Patent Publication Number: US-10789897-B2

Title: Method for regulating color shift in white balance procedure of four-color display device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority of Chinese patent application CN201610715499.3, entitled “Method for regulating color shift in white balance procedure of four-color display device” and filed on Aug. 24, 2016, the entirety of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present disclosure relates to the technical field of liquid crystal display, and in particular, to a method for regulating color shift in a white balance procedure of a four-color display device. 
     BACKGROUND OF THE INVENTION 
     At present, three-color pixel system is commonly used in color display system. Taking a liquid crystal display (LCD) device as an example, each pixel unit is generally composed of three sub pixel units, i.e., a red (R) sub pixel unit, a green (G) sub pixel unit, and a blue (B) sub pixel unit. In a new four-color pixel system, one sub pixel unit is added to the traditional three-color pixel system, and thus a color performance of the four-color pixel system can be improved. 
     With the development of the four-color pixel system, the three-color to four-color conversion technology is basically mature, and the corresponding product has entered into actual using stage. However, since there is no effective white balance regulation technology for the product with the four-color pixel system, the display effect of the product is not desirable. As a result, the advantage of four-color pixel system is not fully played. 
       FIG. 1  shows Gamma curves of a four-color pixel system in the prior art, and  FIG. 2  shows a standard Gamma curve when white color is displayed. As shown in  FIG. 1 , curves  1 ,  2 ,  3 , and  4  are respectively Gamma curves of an LCD with a four-color pixel system when red color, green color, blue color, and white color are displayed. The standard Gamma curve when white color is displayed should fall within a range of −0.2-0.3 with 2.2 as a mid value, so that the brightness change of the white color is in accordance with the perceptual curve of eye, as shown in  FIG. 2 . The brightness change of the four curves as shown in  FIG. 1  all seriously deviate from the aforesaid range at relatively high gray-scale values, and thus an over-bright display effect would be generated. 
     In order to solve the above technical problem, Patent Document “White Balance Method of Four-color Pixel System” (CN105096890A) provides the following method: lightening a plurality of sub pixel units according to two different combinations respectively based on an input white gray-scale value so as to display a white color; and regulating gray-scale values of each of the sub pixel units, and taking the gray-scale values of each of the sub pixel units, which enable the white color displayed by two different combinations to meet respective preset conditions, as output four color gray-scale values corresponding to the input white gray-scale value. 
     Gray-scale values of the red sub pixel unit, the green sub pixel unit, and the blue sub pixel unit, which enable a brightness of a white color displayed by a first combination to be equal to a brightness of a white color corresponding to the input white gray-scale value and in accordance with a Gamma curve, and a chromaticity coordinate of a white color displayed by a second combination to be equal to a chromaticity coordinate of a benchmark white color, are taken as an output gray-scale values corresponding to the input white gray-scale value. An output gray-scale value of the fourth sub pixel unit is obtained according to a three-color to four-color calculation algorithm of the four-color pixel system and based on the output gray-scale values of the red sub pixel unit, the green sub pixel unit, and the blue sub pixel unit. 
     During a regulation procedure of the gray-scale value of each sub pixel unit, a mapping relationship between the gray-scale value of the fourth sub pixel unit and the gray-scale values of the red sub pixel unit, the green sub pixel unit, and the blue sub pixel unit is maintained in accordance with the three-color to four-color calculation algorithm of the four-color pixel system. During the regulation procedure of the gray-scale value of each sub pixel unit, the gray-scale value of the fourth sub pixel unit is maintained unchanged. 
     The first combination comprises the red sub pixel unit, the green sub pixel unit, the blue sub pixel unit, and the fourth sub pixel unit, and the second combination comprises the red sub pixel unit, the green sub pixel unit, and the blue sub pixel unit. The brightness of the white color displayed by the first combination is obtained according to stimulus values Y of each of the sub pixel units of the first combination, and the chromaticity coordinate of the white color displayed by the second combination is obtained according to tri-stimulus values XYZ of each of the sub pixel units of the second combination. 
     The gray-scale value of each sub pixel unit is regulated according to formulas as follows: 
     
       
         
