Abstract:
Aspects of the present invention are related to systems and methods for improving content visibility on a liquid crystal display (LCD) under low-contrast viewing conditions. According to one aspect of the present invention an enhanced image may be formed by combining a key-feature map associated with an input image and a brightness-boosted version of the input image.

Description:
FIELD OF THE INVENTION 
       [0001]    Embodiments of the present invention relate generally to image enhancement and, in particular, to methods and systems for improving content visibility on a liquid crystal display (LCD) under low-contrast viewing conditions. 
       BACKGROUND 
       [0002]    Low-contrast viewing conditions may negatively impact, for example, through eyestrain and fatigue, the viewing experience of a user of an LCD device, for example, an LCD television, an LCD mobile device and other devices comprising an LCD display. 
         [0003]    Low-contrast viewing conditions may arise when a device is used in an aggressive power-reduction mode, wherein the LCD backlight power level may be dramatically reduced making the image/video content appear dark and less visible to a viewer. The contrast of the image/video may be vastly reduced, or in some cases, pegged at black, and many image features that may convey important scene content may fall below the visible threshold. 
         [0004]    Low-contrast viewing conditions may also arise when an LCD display is viewed under high ambient light, for example, direct sunlight. In these situations, the minimum display brightness that a viewer may perceive may be elevated due to the high ambient light in the surroundings. The image/video may appear “washed out” where it is intended to be bright, and the image/video may appear featureless in darker regions. 
         [0005]    For both of the above-described low-contrast viewing scenarios, and other low-contrast viewing scenarios, the tonal dynamic range of the image/video may be compressed and the image contrast may be greatly reduced, thereby degrading the viewing experience of the user. Due to increasing consumer concern for reduced energy costs and demand for device mobility, it may be desirable to provide improved digital imagery and video quality to enhance the viewing experience under low-contrast viewing conditions. 
       SUMMARY 
       [0006]    Some embodiments of the present invention comprise methods and systems for improving content visibility on a liquid crystal display (LCD) under low-contrast viewing conditions. 
         [0007]    According to one aspect of the present invention, a key-feature estimator may estimate a key-feature image, also referred to as a key-feature map, associated with an input image, a brightness booster may generate a brightened image associated with the input image and a combiner may combine the key-feature image and the brightened image to form an enhanced image that may exhibit improved content visibility when displayed on an LCD display and viewed under low-contrast viewing conditions. The key-feature image may identify pixels, in the input image, at which there is a large gradient and a well-defined object contour. 
         [0008]    According to another aspect of the present invention, the key-feature estimator may estimate the gradient at pixels in a grayscale image associated with the input image using a large-spatial-support gradient calculator. 
         [0009]    According to another aspect of the present invention, the brightness booster may determine a boosting factor based on at least one of a power level associated with the LCD display, an ambient-light level associated with the LCD display and a measure of the input-image content. 
         [0010]    The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL DRAWINGS 
         [0011]      FIG. 1  is a picture depicting an exemplary image under a low back-light-power viewing condition; 
           [0012]      FIG. 2  is a picture depicting an exemplary image under a high ambient-light viewing condition; 
           [0013]      FIG. 3  is a chart showing exemplary embodiments of the present invention comprising a brightness booster for boosting the brightness level of an input image, a key-feature estimator for estimating a key-feature map associated with the input image and a combiner for combining the brightness-boosted image and the key-feature map; 
           [0014]      FIG. 4  is a chart showing exemplary embodiments of the present invention comprising a gradient estimator comprising a large-spatial-support gradient calculator; 
           [0015]      FIG. 5  is a picture depicting an exemplary large-spatial support, associated with a pixel location, used in a gradient calculation according to embodiments of the present invention; 
