Abstract:
A method of zero-D dimming backlights for 3D or multi-view displays using right and left image data. The method receives right and left pixel luminance values for the right and left image data, and it remaps the right and left pixel luminance values using a factor based upon an average luminance value or based upon a luminance percentile value or a modified factor that further includes the absolute difference between left and right pixel luminance values. The factor or modified factor are selectively used to remap particular right and left pixel luminance values based upon a disparity consideration. The method results in power savings by dimming the backlight without perceptible or substantial loss in brightness of the display.

Description:
BACKGROUND 
     Typical zero-D backlight dimming transforms (remaps) or opens the pixels of a liquid crystal display (LCD) such that a dimmed backlight plus transformed image results in a similar brightness perception compared to a 100% on backlight and normal image. The advantage of this zero-D dimming is backlight power savings, an important benefit in portable battery operated displays. Pixel correction algorithms have been developed to implement zero-D dimming. These algorithms involve statistical analysis of pixel brightness and can be based on, for example, average frame luminance or luminance percentile statistics. The power savings capability with these algorithms has been experimentally demonstrated and can be as much as threefold while still maintaining reasonable display brightness perception. However, these algorithms have been developed for use with single view displays. 
     Accordingly, a need exists for zero-D dimming for 3D or multi-view displays. 
     SUMMARY 
     A method, consistent with the present invention, provides zero-D dimming for a display using right and left image data. The method includes receiving right image data having right pixel luminance values and left image data having left pixel luminance values. The right and left pixel luminance values are modified using a first factor based upon an average or a percentile statistics based luminance value and a second factor based upon differences between the right pixel luminance values and the left pixel luminance values. The modifying step includes selectively using the first factor or second factor considering an analysis of possible disparity reduction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are incorporated in and constitute a part of this specification and, together with the description, explain the advantages and principles of the invention. In the drawings, 
         FIG. 1  is a schematic side view of an illustrative display apparatus; 
         FIGS. 2A and 2B  are schematic side views of an illustrative autostereoscopic 3D display apparatus in operation; 
         FIG. 3  is a graph illustrating a linear pixel remapping algorithm; and 
         FIG. 4  is a flow chart of a remapping algorithm. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention include a method of uniform or zero-D dimming of backlights for 3D displays that maintains disparity information between luminance of right and left image data. 
     3D Displays 
       FIG. 1  is a schematic side view of an illustrative 3D autostereoscopic display apparatus  10 . The display apparatus includes an LCD panel  20  having a required frame response time and a backlight  30  positioned to provide light to the LCD panel  20 . The backlight  30  includes a right eye image solid state light source  32  and a left eye image solid state light source  34  capable of being modulated between light sources  32  and  34  at a rate of at least 90 Hertz. Backlight  30  includes a light guide  35  having an edge  33  for receiving light from light source  34  and an edge  31  for receiving light from light source  32 . Light sources  32  and  34  can be implemented with, for example, light emitting diodes (LEDs). A double sided prism film  40  having prisms on one side and lenslets on the other is disposed between the LCD panel  20  and the backlight  30 . 
     A synchronization driving element  50  is electrically connected to the backlight  30 , light sources  32  and  34 , and the LCD panel  20 . The synchronization driving element  50  synchronizes activation and deactivation (modulation) of the right eye image solid state light source  32  and the left eye image solid state light source  34  as image frames are provided at a rate of 90 frames per second or greater to the LCD panel  20  to produce a flicker-free video or rendered computer graphics. An image source  60  is connected to the synchronization driving element  50  and provides the images frames (right eye images and left eye images) to the LCD panel  20 . The image provided by image source  60  can include video or computer rendered graphics. The synchronization driving element  50  can include a video interface such as, for example, a Westar VP-7 video adaptor (Westar Display Technologies, Inc., St. Charles, Mo.) coupled to custom solid state light source drive electronics. 