           
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     wherein L v (W i ) represents a brightness of a white color with a gray-scale value being i; xi, yi represent chromaticity coordinates of the white color; R i , G i , B i , and M i  represent four color gray-scale values obtained through an input three color gray-scale value after conversion according to the three-color to four-color calculation algorithm; R o , G o , B o , and M o  represent output four color gray-scale values; X(·), Y(·), and Z(·) represent tri-stimulus values of each sub pixel unit; and f represents a mapping relationship from the gray-scale values of the red sub pixel unit, the green sub pixel unit, and the blue sub pixel unit to the gray-scale value of the fourth sub pixel unit which is in accordance with the three-color to four-color calculation algorithm. 
     The brightness of the white color displayed by the first combination is obtained according to stimulus values Y of the red sub pixel unit, the green sub pixel unit, the blue sub pixel unit, and the fourth sub pixel unit, and the chromaticity coordinate of the white color displayed by the second combination is obtained according to tri-stimulus values XYZ of the four sub pixel units. 
     The gray-scale value of each sub pixel unit is regulated according to formulas as follows: 
     
       
         
           
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     wherein L v (W i ) represents a brightness of a white color with a gray-scale value being i; xi, yi represent chromaticity coordinates of the white color; R i , G i , B i , and M i  represent four color gray-scale values obtained through an input three color gray-scale value after conversion according to the three-color to four-color calculation algorithm; R o , G o , B o , and M o  represent output four color gray-scale values; X(·), Y(·), and Z(·) represent tri-stimulus values of each sub pixel unit; and f represents a mapping relationship from the gray-scale values of the red sub pixel unit, the green sub pixel unit, and the blue sub pixel unit to the gray-scale value of the fourth sub pixel unit which is in accordance with the three-color to four-color calculation algorithm. 
     However, in a process for regulating white balance of a four-color pixel system, the following problems will occur. It may lead to a detrimental result that a mixed color of red and blue, i.e., a violet color in a low gray-scale range shifts out of a correct range of human eye perception when gray-scales of a white color are generally regulated near targets (x,y). That is, under the condition that a chromaticity of the white color is normal, as to the violet color, a red light shift phenomenon occurs at low gray-scales, which shifts out of a range of the violet color. Such a problem does not occur in a traditional three-color white balance system. However, in the four-color pixel system, the fourth sub pixel is used to display the white color together, and the fourth sub pixel is usually not involved in the display of a two-color mixed image such as the violet color, which results in this kind of abnormal phenomenon, as shown in  FIGS. 3 and 4 . 
     SUMMARY OF THE INVENTION 
     Aiming at the above problems in the prior art, i.e., in a four-color pixel system, after a white balance is matched, a phenomenon that a color shifts out of a correct range of human eye perception occurs, the present disclosure provides a method for regulating color shift in white balance procedure of a four-color display device. 
     S 10 : obtaining brightness of a white color displayed by a combination according to stimulus values Y of a red sub pixel unit, a green sub pixel unit, a blue pixel unit, and a fourth sub pixel unit, which equals to brightness of a corresponding standard white gamma curve in gray-scale values from 0 to 255; and 
     S 20 : balancing a white color and one shifting color, or the white color and two shifting colors, or the white color and three shifting colors using a weighting factor in case of a two-color balance, a three-color balance, or a four-color balance. 
     Preferably, in step  10 , a gray-scale value of each sub pixel unit is regulated according to a following expression:
 
 L   v ( W   i )= Y ( R   o )+ Y ( G   o )+ Y ( B   o )+ Y ( M   o )
 
wherein Lv (Wi) represents brightness of a white color with a gray-scale value being i; Ro, Go, Bo, and Mo represent output four color gray-scale values; Y(R o ), Y(G o ), Y(B o ), and Y(M o ) respectively represent stimulus values of each sub pixel unit.
 
