video signal, “x max ” represents a maximum of the gray-scale level of the input video signal, “γ” represents a gamma determined from characteristics of the display unit  16 , and “PL” represents a maximum of the display luminance signal settable by the display luminance set unit  10 . 
       FIG. 4  shows one example of the global-gamma signal “G”. This example represents display characteristics of the display unit  16  for the gray-scale level “x” in case of “PL=pl” and “PL&gt;pl”. In order to prevent a loss of gradation at high gray-scale level, a high gray-scale level part of the global-gamma signal “G” is desirably rounded. 
     The local-gamma calculation unit  12  divides the input video signal into small areas of (M×N) units (S 34 ). A shape of small area can be variously supposed. In the first embodiment, by dividing the input video signal into M units along a horizontal direction and N units along a vertical direction, rectangle small areas of (M×N) units are supposed. Moreover, the small area may be a non-linear shape area by clustering the input video signal into “a person” or “an object”. 
     The local-gamma calculation unit  12  calculates a histogram H mn  of gray-scale level of pixels included in (m,n)-th small area (S 35 ). As to the histogram, bins of “x max ” units as the maximum of gray-scale level are desirably prepared. 
     At S 35 , the histogram H mn  is calculated from gray-scale levels of pixels included in (m,n)-th small area. However, this search area may be extended to a surrounding area including the (m,n)-th small area. For example, by referring to gray-scale levels of pixels included in the surrounding area such as (m−1,n−1)-th area, (m, n−1)-th area, (m+1,n−1)-th area, (m−1,n)-th area, (m,n)-th area, (m+1,n)-th area, (m−1,n+1)-th area, (m,n+1)-th area and (m+1,n+1)-th area, the histogram of gray-scale level of (m,n)-th area may be calculated. In this way, by creating the histogram H mn , difference of shape of histogram among all areas becomes small. Accordingly, occurrence of uneven luminance due to difference of the local-gamma signal (calculated at next step S 36 ) can be suppressed. 
     By using the histogram H mn , the local-gamma calculation unit  12  calculates a local-gamma signal L mn  holding local characteristics of gradation of (m,n)-th small area, and sends it to the conversion unit  15  (S 36 ). The local-gamma signal L mn  is calculated by following equation (2). 
     
       
         
           
             
               
                 
                   
                       
                   
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     In the equation (2), a first term represents a square-error of brightness in case of the display luminance signal “pl and PL”. A second term represents a square-error of gradient of brightness in case of the display luminance signal “pl and PL”. “E(x,pl)” represents an evaluation value of L mn  in case of the display luminance signal “pl”. “x” represents a gray-scale level of the input video signal, “R(x,PL)” represents a brightness of gray-scale level “x” in case of the display luminance signal “PL”. “r(x,pl)” represents a brightness of gray-scale level “x” in case of the display luminance signal “pl”. “λ,” represents a coefficient of linear combination of two weighted square-errors, which is desirably a value smaller than 0.5. Furthermore, “wα(x) and wβ(x)” represent a function of linear combination of the first term and the second term. 
     In the first embodiment, a method for calculating L mn (x) is explained. On condition that “x max =256”, an output gray-scale level L mn (x 0,1 ) of 128 gray-scale level as an internally dividing point between 0 gray-scale level and 256 gray-scale level is calculated. Next, an output gray-scale level L mn (x 1,1 ) of 64 gray-scale level as an internally dividing point between 0 gray-scale level and 128 gray-scale level is calculated, and an output gray-scale level L mn (x 1,3 ) of 1.92 gray-scale level as an internally dividing point between 129 gray-scale level and 256 gray-scale level. Hereinafter, above-mentioned processing is repeated until gray-scale levels of all internally dividing points are determined. In this case, a hierarchical number of the repeat processing is represented by “l”, a position number of input/output gray-scale level is represented by “p”, and an input gray-scale level of each hierarchy is represented as “x l,p ”. 
     Now, as to an internally dividing point x l,p  between two input gray-scale levels x i,p−1  and x i,p+1 , calculation of an output gray-scale level L mn (x l,p ) of the point x l,p  is thought about. In this case, by representing the histogram H mn  as two partial histograms counting frequency of gray-scale levels “x i,p−1 ˜x i,p ” and “x i,p ˜x i,p+1 ”, weights wα(x) and wβ(x) of gray-scale levels “x i,p−1 ˜x i,p+1 ” are calculated by following equation (3). 
     
       
         
           
             
               
                 
                   
                     
                       
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     In the equation (3), H mn (i) represents frequency of appearance of gray-scale level “i” in (m,n)-th small area. 
       FIG. 5  is a graph showing calculation example of L mn  in (m,n)-th small area in case of the display luminance signal “pl”. In this graph, a horizontal axis represents gray-scale levels, a solid line in a vertical axis represents frequency of appearance, and a broken line in the vertical axis represents output characteristic of a local-gamma signal L mn . As shown in the local-gamma signal L mn  of  FIG. 5 , as to input gray-scale levels having high frequency of appearance in the (m,n)-th small area, the gray-scale level and its gradient are larger. 
     By using the input video signal, the luminance quantity calculation unit  153  calculates luminance quantity “Y” representing distribution of spatial luminance in the image, and sends it to the blend ratio calculation unit  154  (S 37 ). In this case, in order to calculate distribution of a local luminance, from gray-scale levels of pixels surrounding a pixel to be processed, an average, a center value, a maximum, a minimum or a standard deviation may be calculated. 
     Furthermore, the luminance quantity Y may be calculated for each pixel to be processed, or calculated for a surrounding area of the pixel. In the luminance quantity calculation unit  153 , the input video signal is processed by an edge-detective spatial smoothing filter. In this case, the processed video signal represents a smoothed image including edges of the input video signal. As to the smoothed image, the luminance quantity Y(u,v) of each pixel is calculated by following equation (4). 
     