           [0016]      FIG. 6  is a picture depicting an exemplary input image; 
           [0017]      FIG. 7  is a picture depicting a raw gradient map, determined according to embodiments of the present invention, for the exemplary input image shown in  FIG. 6 ; 
           [0018]      FIG. 8  is a picture depicting a gradient map after suppressing low-amplitude gradients, according to embodiments of the present invention, in the raw gradient map shown in  FIG. 7 ; 
           [0019]      FIG. 9  is a picture depicting a reversed gradient map generated by polarity reversion, according to embodiments of the present invention, applied to the exemplary gradient map shown in  FIG. 8 ; 
           [0020]      FIG. 10  is a picture depicting a contrast-enhanced gradient map, generated according to embodiments of the present invention, associated with the reversed gradient map shown in  FIG. 9 ; 
           [0021]      FIG. 11  is a picture depicting the effect of gradient smoothing applied to the exemplary contrast-enhanced gradient map shown in  FIG. 10 ; 
           [0022]      FIG. 12  is a chart showing exemplary embodiments of the present invention comprising determining a brightness-boosting factor that maintains the color ratio across three color channels when clipping occurs; 
           [0023]      FIG. 13  is a picture depicting a Non-Photorealistic Rendering (NPR) rendition, according to embodiments of the present invention, of the exemplary input image, at full power consumption, shown in  FIG. 6 ; 
           [0024]      FIG. 14  is a picture depicting an NPR rendition, according to embodiments of the present invention, of the exemplary input image, at 2% power consumption, shown in  FIG. 6 ; 
           [0025]      FIG. 15  is a picture depicting an NPR rendition, according to embodiments of the present invention, of the exemplary input image, viewed in direct sunlight, shown in  FIG. 2 ; and 
           [0026]      FIG. 16  is a chart showing exemplary embodiments of the present invention comprising a brightness booster for boosting the brightness level of an input image, a key-feature estimator for estimating a key-feature map associated with the input image, a combiner for combining the brightness-boosted image and the key-feature map and a blending-parameter selector for determining a blending parameter that is used by the combiner. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0027]    Embodiments of the present invention will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. The figures listed above are expressly incorporated as part of this detailed description. 
         [0028]    It will be readily understood that the components of the present invention, as generally described and illustrated in the figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the methods and systems of the present invention is not intended to limit the scope of the invention but it is merely representative of the presently preferred embodiments of the invention. 
         [0029]    Elements of embodiments of the present invention may be embodied in hardware, firmware and/or software. While exemplary embodiments revealed herein may only describe one of these forms, it is to be understood that one skilled in the art would be able to effectuate these elements in any of these forms while resting within the scope of the present invention. 
         [0030]    Low-contrast viewing conditions may negatively impact, for example, through eyestrain and fatigue, the viewing experience of a user of an LCD device, for example, an LCD television, an LCD mobile device and other devices comprising an LCD display. 
         [0031]    Low-contrast viewing conditions may arise when a device is used in an aggressive power-reduction mode, wherein the LCD backlight power level may be dramatically reduced making the image/video content appear dark and less visible to a viewer. The contrast of the image/video may be vastly reduced, or in some cases, pegged at black, and many image features that may convey important scene content may fall below the visible threshold.  FIG. 1  depicts an exemplary image  10  displayed on a device operating under aggressive power-mode reduction. 
         [0032]    Low-contrast viewing conditions may also arise when an LCD display is viewed under high ambient light, for example, direct sunlight. In these situations, the minimum display brightness that a viewer may perceive may be elevated due to the high ambient light in the surroundings. The image/video may appear “washed out” where it is intended to be bright, and the image/video may appear featureless in darker regions.  FIG. 2  depicts an exemplary image  20  viewed with a mobile phone under high ambient lighting (direct sunlight). 