     Preferably, in step S 20 , in situation of the two-color balance, the specific procedure of regulating the color shift in the white balance procedure of the four-color display device is as follows: 
     dividing gray-scale values from 0 to 255 into two sections: 0 to n and n+1 to 255, two weighting factors a and b corresponding to the section o to n, another two weighting factors c and d corresponding to the section n+1 to 255; 
     in situation of two-color balance, not only the white color is matched, but also shifting color is balanced; 
     assuming target chromaticities of the white color as x 1  and y 1 , and target chromaticities of a shifting color as x 2  and y 2 ; 
     thus, a×x 1 +b×x 2 =(X(R o )+X(G o )+X(B o )+X(M o ))/S, 
     a×y 1 +b×y 2 =(Y(R o )+Y(G o )+Y(B o )+Y(M o ))/S, when 0≤input gray-scale value≤n; and 
     c×x 1 +d×x 2 =(X(R o )+X(G o )+X(B o )+X(M o ))/S, 
     c×y 1 +d×y 2 =(Y(R o )+Y(G o )+Y(B o )+Y(M o ))/S, when n+1≤input gray-scale value≤255, 
     wherein a+b=1, c+d=1, and R o , G o , B o , and M o  represent output four color gray-scale values; and 
     
       
         
           
               
             
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     wherein X(·),Y(·), and Z(·) respectively represent X, Y, and Z stimulus values of each sub pixel unit, and f represents a mapping relationship from the gray-scale values of the red sub pixel unit, the green sub pixel unit, and the blue sub pixel unit to the gray-scale value of the fourth sub pixel unit which is in accordance with the three-color to four-color calculation algorithm. 
     Preferably, in situation of the two-color balance, the procedure for acquiring the four-color gray-scale values R o , G o , B o , and M o  in a specific procedure for regulating the color shift in the white balance procedure of the four-color display device can be converted to a procedure for acquiring a minimum Delta. The specific procedure is as follows: 
     Delta1=((X(R o )+X(G o )+X(B o )+X(M o ))/S−x 1 ) 2 +((Y(R o )+Y(G o )+Y(B o )+Y(M o ))/S−y 1 ) 2    
     Delta2=((X(R o )+X(B o ))/S−x 2 ) 2 +((Y(R o )+Y(B o ))/S−y 2 ) 2    
     Delta=aλDelta1+bλDelta2, wherein a+b=. 
     Preferably, step S 20  of the two-color balance comprises matching the white color, and meanwhile balancing a color shift of violet, a color shift of orange, a color shift of yellow, or a color shift of cyan. 
     Preferably, in step S 20 , in situation of the three-color balance, the specific procedure of regulating the color shift in the white balance procedure of the four-color display device is as follows: 
     dividing gray-scale values from 0 to 255 into two sections: 0 to n and n+1 to 255, three weighting factors e, f, and g corresponding to the section o to n, another three weighting factors h, i, and j corresponding to the section n+1 to 255; 
     in situation of the three-color balance, not only the white color is matched, but also two shifting colors are balanced; 
     assuming target chromaticities of the white color as x 1  and y 1 , and target chromaticities of two shifting colors as x 2 , y 2  and x 3 , y 3 ; 
     thus, e×x 1 +f×x 2 +g×x 3 =(X(R o )+X(G o )+X(B o )+X(M o )/S, 
     e×y 1 +f×y 2 +g×y 3 =(Y(R o )+Y(G o )+Y(B o )+Y(M o ))/S, when 0≤input gray-scale value≤n; and 
     h×x 1 +i×x 2 +j×x 3 =(X(R o )+X(G o )+X(B o )+X(M o ))/S, 
     h×y 1 +i×y 2 +j×y 3 =(Y(R o )+Y(G o )+Y(B o )+Y(M o ))/S, when n+1≤input gray-scale value≤255 
     wherein e+f+g=1, h+i+j=1, and R o , G o , B o , and M o  represent output four color gray-scale values; 
     and 
     
       
         
           
               
