Abstract:
A method of compensating the color gamut of a display includes establishing a plurality of color gamut boundaries of four color hues, generating m sets of original luminance, chrominance and hue values according to m sets of tricolor grey level values, adjusting the chrominance of n sets of luminance, chrominance and hue values of the m sets of luminance, chrominance and hue values exceeding the plurality of color gamut boundaries with four color hues to generate n sets of corrected luminance, chrominance and hue values, generating m sets of four color grey levels according to the n sets of corrected luminance, chrominance and hue values and (m−n) sets of uncorrected luminance, chrominance and hue values, and displaying images on the display according to the m sets of four color grey levels.

Description:
BACKGROUND 
       [0001]    1. Field 
         [0002]    The disclosure relates to a method of compensating color gamut of a display, especially a method of transforming the tricolor image values of the display into four color image values to compensate color gamut of the display. 
         [0003]    2. Description of the Prior Art 
         [0004]    Liquid crystal displays (LCDs) and light emitting diode (LED) displays are widely used nowadays. Because their slim shapes, low power dissipation and low radiation, LCDs and LED displays are widely applied on mobile electronic devices such as notebooks, monitors, and PDAs (personal digital assistants). In general, tricolor (red, green and blue colors) light emitting elements are applied in the related art LCDs and LED displays, to display images with high luminance and chrominance. For display devices, saving power is an important issue. Thus, four color (red, green, blue and white colors) displays capable of raising transmittance and reducing backlight power consumption are developed. The four color display raises luminance through high transmittance of the white color backlight, and saves power by improving light emitting efficiency. 
         [0005]    However, when the number of colors of sub-pixels increases from three to four, the layout area of conventional red, green, and blue color sub-pixels will be reduced. Moreover even the brightest red, green and blue colors are darker than white, lowering the effect of light emitted by red, green and blue color sub-pixels. That is, the addition of white color sub-pixels will degrade luminance and chrominance of other color sub-pixels. 
       SUMMARY 
       [0006]    An embodiment of the disclosure relates to a method of compensating color gamut of a display. The method comprises establishing a plurality of color gamut boundaries of four color hues, generating m sets of original luminance, chrominance and hue values according to m sets of tricolor grey level values, adjusting chrominance values of n sets of original LCH values of the m sets of original LCH values exceeding the plurality of color gamut boundaries to generate n sets of calibrated LCH values, generating m sets of four color grey level values according to the n sets of calibrated LCH values and (m−n) sets of original LCH values, and displaying an image on the display according to the m sets of four color grey level values. m and n are positive integers, and m≧n. 
         [0007]    Another embodiment of the disclosure relates to a display comprising a plurality of pixels and a signal transformation unit. Each pixel of the plurality of pixels comprises four sub-pixels for displaying an image according to four color image values. The signal transformation unit is used for transforming tricolor image values of the pixels into four color image values of the pixels. Among m pixels of the pixels having color saturation values of tricolor image values between 0.7 and 1, a ratio of Wmax and max{min[R,G,B]} of n pixels having similar color saturation values is not larger than 1. Wmax denotes a largest value of n white color image values of four color image values of the n pixels, and max{min[R,G,B]} denotes a largest value of n smallest tricolor values of the tricolor image values of the n pixels. 
         [0008]    Another embodiment of the disclosure relates to a display comprising a plurality of pixels and a signal transformation unit. Each pixel of the plurality of pixels comprises four sub-pixels for displaying an image according to four color image values. The signal transformation unit is used for transforming tricolor image values of the pixels into four color image values of the pixels. Among m pixels of the pixels having color saturation values of tricolor image values between 0.2 and 0.55, a ratio of Wmax and max{min[R,G,B]} of n pixels having similar color saturation values is not smaller than 1, Wmax denotes a largest value of n white color image values of four color image values of the n pixels, and max{min[R,G,B]} denotes a largest value of n smallest tricolor values of the tricolor image values of the n pixels. 
         [0009]    Another embodiment of the disclosure relates to a display comprising a plurality of pixels and a signal transformation unit. Each pixel of the plurality of pixels comprises four sub-pixels for displaying an image according to four color image values. The signal transformation unit is used for transforming tricolor image values of the pixels into four color image values of the pixels. Among m pixels of the pixels having color saturation values of tricolor image values between 0.55 and 0.7, a ratio of Wmax and max{min[R,G,B]} of n pixels having similar color saturation values is not equal to 1, Wmax denotes a largest value of n white color image values of four color image values of the n pixels, and max{min[R,G,B]} denotes a largest value of n smallest tricolor values of the tricolor image values of the n pixels. 
         [0010]    These and other objectives of the disclosure will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1A  is a flowchart showing compensating color gamut of a four color display according to a first embodiment of the disclosure. 
           [0012]      FIG. 1B  shows an implementation of adjusting the chrominance value of a set of original LCH values in the step  106  of  FIG. 1A . 
           [0013]      FIG. 2  is a flowchart showing compensating color gamut of a four color display according to a second embodiment of the disclosure. 
           [0014]      FIG. 3  is a flowchart showing compensating color gamut of a four color display according to a third embodiment of the disclosure. 