         [0033]    For both of the above-described low-contrast viewing scenarios, and other low-contrast viewing scenarios, the tonal dynamic range of the image/video may be compressed and the image contrast may be greatly reduced, thereby degrading the viewing experience of the user. Due to increasing consumer concern for reduced energy costs and demand for device mobility, it may be desirable to provide improved digital imagery and video quality to enhance the viewing experience under low-contrast viewing conditions. 
         [0034]    Some embodiments of the present invention described in relation to  FIG. 3  may increase the visibility of image/video features in low-contrast viewing conditions by highlighting key image features with Non-Photorealistic Rendering (NPR) techniques. Some of these embodiments may comprise an image-enhancement system  30  comprising a brightness booster  32 , a key-feature estimator  34 , a combiner  36  and a code-value mapper  38 . The image-enhancement system  30  may receive an input image  31  and may make the input image  31  available to the brightness booster  32  and the key-feature estimator  34 . In some embodiments of the present invention, the input image  31  may be a color image, for example, an RGB image. In alternative embodiments, the input image  31  may be a gray-scale image. The input image  31  may be a still image or a frame of a video sequence. 
         [0035]    The brightness booster  32  may boost the brightness of the input image  31  using a brightness preservation technique, and the brightness booster  32  may generate a brightened image  33  that may be made available to the combiner  36 . In some embodiments of the present invention, the brightness booster  32  may boost the brightness of the input image  31  based on information related to an LCD backlight associated with an LCD display on which the enhanced image may be displayed. 
         [0036]    The key-feature estimator  34  may estimate a key-feature image  35 , also referred to as a key-feature map, from the input image  31  and may make the key-feature image  35  available to the combiner  36 . 
         [0037]    The combiner  36  may blend the brightened image  33  and the key-feature image  35  to form a blended image  37  which may be made available to the code-value mapper  38 . The code-value mapper  38  may form a key-feature-highlighted (KFH) image  39  by mapping the code-values generated by the combiner  36  into code values appropriate for an LCD, for example, to the range of [0,255]. In some embodiments, the KFH image  39  may be made directly available to the LCD for display. The KFH image  39  may also be referred to as an NPR image. 
         [0038]    In some embodiments of the present invention described in relation to  FIG. 4 , the key-feature estimator  34  may comprise a low-pass filter  40  and a down-sampler  42  for reducing, if necessary, the resolution of the input image to a resolution that may allow near real-time processing. Exemplary low-pass filters may include neighborhood pixel-value averaging, Gaussian smoothing, median blur filtering and other low-pass filters known in the art. In some embodiments of the present invention, a low-pass filter may be selected based on computational limitations and/or system resources. Exemplary down-samplers may comprise removal of image rows, removal of image columns, bilinear image resizing, bicubic image resizing, Gaussian pyramid down-samplers and other down-samplers known in the art. In some embodiments of the present invention, a down-sampler may be selected based on computational limitations and/or system resources. In alternative embodiments (not shown), a key-feature estimator may not reduce the resolution of the input image, and may, therefore, not comprise a low-pass filter and a down-sampler. 
         [0039]    The down-sampled image  43  may be made available to a bilateral filter  44  which may smooth less-textured areas. Major contours of objects within an image may convey important image information, while less-textured areas may be perceptually less important to a viewer. Thus bilateral filtering may be used to remove unnecessary gradient information, while retaining key edge information corresponding to object contours. 