             
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     wherein X(·),Y(·), and Z(·) respectively represent X, Y, and Z stimulus values of each sub pixel unit, and f represents a mapping relationship from the gray-scale values of the red sub pixel unit, the green sub pixel unit, and the blue sub pixel unit to the gray-scale value of the fourth sub pixel unit which is in accordance with the three-color to four-color calculation algorithm. 
     Preferably, step S 20  of the three-color balance comprises matching the white color, and meanwhile balancing color shifts of violet and orange, or color shifts of violet and yellow, color shifts of violet and cyan, color shifts of orange and yellow, color shifts of orange and cyan, or color shifts of yellow and cyan. 
     Preferably, in step S 20 , in situation of the four-color balance, the specific procedure of regulating the color shift in the white balance procedure of the four-color display device is as follows: 
     dividing gray-scale values from 0 to 255 into two sections: 0 to n and n+1 to 255, four weighting factors k, l, m, and n corresponding to the section o to n, another four weighting factors o, p, q, and r corresponding to the section n+1 to 255; 
     in situation of the four-color balance, not only the white color is matched, but also three shifting colors are balanced; 
     assuming target chromaticities of the white color as x 1  and y 1 , and target chromaticities of three shifting colors as (x 2 , y 2 ), (x 3 , y 3 ), and x 4 , y 4 ; 
     thus, k×x 1 +l×x 2 +m×x 3 +n×x 4 =(X(R o )+X(G o )+X(B o )+X(M o ))/S, 
     k×y 1 +l×y 2 +m×y 3 +n×y 4 =(Y(R o )+Y(G o )+Y(B o )+Y(M o ))/S, when 0≤input gray-scale value≤n′; and 
     o×x+p×x 2 +q×x 3 +r×x 4 =(X(R o )+X(G o )+X(B o )+X(M o ))/S, 
     o×y 1 +p×y 2 +q×y 3 +r×y 4 =(Y(R o )+Y(G o )+Y(B o )+Y(M o ))/S, when n+1≤input gray-scale value≤255 
     wherein k+l+m+n=1, o+p+q+r=1, and R, G o , B o , and M o  represent output four color gray-scale values; 
     and 
                   {             M   o     =     f   ⁡     (       R   o     ,     G   o     ,     B   o       )                         L   v     ⁡     (     W   i     )       /       L   v     ⁡     (     W   255     )         =       (     i   /   255     )     2.2                 S   =             X   ⁡     (     R   o     )       +     Y   ⁡     (     R   o     )       +     Z   ⁡     (     R   o     )       +     X   ⁡     (     G   o     )       +     Y   ⁡     (     G   o     )       +     Z   ⁢     (     G   o     )       +                 X   ⁢     (     B   o     )       +     Y   ⁡     (     B   o     )       +     Z   ⁡     (     B   o     )       +     X   ⁡     (     M   o     )       +     Y   ⁡     (     M   o     )       +     Z   ⁡     (     M   o     )                               
wherein X(·),Y(·), and Z(·) respectively represent X, Y, and Z stimulus values of each sub pixel unit, and f represents a mapping relationship from the gray-scale values of the red sub pixel unit, the green sub pixel unit, and the blue sub pixel unit to the gray-scale value of the fourth sub pixel unit which is in accordance with the three-color to four-color calculation algorithm.
 
     Preferably, step S 20  of the four-color balance comprises matching the white color, and meanwhile balancing color shifts of violet, orange, and yellow, color shifts of violet, orange, and cyan, color shifts of violet, yellow and cyan, or color shifts of orange, yellow, and cyan. 
     Preferably, in situation of the two-color balance, the three-color balance, or the four-color balance, a value constraint for the n during the specific procedure for regulating the color shift in the white balance procedure of the four-color display device is: n is less than a gray-scale value corresponding to an inflexion of a chromaticity curve after white balance in a four-color pixel system. 
     Compared with the prior art, one embodiment or more embodiments of the above technical solutions can have following advantages or beneficial effects. 
     In situation of the two-color balance, i.e., the condition that shift of one color occurs, a first target color (the white color) and a second target color are weighed using a weighting factor to achieve the purpose that no distortion of the two colors is perceptible by naked eye, so that the display effect of the four-color display device can be improved. 
     In situation of the three-color balance or the four-color balance, i.e., the condition that shift of two or three colors occurs, through the adjustment of the weighting factor, each color deviates from the target in different degrees, but it will not deviate too much and will not cause distortion which is perceptible by naked eye. 
     Other advantages, objectives, and features of the present disclosure will be further explained in the following description, and partially become self-evident therefrom, or be understood through the embodiments of the present disclosure. The objectives and advantages of the present disclosure will be achieved through the structure specifically pointed out in the description, claims, and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will be illustrated in detail hereinafter with reference to the embodiments and the accompanying drawings. In the drawings: 
         FIG. 1  schematically shows standard Gamma curves of a four-color pixel system in the prior art; 
         FIG. 2  schematically shows a standard Gamma curve when white color is displayed; 
     
    
    