           [0015]      FIG. 4  is a flowchart showing compensating color gamut of a four color display according to a fourth embodiment of the disclosure. 
           [0016]      FIG. 5  shows a display according to a fifth embodiment of the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Some phrases are referred to specific elements in the present specification and claims, please notice that the manufacturer might use different terms to refer to the same elements. However, the definition between elements is based on their functions instead of their names. Further, in the present specification and claims, the term “comprising” is open type and should not be viewed as the term “consisted of.” 
         [0018]    The embodiments and figures are provided as follows in order to illustrate the disclosure in detail, but the claimed scope of the disclosure is not limited by the provided embodiments and figures. 
         [0019]    Please refer to  FIG. 1A , which is a flowchart showing compensating color gamut of a four color display according to a first embodiment of the disclosure. The descriptions of the flowchart in  FIG. 1A  are as follows: 
         [0020]    Step  102 : establish a plurality of color gamut boundaries of four color hues; 
         [0021]    Step  104 : generate m sets of original luminance, chrominance and hue (LCH) values according tom sets of tricolor grey level values; 
         [0022]    Step  106 : adjust chrominance values of n sets of original LCH values of the m sets of original LCH values exceeding the plurality of color gamut boundaries established in Step  102  to generate n sets of calibrated LCH values; 
         [0023]    Step  108 : generate m sets of four color grey level values according to the n sets of calibrated LCH values generated in Step  106  and (m−n) sets of original LCH values; 
         [0024]    Step  110 : display an image on the display according to the m sets of four color grey level values generated in Step  108 ; 
         [0025]    In Step  102 , the plurality of color gamut boundaries of four color hues can be a plurality of color gamut boundaries of red, green, blue and white color hues or another four colors. In Step  104 , tricolor can be red, green, and blue colors. And the m sets of tricolor grey level values are transformed into the m sets of original luminance, chrominance and hue (LCH) values. However in a four color display, after transforming the m sets of tricolor grey level values into the m sets of original LCH values, the chrominance may be distorted, thus Step  106  should be performed. That is, if a set of original LCH values exceeds color gamut boundaries of four color hues, the chrominance value of the set of original LCH values should be adjusted to be on a corresponding color gamut boundary of four color hues, to generate a set of calibrated LCH values. Therefore if n sets of original LCH values among m sets of original LCH values exceed color gamut boundaries of four color hues, the chrominance values of the n sets of original LCH values are adjusted to be on corresponding color gamut boundaries of four color hues, to generate n sets of calibrated LCH values. m and n are positive integers, and After above calibration, in Step  108 , the m sets of four color grey level values can be generated according to the n set of calibrated LCH values and the (m−n) sets of original LCH values not exceeding the color gamut boundaries of four color hues. Therefore, the four color display can display images according to them sets of four color grey level values. And the images displayed by the four color display will have correct luminance, chrominance and hue. 
         [0026]    Please refer to  FIG. 1B , which shows an implementation of adjusting the chrominance value of a set of original LCH values in step  106  of  FIG. 1A . In  FIG. 1B , the horizontal axis represents the chrominance value of a set of original LCH values, the vertical axis represents the luminance value of the set of original LCH values, and the axis perpendicular to the horizontal and vertical axes represents the hue of the original LCH values. The area  130  is within a color gamut boundary of four color hues established in Step  106 . When the set of original LCH values  140  is outside of the area  130 , the coordinate of the set of original LCH values  140  is adjusted along the horizontal axis to be on the edge of the area  130  as shown by the dotted circle depicted in  FIG. 1B  to generated a set of calibrated LCH values. Thus, the set of calibrated LCH values  140  no longer exceeds the area  130 . 
         [0027]    That is, in step  106  of  FIG. 1A , the n sets of original LCH values having coordinates outside of the color gamut boundaries of four color hues are calibrated by adjusting their chrominance values without changing their luminance values and hue values. 
         [0028]    Please refer to  FIG. 2 , which is a flowchart showing compensating color gamut of a four color display according to a second embodiment of the disclosure. The descriptions of the flowchart in  FIG. 2  are as follows: 
         [0029]    Step  202 : establish a plurality of color gamut boundaries of four color hues; 
         [0030]    Step  204 : transform m sets of tricolor grey level values into m sets of first three stimulating values (tristimulus value); 
         [0031]    Step  206 : transform the m sets of first three stimulating values into m sets of original LCH values; 
         [0032]    Step  208 : adjust chrominance values of n sets of original LCH values of the m sets of original LCH values exceeding the plurality of color gamut boundaries established in Step  202  to generate n sets of calibrated LCH values; 
         [0033]    Step  210 : generate m sets of four color grey level values according to the n sets of calibrated LCH values generated in Step  208  and (m−n) sets of original LCH values; 
         [0034]    Step  212 : display an image on the display according to the m sets of four color grey level values generated in Step  210 ; 
         [0035]    The difference between the first and second embodiments is that, in the second embodiment, the m sets of tricolor grey level values are transformed into the m sets of first three stimulating values, and then the m sets of first three stimulating values are transformed into the m sets of original LCH values. The formulas (1) to (3) are shown below to show how to transform tricolor grey level values into first three stimulating values. 
         [0000]        X= 0.49 R+ 0.31 G+ 0.20 B   (1)
 