         [0040]    The results  45  of the bilateral filtering may be converted to gray-scale values by a gray-scale converter  46 , and gradient estimation may be performed on the gray-scale image  47  by a large-spatial-support gradient calculator  48 . Commonly used edge detectors, for example, the Sobel operator, the Canny edge detector and the Laplacian operator, may not effectively detect edges associated with major contours. Use of these common edge detectors may result in broken lines on major object contours. Additionally, minor edges may be detected in less-textured image areas, which may not be desirable in KFH rendering. Further, object boundaries in a gradient map generated using one of the commonly used edge detectors may not be well defined. Embodiments of the present invention may compute image gradients using a large spatial support and may retain, as edge pixels, only pixels with a large gradient value. 
         [0041]    In some embodiments of the present invention, the large-spatial-support gradient calculator  48  may comprise a horizontal-gradient calculator and a vertical-gradient calculator. At each pixel in the gray-scale image  47 , a horizontal-gradient value may be determined by the horizontal-gradient calculator and a vertical-gradient value may be determined by the vertical-gradient calculator. A gradient value may be assigned to a pixel based on the determined horizontal-gradient value and the determined vertical-gradient value associated with the pixel. In some embodiments, the gradient value assigned to a pixel may be the largest of the horizontal-gradient value and the vertical-gradient value associated with the pixel. 
         [0042]    In some embodiments of the present invention, the horizontal-gradient value associated with a pixel may be determined by computing a first-order derivative at the pixel with respect to several horizontal neighbors in each direction, to the left and to the right, of the pixel. The largest derivative value in each direction may be added together to form the horizontal-gradient value associated with the pixel. Similarly, the vertical-gradient value associated with a pixel may be determined by computing a first-order derivative at the pixel with respect to several vertical neighbors in each direction, above and below, the pixel. The largest derivative value in each direction may be added together to form the vertical-gradient value associated with the pixel. In some embodiments of the present invention the size of the one-dimensional search window associated with a direction (left, right, above, below) may be three pixels.  FIG. 5  illustrates the large spatial support for an exemplary embodiment in which the one-dimension search window is three pixels. In this example, for a pixel denoted p 0    80 , the horizontal-gradient value, grad H p 0 ), may be determined according to: 
         [0000]        grad   H ( p   0 )=max[ D   1 ( p   0   ,ph   1 ), D   1 ( p   0   ,ph   2 ), D   1 ( p   0   ,ph   3 )]+max [D   1 ( p   0   ,ph   −1 ), D   1 ( p   0   ,ph   −2 ), D   1 ( p   0   ,ph   −3 )] 
         [0000]    and the vertical-gradient value, grad V (p 0 ), may be determined according to: 
         [0000]        grad   V ( p   0 )=max[ D   1 ( p   0   ,pv   1 ), D   1 ( p   0   pv   2 ), D   1 ( p   0   pv   3 )]+max [D   1 ( p   0   ,pv   −1 ), D   1 ( p   0   ,pv   −2 ), D   1 ( p   0   pv   −3 )] 
         [0000]    where D 1 (•, •) may denote the first-order derivative and ph 1    81 , ph 2    82  and ph 3    83  are the pixels in the one-dimensional search window to the right of the pixel p 0    80 , ph −1    84 , ph −2    85  and ph −3    86  are the pixels in the one-dimensional search window to the left of the pixel p 0    80 , pv 1    87 , pv 2    88  and pv 3    89  are the pixels in the one-dimensional search window below the pixel p 0    80  and pv −1    90 , pv −2    91  and pv −3    92  are the pixels in the one-dimensional search window above the pixel p 0    80 . The final raw gradient value, grad (p 0 ), associated with the pixel p 0    80  may be determined according to: 
         [0000]        grad ( p   0 )=max[ grad   H ( p   0 ), grad   V ( p   0 )], 
         [0000]    thereby producing a raw gradient map  49 . 
         [0043]      FIG. 6  shows an exemplary image  100 , and  FIG. 7  shows the resulting raw gradient map  110  determined according to the above-described embodiments of the present invention for the exemplary image  100  shown in  FIG. 6 . In this example, a three-pixel search window was used. 
         [0044]    The raw gradient map  49  may contain noisy details. Therefore, the raw gradient map  49  may be made available to a low-amplitude gradient suppressor  50  which may remove low-amplitude gradients. In some embodiments of the present invention, the low-amplitude gradient suppressor  50  may comprise a comparator that compares the gradient amplitude to a threshold according to: 
         [0000]    
       