     In  FIG. 3 , a solid line shows a normal chromaticity curve of violet color (X axis), and a dotted line shows a chromaticity curve after a white balance in a four-color pixel system (X axis); 
     In  FIG. 4 , a solid line shows a normal chromaticity curve of violet color (Y axis), and a dotted line shows a chromaticity curve after a white balance in a four-color pixel system (Y axis); and 
       FIG. 5  is a flow chart of a method for regulating color shift in a white balance procedure of a four-color display device in the present disclosure. 
     In the drawings, the same components are represented by the same reference signs, and the size of each component does not represent the actual size of the corresponding component. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The present disclosure will be illustrated further with reference to the drawings. 
     In  FIG. 3 , a solid line shows a normal chromaticity curve of violet color (X axis), and a dotted line shows a chromaticity curve after a white balance in a four-color pixel system (X axis). In  FIG. 4 , a solid line shows a normal chromaticity curve of violet color (Y axis), and a dotted line shows a chromaticity curve after a white balance in a four-color pixel system (Y axis). 
     Embodiment 1 
     As shown in  FIG. 5 , the present embodiment provides a method for regulating a color shift in a white balance procedure of a four-color display device, which is carried out in accordance with following steps. 
     Brightness of a white color displayed by a combination is obtained according to stimulus values Y of a red sub pixel unit, a green sub pixel unit, a blue pixel unit, and a fourth sub pixel unit, which equals to brightness of a corresponding standard white gamma curve in gray-scale values from 0 to 255.
 
 L   v ( W   i )= Y ( R   o )+ Y ( G   o )+ Y ( B   o )+ Y ( M   o )
 
wherein Lv (W i ) represents brightness of a white color with a gray-scale value being i; Ro, Go, Bo, and Mo represent output four color gray-scale values; and Y(R o ), Y(G o ), Y(B o ), and Y(M o ) respectively represent stimulus values of each sub pixel unit.
 
     Gray-scale values from 0 to 255 are divided into two sections: 0 to n and n+1 to 255. Two weighting factors a and b correspond to the section o to n, and another two weighting factors c and d correspond to the section n+1 to 255. 
     In situation of the two-color balance, not only the white color is matched, but also a shifting color is balanced. 
     Target chromaticities of the white color are assumed as x 1  and y 1 , and target chromaticities of the shifting color are determined as x 2  and y 2 ; 
     thus, a×x 1 +b×x 2 =(X(R o )+X(G o )+X(B o )+X(M o ))/S, 
     a×y 1 +b×y 2 =(Y(R o )+Y(G o )+Y(B o )+Y(M o ))/S, when 0≤input gray-scale value≤n; and 
     c×x 1 +d×x 2 =(X(R o )+X(G o )+X(B o )+X(M o ))/S, 
     c×y 1 +d×y 2 =(Y(R o )+Y(G o )+Y(B o )+Y(M o ))/S, when n+1 input gray-scale value≤255, 
     wherein a+b=1, c+d=1, and R o , G o , B o , and M o  represent output four color gray-scale values; 
     and 
                   {             M   o     =     f   ⁡     (       R   o     ,     G   o     ,     B   o       )                         L   v     ⁡     (     W   i     )       /       L   v     ⁡     (     W   255     )         =       (     i   /   255     )     2.2                 S   =             X   ⁡     (     R   o     )       +     Y   ⁡     (     R   o     )       +     Z   ⁡     (     R   o     )       +     X   ⁡     (     G   o     )       +     Y   ⁡     (     G   o     )       +     Z   ⁢     (     G   o     )       +                 X   ⁢     (     B   o     )       +     Y   ⁡     (     B   o     )       +     Z   ⁡     (     B   o     )       +     X   ⁡     (     M   o     )       +     Y   ⁡     (     M   o     )       +     Z   ⁡     (     M   o     )                               
wherein X(·),Y(·), and Z(·) respectively represent X, Y, and Z stimulus values of each sub pixel unit, and f represents a mapping relationship from the gray-scale values of the red sub pixel unit, the green sub pixel unit, and the blue sub pixel unit to the gray-scale value of the fourth sub pixel unit which is in accordance with the three-color to four-color calculation algorithm.
 
