Abstract:
Methods and apparatus are provided for a color liquid crystal display (CLCD). The apparatus comprises a processor coupled to the CLCD for receiving a character code and a color code and translating them into character and color pixel arrays that are overlaid and summed to produce a composite pixel array corresponding to the CLCD pixel array, where each entry in the composite array is used in conjunction with a color table to establish drive levels for each pixel in the CLCD. The character pixel array includes gray level color mixing and the color pixel array includes spatial shading color mixing, so that the composite array uses both techniques to determined the individual CLCD pixel drive levels, thereby providing a wider range of color choices without significant color dependence on viewing angle.

Description:
TECHNICAL FIELD  
       [0001]     The present invention generally relates to liquid crystal displays, and more particularly to color generation for liquid crystal displays.  
       BACKGROUND  
       [0002]     Liquid crystal displays able to show alphanumeric and/or graphical information in various colors are well known in the art. Such liquid crystal color displays are used in avionics, computers, telephones, medical imaging, vehicles, and various other applications. In many cases the displayed colors may convey functional information. For example, and not intended to be limiting, text, numbers, and/or symbols, or a combination thereof may signify a substantially ‘safe’ condition when presented in green, a ‘caution’ condition when presented in yellow or amber, and a potential ‘danger’ condition when presented in red. In such instances, the color of the image is intended to convey information to the user, in addition to or as a supplement to the information provided by the content of the image. Thus, color fidelity including color fidelity as a function of viewing angle or other factors, can be important. For example, if the color perceived by the viewer changes depending upon, for example, viewing angle, or the image contrast or luminance, this can potentially lead to mistaken interpretation of the displayed information. In addition, various users desire that the colors presented conform to particular standards. Thus, having a large number of color choices may also be important.  
         [0003]     While present day color liquid crystal displays are very useful they do suffer certain drawbacks. For example, the viewing angle over which color fidelity is reasonably preserved may be undesirably narrow, and/or the absolute color provided by the display can vary depending upon the drive intensity, and/or the number of possible colors that can be displayed may be undesirably limited, and/or the display brightness may be weak and insufficient to permit easy viewing in sunlight or other bright light conditions, and so forth. Further, color fidelity, color choice, luminance or brightness, viewing angle, and other properties often mutually interact so that prior art approaches for improving one property may cause degradation in another property.  
         [0004]     Accordingly, it is desirable to provide an improved color generation apparatus and method for color liquid crystal displays, especially for displays suitable for use in avionics systems. In addition there is an ongoing need to provide a display and method of driving the display that maximizes the number of available color choices and useful viewing angles, without significantly detracting from the display brightness and life. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.  
       BRIEF SUMMARY  
       [0005]     An apparatus is provided for a color liquid crystal display (CLCD). The apparatus comprises a processor coupled to the CLCD for receiving a character code and a color code and translating them into character and color pixel arrays that are overlaid and summed to produce a composite pixel array corresponding to the CLCD pixel array, where each entry in the composite array is used in conjunction with a color table to establish drive levels for each pixel in the CLCD. The character pixel array includes gray level color mixing as well as defining the character size and shape on the CLCD, and the color pixel array includes spatial shading color mixing, so that the composite array uses both techniques to determine the individual CLCD pixel drive levels, thereby providing a wider range of color choices without significant color dependence on viewing angle.  