         [0000]        Y= 0.17697 R+ 0.81240 G+ 0.01063 B   (2)
 
         [0000]        Z= 0.00 R+ 0.01 G+ 0.99 B   (3)
 
         [0036]    In formulas (1) to (3), R denotes a red color grey level value of a set of tricolor grey level values, G denotes a green color grey level value of the set of tricolor grey level values, B denotes a blue color grey level value of the set of tricolor grey level values, X denotes a first value of a set of first three stimulating values corresponding to the set of tricolor grey level values, Y denotes a second value of the set of first three stimulating values, and Z denotes a third value of the set of first three stimulating values. 
         [0037]    Since the difference between the first and second embodiments is only on how to transform tricolor grey level values into original LCH values, the images displayed by the four color display in the second embodiment will likewise have correct luminance, chrominance and hue. 
         [0038]    Please refer to  FIG. 3 , which is a flowchart showing compensating color gamut of a four color display according to a third embodiment of the disclosure. The descriptions of the flowchart in  FIG. 3  are as follows: 
         [0039]    Step  302 : establish a plurality of color gamut boundaries of four color hues; 
         [0040]    Step  304 : generate m sets of original luminance, chrominance and hue (LCH) values according tom sets of tricolor grey level values; 
         [0041]    Step  306 : adjust chrominance values of n sets of original LCH values of the m sets of original LCH values exceeding the plurality of color gamut boundaries established in Step  302  to generate n sets of calibrated LCH values; 
         [0042]    Step  308 : transform the n sets of calibrated LCH values generated in Step  306  and the (m−n) sets of original LCH values into m sets of second three stimulating values; 
         [0043]    Step  310 : transform the m sets of second three stimulating values into m sets of four color grey level values; 
         [0044]    Step  312 : display an image on the display according to the m sets of four color grey level values generated in Step  310 ; 
         [0045]    The difference between the first and third embodiments is that, in the third embodiment, the n sets of calibrated LCH values and the (m−n) sets of original LCH values are transformed into the m sets of second three stimulating values, and then the m sets of second three stimulating values are transformed into the m sets of four color grey level values. The mathematical operations of transforming the second three stimulating values into the m sets of four color grey level values can be obtained by the inverse transformations of formulas (1) to (3), thus will not be further described. 
         [0046]    Since the difference between the first and third embodiments is only on how to transform the n sets of calibrated LCH values and the (m−n) sets of original LCH values into the m sets of four color grey level values, the images displayed by the four color display in the third embodiment will likewise have correct luminance, chrominance and hue. 
         [0047]    Please refer to  FIG. 4 , which is a flowchart showing compensating color gamut of a four color display according to a fourth embodiment of the disclosure. The descriptions of the flowchart in  FIG. 4  are as follows: 
         [0048]    Step  402 : establish a plurality of color gamut boundaries of four color hues; 
         [0049]    Step  404 : transform m sets of tricolor grey level values into m sets of first three stimulating values; 
         [0050]    Step  406 : transform the m sets of first three stimulating values into m sets of original LCH values; 
         [0051]    Step  408 : adjust chrominance values of n sets of original LCH values of the m sets of original LCH values exceeding the plurality of color gamut boundaries established in Step  402  to generate n sets of calibrated LCH values; 
         [0052]    Step  410 : transform the n sets of calibrated LCH values generated in Step  408  and the (m−n) sets of original LCH values into m sets of second three stimulating values; 
         [0053]    Step  412 : transform the m sets of second three stimulating values into m sets of four color grey level values; 
         [0054]    Step  414 : display an image on the display according to the m sets of four color grey level values; 
         [0055]    The difference between the second and fourth embodiments is that, in the fourth embodiment, the n sets of calibrated LCH values and the (m−n) sets of original LCH values are transformed into the m sets of second three stimulating values, and then the m sets of second three stimulating values are transformed into the m sets of four color grey level values. The mathematical operations of transforming the second three stimulating values into the m sets of four color grey level values can be obtained by the inverse transformations of formulas (1) to (3), thus will not be further described. Since the difference between the second and fourth embodiments is only on how to transform the n sets of calibrated LCH values and the (m−n) sets of original LCH values into the m sets of four color grey level values, the images displayed by the four color display in the fourth embodiment will likewise have correct luminance, chrominance and hue. 
         [0056]    Please refer to  FIG. 5 , which shows a display  500  according to a fifth embodiment of the disclosure. As shown in  FIG. 5 , the display  500  comprises a plurality of pixels  510  and a signal transformation unit  520 . Each pixel  510  comprises four sub-pixels for displaying an image according to four color image values. The signal transformation unit  520  is used for transforming tricolor image values of the pixels into four color image values of the pixels  510 . The four colors can be the red, green, blue and white colors, and the tricolor can be the red, green and blue colors as described in the first embodiment. Among m pixels of the pixels  510  having color saturation values of tricolor image values between 0.7 and 1, a ratio of Wmax and max{min[R,G,B]} of n pixels having similar color saturation values is not larger than 1. Wmax denotes a largest value of n white color image values of four color image values of the n pixels, and max{min[R,G,B]} denotes a largest value of n smallest tricolor values of the tricolor image values of the n pixels. R, G and B respectively denote red, green and blue image values in the tricolor image value. The operation expression of the color saturation value is shown in formula (4) below. 
         [0000]    
       