     The embodiment can have following beneficial effects. In situation of the two-color balance, i.e., the condition that shift of one color occurs, a first target color (the white color) and a second target color are weighed using a weighting factor to achieve the purpose that no distortion of the two colors is perceptible by naked eye, so that the display effect of the four-color display device can be improved. 
     Embodiment 2 
     The present embodiment is a further explanation of the method for regulating the color shift in the white balance procedure of the four-color display device according to embodiment 1. In the process of the two-color balance, a white color is matched, and meanwhile color shift of violet is regulated. 
     Embodiment 3 
     The present embodiment is a further explanation of the method for regulating the color shift in the white balance procedure of the four-color display device according to embodiment 1. In the process of the two-color balance, a white color is matched, and meanwhile color shift of orange is regulated. 
     Embodiment 4 
     The present embodiment is a further explanation of the method for regulating the color shift in the white balance procedure of the four-color display device according to embodiment 1. In the process of the two-color balance, a white color is matched, and meanwhile color shift of yellow is regulated. 
     Embodiment 5 
     The present embodiment is a further explanation of the method for regulating the color shift in the white balance procedure of the four-color display device according to embodiment 1. In the process of the two-color balance, a white color is matched, and meanwhile color shift of cyan is regulated. 
     Embodiment 6 
     As shown in  FIG. 5 , the present embodiment provides a method for regulating color shift in a white balance procedure of a four-color display device, which is carried out in accordance with following steps. 
     Brightness of a white color displayed by a combination is obtained according to stimulus values Y of a red sub pixel unit, a green sub pixel unit, a blue pixel unit, and a fourth sub pixel unit, which equals to brightness of a corresponding standard white gamma curve in gray-scale values from 0 to 255.
 
 L   v ( W   i )= Y ( R   o )+ Y ( G   o )+ Y ( B   o )+ Y ( M   o ),
 
wherein Lv (Wi) represents brightness of a white color with a gray-scale value being i; Ro, Go, Bo, and Mo represent output four color gray-scale values; and Y(R o ), Y(G o ), Y(B o ), and Y(M o ) respectively represent stimulus values of each sub pixel unit.
 
     Gray-scale values from 0 to 255 are divided into two sections: 0 to n and n+1 to 255. Three weighting factors e, f, and g correspond to the section o to n, and another three weighting factors h, i, and j correspond to the section n+1 to 255. 
     In situation of the three-color balance, not only the white color is matched, but also the two shifting colors are balanced. 
     Target chromaticities of the white color are assumed as x 1  and y 1 , and target chromaticities of the two shifting colors are assumed as x 2 , y 2 , and x 3 , y 3 ; 
     thus, e×x 1 +f×x 2 +g×x 3 =(X(R o )+X(G o )+X(B o )+X(M o ))/S, 
     e×y 1 +f×y 2 +g×y 3 =(Y(R o )+Y(G o )+Y(B o )+Y(M o )/S, when 0≤input gray-scale value≤n; and 
     h×x 1 +i×x 2 +j×x 3 =(X(R o )+X(G o )+X(B o )+X(M o ))/S, 
     h×y 1 +i×y 2 +j×y 3 (Y(R o )+Y(G o )+Y(B o )+Y(M o ))/S, when n+1≤input gray-scale value≤255, 
     wherein e+f+g=1, h+i+j=1, and R o , G o , B o , and M o  represent output four color gray-scale values; 
     and 
                   {             M   o     =     f   ⁡     (       R   o     ,     G   o     ,     B   o       )                         L   v     ⁡     (     W   i     )       /       L   v     ⁡     (     W   255     )         =       (     i   /   255     )     2.2                 S   =             X   ⁡     (     R   o     )       +     Y   ⁡     (     R   o     )       +     Z   ⁡     (     R   o     )       +     X   ⁡     (     G   o     )       +     Y   ⁡     (     G   o     )       +     Z   ⁢     (     G   o     )       +                 X   ⁢     (     B   o     )       +     Y   ⁡     (     B   o     )       +     Z   ⁡     (     B   o     )       +     X   ⁡     (     M   o     )       +     Y   ⁡     (     M   o     )       +     Z   ⁡     (     M   o     )                               
wherein X(·),Y(·), and Z(·) respectively represent X, Y, and Z stimulus values of each sub pixel unit, and f represents a mapping relationship from the gray-scale values of the red sub pixel unit, the green sub pixel unit, and the blue sub pixel unit to the gray-scale value of the fourth sub pixel unit which is in accordance with the three-color to four-color calculation algorithm.
 