         [0006]     A method is provided for driving a color liquid crystal display (CLCD) to show one or more predetermined characters in a predetermined color. The method comprises, in either order, receiving a character code defining the character to be displayed and a color code defining the predetermined color, then in either order, determining a character pixel pattern from the character code and determining a spatial color pixel pattern from the color code, then combining the character pixel pattern and the spatial pixel pattern to produce a composite pixel pattern having combined pixel values at least for each pixel within a pixel pattern outline of the predetermined character, then, using the pixel values, obtaining red (R), green (G) and blue (B) pixel drive amounts for each pixel, and sending the pixel drive amounts to the pixels of the CLCD.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and  
         [0008]      FIGS. 1A and 1B  are simplified plan and side views of an observer positioned with respect to a liquid crystal display;  
         [0009]      FIG. 2  shows a simplified electrical schematic of a display drive system, coupled to a color liquid crystal display; according to the present invention;  
         [0010]      FIGS. 3A-3E  are simplified plan views of a portion of the liquid crystal display different condition of excitation;  
         [0011]      FIGS. 4-6  show various look-up tables for implementing the present invention according to a preferred embodiment for an exemplary color;  
         [0012]      FIG. 7  shows a simplified flow chart illustrating the method of the present invention;  
         [0013]      FIG. 8  shows a 1976 u′, v′ CIE Chromaticity Diagram on which the present invention&#39;s viewing angle shift and color matching capability are compared to prior art approaches, for an exemplary color; and  
         [0014]      FIG. 9  is a table wherein the experimental results illustrated graphically in  FIG. 8  are presented in numeric and descriptive form. 
     
    
     DETAILED DESCRIPTION  
       [0015]     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description.  
         [0016]      FIGS. 1A  shows simplified plan view  20  and  1 B shows simplified side view  30  of observer  21  positioned with respect to liquid crystal display  22 . Display  22  emits light at different angles as indicated by rays  24 - 26  in  FIG. 1A  and rays  34 - 36  in  FIG. 1B .  FIG. 1A  shows observer  21  in different azimuthal positions, for example along arc  23 , receiving ray  24 , or ray  25  or ray  26  depending upon the observer&#39;s position. Rays  25 ,  26  make angles  27 ,  28  with respect to central ray  24  in  FIG. 1A .  FIG. 1B  shows observer  21  in different vertical positions, for example along arc  33 , receiving ray  34 , or ray  35  or ray  36  depending upon the observer&#39;s position. Rays  35 ,  36  make angles  37 ,  38  with respect to central ray  34  in  FIG. 1B . One of the problems often associated with prior art color liquid crystal displays is that the color perceived by observer  21  can change depending upon the magnitude of angles  27 ,  28 ,  37 ,  38 .  
         [0017]      FIG. 2  shows simplified electrical schematic of display drive system  50 , coupled to color liquid crystal display (CLCD)  22 , according to an embodiment of the present invention. The depicted CLCD  22  includes several layers or regions, for example, and not intended to be limiting, backlight  52 , thin film transistor (TFT) drive array layer  55 , liquid crystal region layer  56  (hereafter active region  56 ) and color filter layer  57 . Backlight  52  receives power via lead or connection  53  and produces substantially white light directed toward layers  55 ,  56 ,  57 . In the preferred embodiment, backlight  52  employs an array of white light emitting diodes. TFT layer  55  receives drive signals from graphics processor  60  via leads or bus  61  and provides the appropriate signals to CLCD layer  56  to cause its light transmission to vary pixel by pixel. Filter layer  57  contains pixel-size regions for each primary color: red, green and blue. Each pixel region in layer  57  corresponds in size and location to an individual TFT on TFT array layer  55 . The alignment of the liquid crystal in region  56  is electrically switched by the drive voltage to the TFT. When the liquid crystal in region  56  is electrically aligned between the TFT active pixels in layer  55  and the overlying portion of color filter layer  57 , light is emitted from CLCD  22  in direction  54  toward observer  21 , and is red, green or blue depending upon the color of the filter portion over the individual TFT. Thus, by selectively energizing the corresponding TFT in layer  55  under the red, green or blue pixels of filter layer  57 , a large number of different colored light combinations may be emitted by CLCD  22 . As will be explained in more detail in connection with  FIGS. 3-6 , different combinations of colored pixels are energized to cause display  22  to present various messages.  