         
           
             
               
                 grad 
                 suppress 
               
                
               
                 ( 
                 
                   p 
                   0 
                 
                 ) 
               
             
             = 
             
               { 
               
                 
                   
                     
                       
                         
                           grad 
                            
                           
                             ( 
                             
                               p 
                               0 
                             
                             ) 
                           
                         
                         , 
                       
                     
                     
                       
                         
                           grad 
                            
                           
                             ( 
                             
                               p 
                               0 
                             
                             ) 
                           
                         
                         &gt; 
                         T 
                       
                     
                   
                   
                     
                       
                         0 
                         , 
                       
                     
                     
                       otherwise 
                     
                   
                 
                 , 
               
             
           
         
       
     
         [0000]    where T may denote a threshold and grad suppress  (p 0 ) may denote the low-amplitude-gradient-suppressed gradient map. In some embodiments, the threshold may be set to T=5.0. In alternative embodiments, the low-amplitude gradient suppressor  50  may comprise a zero-crossing detector, and pixel locations associated with zero-crossings may be retained in the gradient map, while non-zero-crossings may be suppressed.  FIG. 8  shows the resulting gradient map  120  after suppressing low-amplitude gradients, by thresholding, in the raw gradient map  110  shown in  FIG. 7 . 
         [0045]    The low-amplitude-gradient-suppressed gradient map  51  may be made available to a gradient-map polarity reverser  52  that may reverse the gradient polarity according to: 
         [0000]        grad   rev ( p   0 )= offset−grad   suppress ( p   0 ), 
         [0000]    where offset may denote an offset parameter that may be associated with white background and grad rev (p 0 ) may denote the reversed gradient map. In some embodiments, the parameter offset may be determined empirically. In some embodiments, offset=120.  FIG. 9  shows the outcome  130  of polarity reversion applied to the exemplary gradient map  120  shown in  FIG. 8 . 
         [0046]    The reversed gradient map  53  may be made available to a gradient-contrast enhancer  54  that may improve the contrast of the reversed gradient map  53  and may map the gradient values to the range of 0 to 255. In some embodiments, the gradient-contrast enhancer  54  may map the reversed gradient values according to: 
         [0000]    
       
         
           
             
               
                 grad 
                 enhanced 
               
                
               
                 ( 
                 
                   p 
                   0 
                 
                 ) 
               
             
             = 
             
               { 
               
                 
                   
                     
                       
                         255 
                         , 
                       
                     
                     
                       
                         
                           
                             grad 
                             rev 
                           
                            
                           
                             ( 
                             
                               p 
                               0 
                             
                             ) 
                           
                         
                         = 
                         offset 
                       
                     
                   
                   
                     
                       
                         0 
                         , 
                       
                     
                     
                       
                         
                           
                             grad 
                             rev 
                           
                            
                           
                             ( 
                             
                               p 
                               0 
                             
                             ) 
                           
                         
                         ≤ 
                         0 
                       
                     
                   
                   
                     
                       
                         
                           
                             
                               grad 
                               rev 
                             
                              
                             
                               ( 
                               
                                 p 
                                 0 
                               
                               ) 
                             
                           
                           + 
                           shift 
                         
                         , 
                       
                     
                     
                       
                         0 
                         &lt; 
                         
                           
                             grad 
                             rev 
                           
                            
                           
                             ( 
                             
                               p 
                               0 
                             
                             ) 
                           
                         
                         &lt; 
                         offset 
                       
                     
                   
                 
                 , 
               
             
           
         
       
     
         [0000]    where shift may denote a contrast shift and grad enhanced  (p 0 ) may denote the contrast-enhanced gradient map. In some embodiments of the present invention, the parameter shift may be determined empirically. In some embodiments, shift=120. 
         [0047]    In some embodiments of the present invention, the gradient-contrast enhancer  54  may produce a binary gradient map according to: 
         [0000]    
       
         
           
             
               
                 grad 
                 enhanced 
               
                
               
                 ( 
                 
                   p 
                   0 
                 
                 ) 
               
             
             = 
             
               { 
               
                 
                   
                     
                       
                         255 
                         , 
                       
                     
                     
                       
                         
                           
                             grad 
                             rev 
                           
                            
                           
                             ( 
                             
                               p 
                               0 
                             
                             ) 
                           
                         
                         = 
                         offset 
                       
                     
                   
                   
                     
                       
                         0 
                         , 
                       
                     
                     
                       
                         
                           
                             grad 
                             rev 
                           
                            
                           
                             ( 
                             
                               p 
                               0 
                             
                             ) 
                           
                         
                         &lt; 
                         offset 
                       
                     
                   
                 
                 . 
               