     The present embodiment can have following beneficial effects. In situation of the three-color balance, i.e., the condition that shift of two colors occurs, through the adjustment of the weighting factor, each color deviates from the target in different degrees, but it will not deviate too much and will not cause distortion which is perceptible by naked eye. 
     Embodiment 7 
     The present embodiment is a further explanation of the method for regulating the color shift in the white balance procedure of the four-color display device according to embodiment 6. In the process of the three-color balance, a white color is matched, and meanwhile color shifts of violet and orange are regulated. 
     Embodiment 8 
     The present embodiment is a further explanation of the method for regulating the color shift in the white balance procedure of the four-color display device according to embodiment 6. In the process of the three-color balance, a white color is matched, and meanwhile color shifts of violet and yellow are regulated. 
     Embodiment 9 
     The present embodiment is a further explanation of the method for regulating the color shift in the white balance procedure of the four-color display device according to embodiment 6. In the process of the three-color balance, a white color is matched, and meanwhile color shifts of violet and cyan are regulated. 
     Embodiment 10 
     The present embodiment is a further explanation of the method for regulating the color shift in the white balance procedure of the four-color display device according to embodiment 6. In the process of the three-color balance, a white color is matched, and meanwhile color shifts of orange and yellow are regulated. 
     Embodiment 11 
     The present embodiment is a further explanation of the method for regulating the color shift in the white balance procedure of the four-color display device according to embodiment 6. In the process of the three-color balance, a white color is matched, and meanwhile color shifts of orange and cyan are regulated. 
     Embodiment 12 
     The present embodiment is a further explanation of the method for regulating the color shift in the white balance procedure of the four-color display device according to embodiment 6. In the process of the three-color balance, a white color is matched, and meanwhile color shifts of yellow and cyan are regulated. 
     Embodiment 13 
     As shown in  FIG. 5 , the present embodiment provides a method for regulating a color shift in a white balance procedure of a four-color display device, which is carried out in accordance with following steps. 
     Brightness of a white color displayed by a combination is obtained according to stimulus values Y of a red sub pixel unit, a green sub pixel unit, a blue pixel unit, and a fourth sub pixel unit, which equals to brightness of a corresponding standard white gamma curve in gray-scale values from 0 to 255.
 
 L   v ( W   i )= Y ( R   o )+ Y ( G   o )+ Y ( B   o )+ Y ( M   o ),
 
wherein Lv (W i ) represents brightness of a white color with a gray-scale value being i; Ro, Go, Bo, and Mo represent output four color gray-scale values; and Y(R o ), Y(G o ), Y(B o ), and Y(M o ) respectively represent stimulus values of each sub pixel unit.
 
     Gray-scale values from 0 to 255 are divided into two sections: 0 to n and n+1 to 255. Four weighting factors k, l, m, and n correspond to the section o to n, another four weighting factors o, p, q, and r correspond to the section n+1 to 255. 
     In situation of the four-color balance, not only the white color is matched, but also the three shifting colors are balanced. 
     Target chromaticities of the white color are assumed as x 1  and y 1 , and target chromaticities of the three shifting colors are assumed as (x 2 , y 2 ), (x 3 , y 3 ), and x 4 , y 4 ; 
     thus, k×x 1 +l×x 2 +m×x 3 +n×x 4 =(X(R o )+X(G o )+X(B o )+X(M o ))/S, 
     k×y 1 +l×y 2 +m×y 3 +n×y 4 =(Y(R o )+Y(G o )+Y(B o )+Y(M o ))/S, when 0≤input gray-scale value≤n′; and 
     o×x 1 +p×x 2 +q×x 3 +r×x 4 =(X(R o )+X(G o )+X(B o )+X(M o )/S, 
     o×y 1 +p×y 2 +q×y 3 +r×y 4 =(Y(R o )+Y(G o )+Y(B o )+Y(M o ))/S, when n+1≤:input gray-scale value≤255, 
     wherein k+l+m+n=, o+p+q+r=1, and R o , G o , B o , and M o  represent output four color gray-scale values, and 
                   {             M   o     =     f   ⁡     (       R   o     ,     G   o     ,     B   o       )                         L   v     ⁡     (     W   i     )       /       L   v     ⁡     (     W   255     )         =       (     i   /   255     )     2.2                 S   =             X   ⁡     (     R   o     )       +     Y   ⁡     (     R   o     )       +     Z   ⁡     (     R   o     )       +     X   ⁡     (     G   o     )       +     Y   ⁡     (     G   o     )       +     Z   ⁢     (     G   o     )       +                 X   ⁢     (     B   o     )       +     Y   ⁡     (     B   o     )       +     Z   ⁡     (     B   o     )       +     X   ⁡     (     M   o     )       +     Y   ⁡     (     M   o     )       +     Z   ⁡     (     M   o     )                               
wherein X(·),Y(·), and Z(·) respectively represent X, Y, and Z stimulus values of each sub pixel unit, and f represents a mapping relationship from the gray-scale values of the red sub pixel unit, the green sub pixel unit, and the blue sub pixel unit to the gray-scale value of the fourth sub pixel unit which is in accordance with the three-color to four-color calculation algorithm.
 