         [0018]     The individual pixels of TFT array layer  55  are driven by display electronics system  60 , which includes processor (CPU)  62 , optional non-volatile memory (NVM)  63 , temporary memory (RAM)  64 , program memory  66 , input-output (I/O) device  68  and graphics processor  70 , all mutually coupled by bus or leads  69  so as to allow intercommunication. User controls  58  are coupled to I/O  68  by bus or leads  59  and graphics processor  70  is coupled to TFT array layer  55  of display  22  by bus or leads  61 . Bus or leads  71  couple font table  72  to graphics processor  70 . As will be more fully explained later, font tables  72  contain information used by graphics processor  70  to activate pixels of the desired color and intensity in the desired location on display  22  to convey the desired information. Display electronics system  60  is also preferably coupled through internal bus  69  and external bus or leads  65  to general systems bus  67  whereby it can receive commands and exchange information of interest to the general system (e.g., an avionics system, not shown). For example, and not intended to be limiting, display system  60  can receive a command from user controls  58  or general bus  67  or a combination thereof to show certain alphanumeric or symbol information such as, for example, current altitude. Based on information received from, for example, program memory  66 , NVM  63 , user input, or controls  58  and/or general systems bus  67 , CPU  62  instructs graphics processor  70  to display altitude information present on general bus  67  in different colors depending upon the altitude value with respect to a predetermined minimum desired altitude. The predetermined minimum altitude may be stored for example in NVM  63  or elsewhere, or set by user controls  58  or a combination thereof. Assume that the minimum desired altitude has been set at 3000 meters. Then, in response to instructions retrieved from program memory  66  and/or general system bus  67 , graphics processor  70  in cooperation with font tables  72 , displays altitudes over 3100 meters in green, altitudes between 3001 and 3100 meters in amber, and altitudes at or below 3000 meters in red. Those of skill in the art will understand that this is merely exemplary and is not intended to be limiting. System  50  is able to provide the commanded characters and/or symbols in the commanded colors with adequate brightness, color fidelity, and viewing angle. The preferred means for accomplishing this is explained more fully in connection with  FIGS. 3-6 .  
         [0019]      FIGS. 3A-3E  show simplified plan views of portions  80 ,  82 ,  84 ,  86 ,  88  respectively of liquid crystal display  22  of  FIGS. 1-2 , under different conditions of excitation. Merely for convenience of explanation and not intended to be limiting, portions  80 - 88  have four columns (A,B,C,D) and six rows ( 1 , 2 , 3 , 4 , 5 , 6 ) of tri-color pixels. Each tri-color pixel has three separately addressable sub-pixels, one red (denoted “R”), one green (denoted “G”) and one blue (denoted “B”). Thus, in each portion  80 - 88  there are 4×6=24 pixels of each color and a total of 3×24=72 individually activated pixels. For convenience of explanation, the following convention is used herein. The letters R, G, B identify the color of the respective pixel and the size of the letters indicates the relative intensity of the drive being supplied and therefore the illumination from that pixel. The larger the letter the brighter the pixel. For example, in  FIG. 3A , all three colors of pixels in column A are being exited at the maximum level so as to have their maximum brightness, while all three colors in column B are excited at a lower level and therefore have lower luminance or brightness. All three colors in column C have still lower excitation and still lower luminance and all three colors in column D are not excited at all and therefore exhibit little or no luminance. For simplicity, in  FIG. 3A , each row has the same configuration: column A is the brightest, column B is less bright, column C is even less bright and column D is OFF. The difference in brightness is achieved by varying the excitation voltage applied to the TFT(s) driving the liquid crystal pixel under the corresponding region of the colored filter layer. Because the R, G, B pixels in each tri-pixel, are equally excited, the resulting light output from columns A-C will be substantially white, but of different intensity in each column; column A brightest, column B less bright, column C still less bright and column D dark. The purpose of display portion  80  in  FIG. 3A  is to illustrate the convention used in FIGS.  3 B-E where different ways of exciting the pixels to obtain different colors and viewing angles are shown.  