             
           
         
       
     
         [0000]      FIG. 10  shows the outcome  140  of gradient-contrast enhancement applied to the exemplary reversed gradient map  130  shown in  FIG. 9 . 
         [0048]    The contrasted-enhanced gradient map  55  may be made available to a gradient smoother  56  that may blur the boundary between foreground edges and white background and may link broken lines. In some embodiments of the present invention, the gradient smoother  56  may comprise a Gaussian low-pass filter. In some embodiments, the kernel size of the Gaussian low-pass filter may be 3×3.  FIG. 11  shows the effect  150  of gradient smoothing applied to the exemplary contrast-enhanced gradient map  140  shown in  FIG. 10 . 
         [0049]    The smoothed gradient map  57  may be made available to an up-scaler  58  that may scale the smoothed gradient map  57  to the original input image resolution. The up-scaled gradient map  59  may be made available to a gradient-map shifter  60  that may shift the background of the gradient map to zero. In some embodiments, the gradient-map shifter  60  may subtract 255 from the up-scaled gradient values to shift the background to zero. The resulting key-feature map  61  may be made available from the key-feature estimator  34  to the combiner  36 . 
         [0050]    In some embodiments of the present invention described in relation to  FIG. 3 , the brightness booster  32  may boost the brightness of the input image  31  using a linear scaling factor, also referred to as a scaling factor, a boosting factor, a brightening factor and a brightness-boosting factor. In some of these embodiments, the linear scaling factor may be determined such that the brightness is preserved under a predetermined percentage of backlight dimming according to: 
         [0000]    
       
         
           
             
               S 
               = 
               
                 
                   ( 
                   
                     1 
                     
                       BL 
                       reduced 
                     
                   
                   ) 
                 
                 
                   1 
                   γ 
                 
               
             
             , 
           
         
       
     
         [0000]    where S may denote the scaling factor, BL reduced  may denote the percentage of backlight dimming and γ may denote the LCD system gamma. In some embodiments, BL reduced  may be a predetermined fixed percentage, for example, 15 percent. In alternative embodiments, the scaling factor, S, may be determined adaptively based on image content. In some of these embodiments, the scaling factor, S, may be computed using the color histogram of the input image. As will be appreciated by a person of ordinary skill in the art, the percentage of backlight dimming, BL reduced , may be determined any of the methods and systems known in the art. For example, the percentage of backlight dimming, BL reduced , may be determined according to the methods and systems disclosed in U.S. patent application Ser. No. 11/465,436, entitled “Systems and Methods for Selecting a Display Source Light Illumination Level,” filed Aug. 17, 2006, which is hereby incorporated by reference herein in its entirety. 
         [0051]    In some embodiments of the present invention, to avoid a clipping problem, the brightness boosting may comprise per-pixel processing described in relation to  FIG. 12 . The boosting factor, S, may be computed  160 , and a determination  162  may be made as to whether or not there are unprocessed pixels. If there are no 163 unprocessed pixels, then the brightness boosting procedure may terminate  164 . If there are 165 unprocessed pixels, then the color-component values, denoted [R, G, B] of the next pixel may be obtained  166 . The largest color-component value, which may be denoted V, may be determined  168 . In some embodiments, V may be determined according to: 
         [0000]        V =max(max( R,G ), B ). 
         [0000]    The largest color-component value, V, may be scaled by the boosting factor, S, and the scaled value may be compared  170  to the maximum code value. In some embodiments of the present invention, the maximum code value may be 255. If the scaled value is less than or equal to  171  the maximum code value, the color value associated with the current pixel may be brightness boosted using the scale value, S, and the brightness-boosted color value may be output  172  for the current pixel. A determination  162  may be made as to whether or not there are unprocessed pixels, and the process may continue. If the scaled value is greater than  173  the maximum code value, then the boosting factor may be re-computed according to: 
         [0000]    
       