         
           
             
               
                 
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         [0057]    In formula (4), max[R,G,B] represents a largest image value of R, G and B, and min[R,G,B] represents a smallest image value of R, G and B. The operations of the aforementioned saturation value S and image values R, G and B can be performed in grey level domain or in gamma domain. That is, the four color image values of the pixels  510  can be four color grey level values or four color gamma values. Similarly, the tricolor image values of the pixel  510  can be tricolor grey level values or tricolor gamma values. 
         [0058]    Besides, If the color saturation values of tricolor image values of m pixels of the pixels  510  are between 0.2 and 0.55, a ratio of Wmax and max{min[R,G,B]} of n pixels having similar color saturation values is not smaller than 1. Moreover, If the color saturation values of tricolor image values of m pixels of the pixels  510  are between 0.55 and 0.7, a ratio of Wmax and max{min[R,G,B]} of n pixels having similar color saturation values is not equal to 1. 
         [0059]    According to the relationship between the color saturation value S and the ratio of Wmax and max{min[R,G,B]} illustrated in the fifth embodiment, whether the color saturation value S is high (between 0.7 to 1), middle (between 0.55 to 0.7) or low (0.2 to 0.55), the relationship between the color saturation value S and the ratio of Wmax and max{min[R, G, B]} is quite smooth, or substantially linear. Therefore, the gray level overlapping effect of the image can be reduced, reducing color distortion of the display  500 . 
         [0060]    Further, the signal transformation unit  520  can be configured to comprise a look-up table  522 , for mapping the tricolor image values of the pixels  510  to four color image values, and the lookup table  522  can further map the tricolor image values of the pixels  510  to backlight luminance, so as to adjust the backlight of the display  500  accordingly. 
         [0061]    In view of above, through utilizing the first to fifth embodiments of the disclosure, the tricolor image values can be transformed into four color image values without distorting the color performance of the four color display. Thus, the four color display can display correct images and save power. 
         [0062]    Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the disclosure. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.