         [0020]     For convenience of explanation and not intended to be limiting, FIGS.  3 B-E illustrate various ways of obtaining an approximately amber output from screen portions  82 - 88 . In order to produce amber, no blue is used; therefore all blue (“B”) pixels are dark (OFF) in these examples. This is not intended to be limiting, but occurs merely because of the colors (yellowish or amber) chosen for purposes of explanation. Persons of skill in the art will understand that if a different example color were chosen, different combinations of the R, G, and B pixels would be used. In  FIG. 3B , a yellowish output is created by turning on all red (R) and green (G) pixels at substantially the same brightness level, as indicated by letters R, G having substantially the same size. For example, red pixel  82 - 1 C 1  and green pixel  82 - 1 C 2  are turned on full while blue pixel  82 - 1 C 3  is dark. This pattern is repeated in each tri-pixel of array  82 . Because the intensity of the individual color pixels is the same, this is referred to as “equal gray level mixing,” that is, there are no intensity variations from tri-pixel to tri-pixel. While maximum drive is used on all R, G pixels (e.g., shown by the largest letter size) this is merely for convenience of illustration. Equal gray level mixing can occur at any drive level as long as the drive levels for the various colors being used are chosen to provide equal light output from red and green (or whatever colors are being used). When maximum drive is used, the brightness of the yellowish color produced in the example of  FIG. 3B  is good, but the number of colors that can be produced is significantly limited.  
         [0021]      FIG. 3C  showing array portion  84 , illustrates the use of different pixel drive levels as another way of producing a yellowish color, in this case an amber or darker yellow. In this example, all red (R) pixels receive maximum drive and produce maximum brightness, but adjacent green (G) pixels receive a lower level of drive and therefore produce less than maximum brightness, as shown by the smaller relative size of the letter “G” compared to the letter “R.” This arrangement is referred to as unequal gray level mixing. This approach offer many more possible colors than the approach of  FIG. 3B , but suffers from the disadvantage that there is a significant color shift with viewing angle. A further difficulty with this approach is that as certain pixels receive less and less drive compared to other pixels, that is as the ratio of drive on the dimmed pixels to the drive on the bright pixels gets smaller and smaller, the brightness degrades and color shift with viewing angle gets worse.  
         [0022]      FIG. 3D  showing array portion  86 , illustrates the use of what is referred to as spatial shading to achieve an approximately amber color. All operating pixels are energized at the same brightness level. In this example, all of the red pixels are ON but only half of the green pixels are ON. Thus, referring by way of example to columns C and D of array  86 , red pixel  86 - 1 C 1  and all other red pixels in column C (and the other columns) are ON, and green pixels  86 - 1 C 2  and  86 - 2 D 2  are ON and green pixels  86 - 2 C 2  and  86 - 1 D 2  are OFF. The ON and OFF green pixels in adjacent columns are staggered to improve the uniformity of illumination. As before, all blue pixels are OFF because the desired color is amber. This approach has a good field of view (little color shift with viewing angle) relative to the others described above but is limited in its ability to provide a wide range of colors or a particularly desired color. Some colors cannot be achieved at all, or only with spatial shading so coarse that the low fill factor of the minor color is visible in the display. This is undesirable.  
         [0023]      FIG. 3E  shows display portion  88  illustrating the preferred arrangement according to the present invention for producing both a wide range of colors of adequate brightness and with good viewing angle color performance. The arrangement of  FIG. 3E  combines gray level and spatial mixing. For example, the arrangement of  FIG. 3E  easily provides the desired amber color by reducing the drive level on the green (G) pixels, as indicated by the smaller size of the letters “G” and illuminating only every other green pixel in a staggered pattern but at a different (e.g., lower) luminance level than used for the red pixels. In this example, the green pixels are driven at about 70% of their maximum luminance while the red pixels are driven to 100%, as indicated by the different size of the “R” and “G” letters on the pixels. Thus, red pixels  88 - 1 A 1  and  88 - 1 B 1  have a higher luminance than green pixel  88 - 1 A 2 , and blue pixels  88 - 1 A 3  and  88 - 1 B 2  are OFF. The staggered pattern of illumination of the green pixels is repeated throughout the array where the desired amber color is needed. To achieve the same color without using spatial shading, the green pixels would have to be driven at about 30% of maximum luminance compared to the red pixels. This large difference in pixel drive levels would cause the color to shift over the field of view. Thus, the combination of gray level and spatial shading implemented in  FIG. 3E  provides superior results.  