       FIGS. 2A and 2B  are schematic side views of display apparatus  10  in operation. In  FIG. 2A  the left eye image solid state light source  34  is illuminated and the right eye image solid state light source  32  is not illuminated. In this state, the light emitted from the left eye image solid state light source  34  transmits through the backlight  30 , through the double sided prism sheet  40 , and LCD panel  20  providing a left eye image directed toward the left eye  1   a  of an viewer or observer. In  FIG. 2B  the right eye image solid state light source  32  is illuminated and the left eye image solid state light source  34  is not illuminated. In this state, the light emitted from the right eye solid state light source  32  transmits through the backlight  30 , through the double sided prism sheet  40 , and LCD panel  20  providing a right eye image directed toward the right eye  1   b  of an viewer or observer. 
     Providing at least 45 left eye images and at least 45 right eye images (alternating between right eye and left eye images and the images are possibly a repeat of the previous image pair) to a viewer per second provides a flicker-free 3D image to the viewer. Accordingly, displaying different right and left viewpoint images either computer rendered or acquired by appropriately positioned still image or video image cameras, alternating in synchronization with the switching of the light sources  32  and  34 , enables the viewer to visually fuse the two different images, creating the perception of depth from the flat panel display. An example of a 3D display is described in more detail in U.S. Pat. No. 7,528,893, which is incorporated herein by reference as if fully set forth. 
     Zero-D Dimming for 3D Displays 
     In an LED backlight display, such as the 3D display described above, the pixel luminance L is obtained by summing the red, green, and blue (RGB) subpixel values as follows: L=0.299R+0.587G+0.144B. Each of the single byte RGB values is between 0 and 255, meaning 0≦L≦255. The weighting coefficients for RGB in the L value are chosen based on how much each of the RGB components contribute to the perceived intensity to a viewer, green being the most and blue the least, although different coefficients can be used. The normalized decimal pulse width modulation (PWM) value and LED backlight duty cycle are equal to the following: PWM=L/255. Dimming the backlight is implemented by PWM of a signal driving the LEDs in the backlight. 
     The pixel luminance algorithm of embodiments of the present invention provides correction of such dimming for 3D displays. In particular, in a 3D display the left and right images need special consideration if the luminance for a particular pixel for the left and right image shows high contrast. To maintain a desirable 3D experience, this contrast needs to be maintained. Therefore, the pixel luminance correction algorithm takes into account the difference in left and right pixel luminance value. 
     A strategy in pixel remapping is depicted in  FIG. 3  where the new or remapped pixel luminance values L n  are plotted against the old pixel luminance values L o . The “no change” situation is indicated by the solid line  62  with unity slope, resulting in new pixel luminance values L n  equal to the old pixel luminance values L o . The dashed line shows a transformation or remapping using two straight lines (segments  64  and  66 ). The inflection point ( 68 ) of the dashed line in  FIG. 3  can be based on the average frame luminance L a  or other suitable statistical parameter, resulting in the value L s  on the scale of the new luminance values L n . As shown by line segment  66 , the pixel differences for values greater than L a  will be compressed. Thus, a disparity between the left and right views that occurs in this region (line segment  66 ) is at risk of being lost due to zero-D dimming. Zero-D dimming for 3D displays modifies the inflection point  68  based on the pixel luminance values for the image pixels with luminance disparity. Disparity refers to the difference in image location of an object seen by the left and right eyes, resulting from the eyes&#39; horizontal separation. Disparity is used to extract depth information from the two-dimensional retinal images in stereopsis. In this case disparity refers to the difference in left and right images based on the horizontal separation giving rise to 3D depth perception. In an alternative implementation, the correction algorithm might not take into consideration all disparity values but rather can only factor in those pixel differences that are noticeably different in terms of the human visual system sensitivity. 
     From  FIG. 3  a pull-up factor F can be defined as the ratio L s /L a . The value of F is greater than one and is set by the display manufacturer or a viewer. The algorithm takes into account the difference between left pixel luminance value L l  and right pixel luminance value L r  for a particular frame of image data to determine a modified pull-up factor F m  in equation (1):
 