       
         
           
             
               
                 
                   
                     
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     In the equation (4), “x(u,v)” represents a gray-scale level of a luminance at a pixel (u,v), “B(u,v)” represents the luminance after filtering x(u,v), “ε” represents a threshold set by a luminance-difference, and “T(u,v)” represents a function on which a convolution function for the specific number of taps “t” is superimposed. In this case, the number of taps “t” and the threshold “ε” are desirably adjusted as a suitable value. For example, “t” is desirably set as a diameter of a small area (M×N divided areas) divided from the input video signal, and “ε” is desirable set as 0.02˜0.20 times of maximum “x max ” of gray-scale level of the input video signal. 
     By using the luminance quantity Y, the blend ratio calculation unit  154  calculates a blend ratio α representing, composition degree between the global-gamma signal and the local-gamma signal, and sends it to the gradation conversion unit  155  (S 38 ). The global-gamma signal is gamma characteristic to raise characteristics of gradation of a dark part, and the local-gamma signal is gamma characteristic to raise characteristics of gradation of a bright part. Accordingly, the blend ratio α had better be calculated so that a ratio of the global-gamma signal is higher when the luminance quantity Y is lower and so that a ratio of the local-gamma signal is higher when the luminance quantity Y is higher. 
     In this case, the blend ratio may be calculated for each pixel, or for each area having a plurality of pixels. In the first embodiment, the blend ratio α(u,v) of each pixel is calculated by following equation (5). 
     
       
         
           
             
               
                 
                   
                     α 
                      
                     
                       ( 
                       
                         u 
                         , 
                         v 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       Y 
                        
                       
                         ( 
                         
                           u 
                           , 
                           v 
                         
                         ) 
                       
                     
                     
                       x 
                       max 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     In the equation (5), “x mn ” represents a maximum of gray-scale level of the input video signal. 
     By using the global-gamma signal G, the local-gamma signal L mn  and the blend ratio α, the gradation conversion unit  155  calculates a video signal “f” having gray-scale level converted from the input video signal, and sends it to the display unit  16  (S 39 ). In the first embodiment, as to a gray-scale level “x(u,v)” corresponding to a pixel position “u” along a horizontal direction and a pixel position “v” along a vertical direction in the input video signal, a video signal “f(u,v)” having gray-scale level converted from the gray-scale level “x(u,v)” is calculated by following equation (6). 
         f ( u,v )=[1−α( u,v )] c   ×G ( x ( u,v ))+α( u,v ) c   ×L   mn ( x ( u,v ))  (6)
 
     In the equation (6), “c” represents a coefficient of blend strength, which is desirably set in a range “0˜1”. At S 39  in  FIG. 3 , as to all pixels in all small areas (M×N divided areas), each gray-scale level is converted by the equation (6), and the video signal “f” is calculated. Moreover, except for linear combination such as the equation (6), the video signal “f” may be calculated by non-linear combination such as operation of inverse proportion. 
       FIG. 6  shows one example of outputs of the global-gamma signal G, the local-gamma signal L mn  and the video signal f for a gray-scale level x of the input video signal. In  FIG. 6 , assume that “α=0.5 and c=1.0”. As shown in  FIG. 6 , in the first embodiment, even if the display luminance signal is lower than a maximum, the gray-scale level can be converted on condition that characteristics of gradation of the dark part improves while maintaining characteristics of gradation of the bright part. 
     The gradation conversion unit  155  decides whether the video signal f of all pixels in (m,n)-th small area is already calculated (S 40 ). If the video signal f of all pixels is already calculated (Yes at S 40 ), processing is forwarded to S 41 . If the video signal f of at least one pixel is not calculated yet (No at S 40 ), processing is forwarded to S 400 . In latter case, the luminance quantity calculation unit  153  selects the at least one pixel, and processing is returned to S 37  (S 400 ). 
     The gradation conversion unit  155  decides whether the video signal f of all pixels in all small areas (M×N divided areas) is already calculated (S 41 ). If the video signal f of all pixels in at least one small area is not calculated yet (No at S 41 ), processing is forwarded to S 400 . The luminance quantity calculation unit  153  selects the at least one small area, and processing is returned to S 35  (S 410 ). If the video signal f of all pixels in all small areas is already calculated (Yes at S 41 ), processing is forwarded to S 42 . 
     The display unit  16  makes the backlight  161  emit based on the display luminance signal pl, and writes the video signal f into the liquid crystal panel  162 . As a result, the image is displayed (S 42 ). 
     As mentioned-above, in the first embodiment, even if a peak luminance of the display is reduced in order to save the power, characteristics of gradation of a high gray-scale level part or a low gray-scale level part having low frequency of appearance improves. Briefly, uneven luminance of a local area can be suppressed. Furthermore, by using the global-gamma uniformly for the display, occurrence of uneven luminance can be suppressed. 
     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.