         
           
             
               
                 S 
                 ′ 
               
               = 
               
                 255 
                 V 
               
             
             , 
           
         
       
     
         [0000]    where S′ may denote the re-computed boosting factor. The color value associated with the current pixel may be brightness boosted using the re-computed boosting factor, S′, and the brightness-boosted color value may be output  176  for the current pixel. A determination  162  may be made as to whether or not there are unprocessed pixels, and the process may continue. In these embodiments, the color ratio across the three color channels is maintained when clipping occurs, and thus color fidelity is maintained. 
         [0052]    In the above-described brightness-boosting methods and systems, a common brightening factor, S, may be used at each pixel, with the exception of pixels for which clipping occurs. In alternative embodiments of the present invention, the brightening factor, S, may be spatially varying according to image content. In some embodiments, the brightening factor, S, may be determined according to: 
         [0000]    
       
         
           
             
               
                 S 
                  
                 
                   ( 
                   
                     x 
                     , 
                     y 
                   
                   ) 
                 
               
               = 
               
                 ( 
                 
                   α 
                   + 
                   
                     exp 
                      
                     
                       ( 
                       
                         - 
                         
                           
                             
                               f 
                                
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                             2 
                           
                           
                             σ 
                             2 
                           
                         
                       
                       ) 
                     
                   
                 
                 ) 
               
             
             , 
             
               α 
               ≥ 
               1 
             
             , 
           
         
       
     
         [0053]    where f(x, y) may be the image brightness at location (x, y), α may be a parameter that controls the range of the brightening factor and σ may be a factor that controls the shape of the Gaussian weighting function. For f(x, y) with a range of [0,255], exemplary parameter values of α and σ are 1.6 and 100, respectively. In these embodiments, the Gaussian weighting function may produce a larger boosting factor, S(x, y), when the brightness f(x, y) is low. Therefore, a pixel with a low-brightness value may be more heavily brightened than a pixel with a larger brightness value. 
         [0054]    In alternative embodiments of the present invention, the image brightness values may be quantized into a plurality of brightness-value bins, and a brightening factor may be associated with each brightness-value bin. Pixels with brightness values within the same brightness-value bin may be brightened by the same factor, the brightening factor associated with the respective bin. In some embodiments, the quantization may be based on a histogram of the brightness values. 
         [0055]    In some embodiments of the present invention, RGB input values may be converted to an alternative color space, for example, a luminance-chrominance-chrominance color space. Exemplary luminance-chrominance-chrominance color spaces may include YCbCr, YUV, Lab and other luminance-chrominance-chrominance color spaces. In these embodiments, the luminance channel may be brightness boosted while the chrominance channels remain unchanged. 
         [0056]    The brightened image  33  generated by the brightness booster  32  and the key-feature image  35  generated by the key-feature estimator  34  may be combined by the combiner  36 . In some embodiments of the present invention, the combiner  36  may combine the brightened image  33  and the key-feature image  35  by adding the two images. In alternative embodiments of the present invention, the combiner  36  may blend the images using a weighted average of the two images according to: 
         [0000]        I   KFH   =βI   boosted +(1−β) I   KFM ,
 