         [0024]      FIG. 4  shows look-up tables or patterns  90 , stored for example, in font tables  72  and/or NVM  63  for use by system  50  in implementing the present invention according to a preferred embodiment. Table  92  is an example of a typical 18×27 character pattern table for the letter “A” used by graphic processor  70 . Each square  93  in table  92  represents a tri-pixel, that is, each square  93  contains R, G, B sub-pixels. Graphic processor  70  (not shown in  FIG. 4 ) turns on one or more sub-pixels in each tri-pixel within outline  94  of array or table  92  to produce, for example, the letter “A.” The numbers  1 ,  2 ,  3  shown on the pixels within outline  94  determine, when passed through color table  98 , the relative drive levels to the R, G, B sub-pixels in order to produce a particular target color. When used without color pattern table  96 , table  92  provides unequal gray level mixing for determining the resulting character color. Persons of skill in the art will understand that the letter “A” is used merely by way of illustration and not intended to be limiting. Any alphanumeric character or other graphic that will fit within table or pattern  92  may be displayed. While character pattern or table  92  is described as being an 18×27 array, this is merely exemplary and not limiting. Persons of skill in the art will understand that an array of any one of numerous sizes consistent with the required character resolution and display size may be used.  
         [0025]     Color pattern or array  96  is similar to array  92  but for implementing spatial shading in order to produce by way of example and not intended to be limiting a particular shade of amber. Array  96  alone produces staggered spatial shading analogous to that shown in  FIG. 3D  where every other green pixel is dark. Persons of skill in the art will understand that for other colors, the entries in the boxes of array  96  will be different. Each box in array  96  corresponds to a tri-pixel box in array  92 . Array or table  96  is shown as being an 8×8 array but this is merely for convenience of explanation and is generally hardware determined. In the preferred arrangement, a type 69000 graphics processor chip manufactured by Asiliant Technologies, San Jose, Calif. was utilized for driving CLCD  22 . The exemplary 8×8 and 18×27 row by column dimensions of tables or patterns  92 ,  96  are suitable for use with the 69000 chip but other row by column arrangements can be used with other graphics processors. For example, with an alternating spatial shading arrangement like that shown in  FIG. 3D , a 2×2 array is sufficient. The entries in each box  97  of table  96  determine the spatial shading employed in display  22  and, in combination with the entries in table or array  92  determine the color of the letter or other alphanumeric or graphic being generated by system  50 . The format of tables  92 ,  96  are desirably such that they may be superposed to produce a result interpretable by color table  98  to generate signals to pixel driver  100  that, in turn, supplies the drive signals to the individual R, G, B pixels in display  22  (pixel driver  100  is equivalent to graphics processor  70  of  FIG. 2 ). Array adder  102  is used to combine tables  92 ,  96 , tri-pixel by tri-pixel, i.e., square by square, as explained below. The functions of array adder  102 , color table  98  and pixel driver  100  are provided by system  60  of  FIG. 2 .  
         [0026]     Arrays or tables  92 ,  96  are conveniently but not essentially combined by superposition, that is, the content of each tri-pixel (square) in table  96  is added algebraically to the content of the corresponding tri-pixel (square) in array  92  in array adder  102  and the result fed to color table  98 . The result of combining arrays  92 ,  96  is illustrated in composite array  110  of  FIG. 5 . The blank squares in array  92  outside of outline  94  are assumed to have value zero. Thus, for those tri-pixels in array  92  outside of outline  94 , the summation in array  110  yields just the alternating  0 ,  4  values of array  96  for the desired amber color. Persons of skill in the art will understand based on the explanation herein that a different pattern would be used to achieve other colors. Within outline  94  where array  92  has various values  1 ,  2 ,  3 , these numbers are added square by square to the numbers  0 ,  4  shown square by square in array  96  to obtain composite array  110 . In composite array  110 , the numbers in the squares within outline  94  have values  1 ,  2 ,  3 ,  5 ,  6 ,  7 . While the foregoing arrangement is preferred, any means for combining a spatial array matrix with a character generator gray level matrix may be used.  