     The present embodiment can have following beneficial effects. In situation of the four-color balance, i.e., the condition that shift of three colors occurs, through the adjustment of the weighting factor, each color deviates from the target in different degrees, but it will not deviate too much and will not cause distortion which is perceptible by naked eye. 
     Embodiment 14 
     The present embodiment is a further explanation of the method for regulating the color shift in the white balance procedure of the four-color display device according to embodiment 13. In the process of the three-color balance, a white color is matched, and meanwhile color shifts of violet, orange and yellow are regulated. 
     Embodiment 15 
     The present embodiment is a further explanation of the method for regulating the color shift in the white balance procedure of the four-color display device according to embodiment 13. In the process of the three-color balance, a white color is matched, and meanwhile color shifts of violet, orange and cyan are regulated. 
     Embodiment 16 
     The present embodiment is a further explanation of the method for regulating the color shift in the white balance procedure of the four-color display device according to embodiment 13. In the process of the three-color balance, a white color is matched, and meanwhile color shifts of violet, yellow and cyan are regulated. 
     Embodiment 17 
     The present embodiment is a further explanation of the method for regulating the color shift in the white balance procedure of the four-color display device according to embodiment 13. In the process of the three-color balance, a white color is matched, and meanwhile color shifts of orange, yellow and cyan are regulated. 
     Embodiment 18 
     The present embodiment is a further explanation of the method for regulating the color shift in the white balance procedure of the four-color display device according to embodiments 1 to 5. The procedure for acquiring four-color gray-scale values R o , G o , B o , and M o  can be converted to a procedure for acquiring a minimum Delta. The specific procedure is as follows.
 
Delta1=(( X ( R   o )+ X ( G   o )+ X ( B   o )+ X ( M   o ))/ S−x   1 ) 2 +(( Y ( R   o )+ Y ( G   o )+ Y ( B   o )+ Y ( M   o ))/ S−y   1 ) 2  
 
Delta2=(( X ( R   o )+ X ( B   o ))/ S−x   2 ) 2 +(( Y ( R   o )+ Y ( B   o )/ S−y   2 ) 2  
 
Delta= a ×Delta1+ b ×Delta2, wherein  a+b=   1 .
 
     Specifically, Delta 1 is a sum of squares of a difference between actual chromaticity coordinates (X(R o )+X(G o )+X(B o )+X(M o ))/S and a target x 1  and a difference between (X(R o )+X(G o )+X(B o )+X(M o ))/S and a target y 1 , or a square of a distance between an actual color and a target color in a chromaticity space. Similarly, Delta 2 is a distance between the actual chromaticity coordinates and targets (x 2 , y 2 ). 
     Embodiment 19 
     The present embodiment is a further explanation of the method for regulating the color shift in the white balance procedure of the four-color display device according to embodiments 1 to 18. A value constraint for n is: n is less than a gray-scale value corresponding to an inflexion of a chromaticity curve after white balance in a four-color pixel system. 
     The present disclosure is illustrated hereinabove with reference to the specific embodiments, which are only examples of the principle and use of the present disclosure. Those skilled in the art can make amendments to the embodiments disclosed herein or provide other arrangements without departing from the spirit and scope of the present disclosure. The technical feature described in one embodiment can also be used in other embodiments.