         [0000]    where β may denote a blending factor, also referred to as a blending parameter, I KFH  may denote the blended image  37 , I boosted  may denote the brightened image  33  generated by the brightness booster  32  and I KFM  may denote the key-feature image  35  generated by the key-feature estimator  34 . In some embodiments of the present invention, the blending factor, β, may be a user selected parameter. In alternative embodiments of the present invention, the blending factor, β, may be a predefined value. 
         [0057]    The blended image  37  values may be mapped by a code-value mapper  38  to the range of display code values. In some embodiments of the present invention, the range of display code values is [0,255]. In some embodiments, the resulting KFH image  39  may be made available from the image-enhancement system  30  to an LCD display. 
         [0058]      FIG. 13  depicts the NPR rendition  190 , according to embodiments of the present invention, of the input image  100 , at full power consumption, shown in  FIG. 6 .  FIG. 14  depicts the NPR rendition  200 , according to embodiments of the present invention, of the input image  100 , at 2% power consumption, shown in  FIG. 6 .  FIG. 15  depicts the NPR rendition  210 , according to embodiments of the present invention, of the input image  20 , viewed in direct sunlight, shown in  FIG. 2 . 
         [0059]    Some embodiments of the present invention, described in relation to  FIG. 16 , may comprise a brightness booster  260 , a key-feature estimator  262 , a blending-parameter selector  264 , a combiner  266  and a code-value mapper  268 . In these embodiments, an input image  252 , a backlight power level  254  and an ambient-light level  256  may be received by the image-enhancement system  250 . The input image may be a color image or a gray-scale image. The input image  252  may be made available to the brightness booster  260  and the key-feature estimator  262 . The backlight power level  254  and the ambient-light level  256  may be made available to the brightness booster  260 . 
         [0060]    The key-feature estimator  262  may produce a key-feature image  263 , also considered a key-feature map, associated with the input image  252 . In some embodiments of the present invention, the key-feature estimator  262  may generate the key-feature map  263  according to previously described embodiments of the present invention. 
         [0061]    The brightness booster  260  may generate a brightened image  261  based on the input image  252  content, the backlight power level  254  and the ambient-light level  256 . 
         [0062]    The blending-parameter selector  264  may determine the blending parameter  265  used by the combiner  266  to blend the brightened image  261  and the gradient map  263 . A user-selected blending parameter  270  may be provided to the blending-parameter selector  264 . In some embodiments of the present invention, the user-selected blending parameter  270  may correspond directly to the blending parameter  265 . In alternative embodiments, the user-selected blending parameter  270  may be an image-quality setting selected by a user and associated with a blending parameter  265  value by the blending-parameter selector  264 . In some embodiments of the present invention, the blending-parameter selector  264  may select a default value for the blending parameter  265  when a user-selected blending parameter  270  is not available. 
         [0063]    The combiner  266  may combine the key-feature image  263  and the brightened image  261  based on the blending parameter  265 . In some embodiments of the present invention, the combiner  266  may linearly blend the key-feature image  263  and the brightened image  261  using the blending parameter  265  as a weighting factor according to: 
         [0000]        I   KFH   =βI   boosted +(1−β) I   KFM ,
 
         [0000]    where β may denote the blending parameter  265 , I KFH  may denote the blended image  267 , I boosted  may denote the brightened image  261  generated by the brightness booster  260  and I KFM  may denote the key-feature image  263  generated by the key-feature estimator  262 . In alternative embodiments, the combiner  266  may combine the key-feature image  263  and the brightened image  261  according to: 
         [0000]        I   KFH   =I   boosted   +I   KFM . 
         [0064]    The blended image  267  values may be mapped by a code-value mapper  268  to the range of display code values. In some embodiments of the present invention, the range of display code values is [0,255]. In some embodiments, the resulting KFH image  269  may be made available from the image-enhancement system  250  to an LCD display. 
         [0065]    Some embodiments of the present invention may comprise an LCD display. Some embodiments of the present invention may comprise an ambient-light sensor. 
         [0066]    Some embodiments of the present invention may comprise a computer program product that is a computer-readable storage medium, and/or media, having instructions stored thereon, and/or therein, that may be used to program a computer to perform any of the features presented herein. 
         [0067]    The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding equivalence of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.