         [0027]     The values in composite array  110  are fed to color table  98 , which is shown in detail in  FIG. 6 . The entries in color table  120  of  FIG. 6  relate the composite array values (abbreviated as “CA values” or “CA #&#39;s”) to the relative drive level for each R, G, B pixel in CLCD  22 . The abbreviation “GL” stands for “gray level” and refers to the relative pixel excitation level for gray level color mixing as explained in connection with  FIG. 3C . If the CA value is ‘0’ or ‘4’, then according to color table  120 , this corresponds to a pixel drive level of ‘0’ for all three colors R, G, B. Thus, all pixels outside of outline  94  will be dark. The values  132 ,  168 ,  172 ,  212 ,  220 ,  252  shown in table  120  of  FIG. 6  for different CA#&#39;s, conveniently refer to driver addresses where the actual pixel drive levels (or intermediate signals controlling the pixel drive levels) are stored. In the example of table  120  and for convenience of explanation the higher the driver address number, the higher the drive level to the pixel, although this is not essential. For example, in table  120  driver address  172  corresponds to greater pixel drive and therefore greater pixel brightness than, say, driver address  132 . Driver address  252  corresponds to the maximum available drive level and  0  corresponds to the minimum (e.g., no drive). For convenience of explanation, the drive address values shown in table  120  may be thought of as expressing relative pixel brightness. However, the relationship between driver address and pixel drive level need not be linear. Persons of skill in the art will understand based on the description herein how such an arrangement can be implemented.  
         [0028]     If the CA value is “1”, this corresponds to unequal gray level two (GL- 2 ) wherein, in our example of an approximately amber “A”, the red pixels are supplied with driver address  172  compared to the green pixels with driver address  132 . The maximum excitation corresponds to driver address  252 . This provides unequal gray level mixing as in  FIG. 3C  for those pixels. Similarly with CA values  2  and  3  where the relative excitation levels are controlled by driver addresses R( 212 ), G( 168 ) and R( 252 ), G( 220 ), respectively, there is also unequal gray level mixing. However, for CA values  5 ,  6 ,  7  spatial mixing is included, in that for this amber example only red pixels are illuminated and all green and blue pixels are dark where CA values  5 ,  6 ,  7  occur in  FIG. 5 . Further, depending upon the CA value, the excitation level of the red pixels is different, specifically CA numbers  5 ,  6 ,  7  correspond to gray levels GL- 2 , GL- 4 , GL- 6  where the relative red pixel excitation levels for the different pixels are expressed by drive addresses  172 ,  212  and  252  respectively with a maximum drive level corresponding to address  252 . It will be appreciated that the present invention provides for a mixture of unequal gray level excitation and spatial shading excitation of the various colored pixels. As will be subsequently explained in more detail, this produces a superior result. Persons of skill in the art will understand based on the description herein, that for other colors, the mix of spatial and unequal gray level excitation levels for the various R, G, and B pixels will be different. Also, the particular pixels being excited will also depend upon the shape of the alphanumeric or graphic being displayed.  