If L l  and L r &lt;L a  then F m =F
 
Else  F   m =(1− F )| L   r   −L   l   |+F   (1)
 
     The remapping equations of the two straight line segments  64  and  66  are given by equation (2): 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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                             L 
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                           L 
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                         = 
                         
                           
                             
                               
                                 ( 
                                 
                                   1 
                                   - 
                                   
                                     
                                       F 
                                       m 
                                     
                                     ⁢ 
                                     
                                       L 
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                               ⁢ 
                               
                                 L 
                                 o 
                               
                             
                             + 
                             
                               
                                 F 
                                 m 
                               
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                                 L 
                                 a 
                               
                             
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                   ( 
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                   ) 
                 
               
             
           
         
       
     
     Equations (1) and (2) are implemented electronically.  FIG. 4  is a flow chart of a remapping algorithm  70  using the results of equations (1) and (2). Algorithm  70  can be implemented in, for example software executed by a processor such as a controller in synchronization driving element  50 , or in a controller using hardware modules or a combination of software and hardware. For example, algorithm  70  along with equations (1) and (2) can be implemented electronically in the Westar VP-7 video adaptor identified above. Table 1 provides sample code to determine disparity values for use in implementing algorithm  70 . The code in Table 1 uses the 75th percentile value for the pixels with luminance disparity as the value L a  for illustrative purposes only; different percentile values can be used for L a  depending upon, for example, a particular display device. 
     Algorithm  70  uses the factors F and F m  to perform the remapping. The factor F is determined from L s  and L a  (step  72 ). The value L s  is set (step  74 ), for example as a percentile value of pixels with luminance disparity, and L a  is determined using pixel statistics (step  76 ), meaning statistical techniques can be applied to the pixel luminance values to determine the optimum or desired value for L a  based upon a visual appearance of the corresponding display or a calculated brightness of the display using various values of L a . The value L a  is also used to derive a PWM value for backlight dimming (step  78 ). The factor F m  is determined from F using |L r −L l | (step  86 ). The algorithm determines the original right and left pixel luminance values L r  and L l  for a particular frame of image data (step  80 ) and remaps them. If L r  or L l  is not greater than L a  (step  82 ), then F is used to modify the right and left pixel luminance values L r,l  (step  84 ). If L r  or L l  is greater than L a  (step  82 ), then F m  is used to modify the right and left pixel luminance values L r,l  (step  88 ). The term “frame” means a full frame of right and left image data for a particular display or any partial frame of the data on the display. 
     Although described with respect to a 3D display, the zero-D dimming can be implemented for any multi-view system using right and left image data. An example of a multi-view display is described in U.S. Patent Application Publication No. 2009/0167639, which is incorporated herein by reference as if fully set forth. 
     
       
         
               
             
               
             
               
               
             
               
             
               
               
             
               
               
             
               
               
             
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Sample Code to Determine Disparity Values 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Left_Image = imread(‘L.bmp’); 
               
               
                 Right_Image = imread(‘R.bmp’); 
               
               
                 Disparity = Left_Image-Right_Image; %For example, a comparison of Left and Right 
               
               
                 Images on a pixel by pixel basis 
               
               
                 Size_of_Images = size(Left_Image); 
               
               
                 Total_number_of_pixels = Size_of_Images(1,1)*Size_of_Images(1,2); 
               
               
                 Pixels_with_Disparity_Originals = find(Left_Image~=Right_Image); 
               
               
                 Size_of_pixels_with_disparity_Originals = size(Pixels_with_Disparity_Originals); 
               
               
                 Number_of_pixels_with_disparity_Originals = 
               
               
                 Size_of_pixels_with_disparity_Originals(1,1); 
               
               
                 Percentage_of_pixels_with_intact_disparity_Originals = 
               
               
                 (Number_of_pixels_with_disparity_Originals/Total_number_of_pixels)*100; 
               
               
                 [c d] = hist(Left_Image(Pixels_with_Disparity_Originals),256); 
               
               
                 count = 0; 
               
               
                 for i = 1:256; 
               
             
          
           
               
                   
                 count = count + c(1,i); 
               
               
                   
                 f(1,i) = count; 
               
             
          
           
               
                 end 
               
               
                 cutoff = Number_of_pixels_with_disparity_Originals*0.75 
               
               
                 %75th percentile point specified but this cutoff point in practice could be any user of 
               
               
                 display specified point 
               
               
                 for i = 1:256; 
               
             
          
           
               
                   
                 if f(1,i) &lt;= cutoff; 
               
             
          
           
               
                   
                 cutoff_value = i−1; 
               
             
          
           
               
                   
                 end 
               
             
          
           
               
                 end 
               
               
                 %cutoff_value is the value that determines which values in the images 
               
               
                 %receive what pull-up factor