         [0029]      FIG. 7  shows a simplified flow chart illustrating method  200  of the present invention. Method  200  begins with start  202  that preferentially occurs whenever system  50  seeks to display a new character or graphic. In step  204 , CPU  62  and/or graphics processor  70  receives the code identifying the desired character, as for example, an ASCII code. In step  205  the pixel pattern needed to display that character is determined, as for example, through use of a look up table or other means stored in font tables  72 . The result is, generally, a character array similar to array  92  of  FIG. 4 , however, this is not essential and any means for character generation may be used. In step  206  the code for the color(s) in which the character is to be presented is received by CPU  62  and/or graphics processor  70 , and in step  207 , analogous to step  206 , the spatial mixing color array (e.g., array  96 ) needed to produce that color is obtained, for example from font tables  72  and/or NVM  63  or elsewhere. The results of steps  205 ,  207  are combined in step  210  where the spatial color array (or equivalent) and the character array (or equivalent) are combined to produce a composite array, such as for example array  110  of  FIG. 5  or equivalent. The composite array values are used in conjunction with a color table such as color table  120  of  FIG. 6  to obtain the relative red (R), green (G), blue (B) pixel drive levels for the individual pixels in CLCD array  22 . In subsequent step  214 , these drive levels are sent by graphics processor  70  to the individual pixels in CLCD array  22  and the process thereafter terminates at END  216 . Step groups  204 - 205  and  206 - 207  may be performed in either order. All that is important is that the results of step groups  204 - 205  and  206 - 207  be available to be combined in step  210 .  
         [0030]     Method  200  may be repeated each time a new character or graphic is to be displayed. If there is no change in the color code and the previous spatial pattern determined in step  205  is still available in memory, then this previously determined spatial pattern may be reused. Conversely, if the character is unchanged, but the color is changed, then a new spatial color pattern is determined and combined with the previously determined character pattern. The foregoing explanation has been presented for the situation where only a single character is being displayed, but this is merely for convenience of description. Those of skill in the art will appreciate based on the description herein that character generation and display can also occur in groups, all the same color or with a mixture of colors. In those situations, the character arrays and spatial color arrays may be combined in groups to produce composite arrays for the groups of characters, analogously to the single character method described above. Thus, the above-described method is useful for multiple as well as single characters.  
         [0031]      FIG. 8  shows 1976 u′, v′ CIE Chromaticity Diagram  220  on which the present invention&#39;s viewing angle shift and color matching capability are compared to prior art approaches, for an exemplary color (amber). Such Chromaticity Diagrams are well known in the art and are described, for example by G. J. and D. G. Chamberlin in  Color: Its Measurement, Computation and Application , Heyden and Sons Press Ltd, 1980, pages 60 ff. The human visible color spectrum is contained within outline  222 . Region  223  is the locus of primary red (R), region  224  the locus of primary green and region  225  the locus of primary blue. White is in the regions of approximately u′˜0.22 and v′˜0.48. Intermediate shades have other u′, v′ values. Marker  226  indicates the exemplary desired color, an amber shade, at about u′˜0.3 and v′˜0.55.  FIG. 9  is a table wherein the experimental results illustrated graphically in  FIG. 8  are presented in numeric and descriptive form.  
         [0032]     Referring now to  FIGS. 8-9 , brackets  228 - 230  in  FIG. 8  shows the results obtained using different methods of color generation and different viewing angles. Azimuthal angles  27  and  28  were varied from 0 to 45 degrees, vertical angle  37  was varied from 0 to 5 degrees and vertical angle  38  was varied from 0 to 35 degrees. Bracket  228  in  FIG. 8  corresponds to line  252  in table  250  of  FIG. 9  wherein color generation employed gray level mixing, such as has been previously described in connection with  FIGS. 3B-3C . It will be noted that this method of color generation was able to achieve target amber color  226  in  FIG. 8 , but as noted in line  252  of  FIG. 9  and shown graphically by bracket  228  in  FIG. 8 , a comparatively large color shift occurs for different viewing angles. As noted earlier, this is undesirable. Thus, although gray level mixing allowed the target color to be achieved, the comparatively large color shift indicates that it is not a desirable candidate for color generation applications where color fidelity as a function of viewing angle is important. Avionics systems are examples of such applications.  
         [0033]     Bracket  229  in  FIG. 8  and line  254  in table  250  of  FIG. 9  illustrates the results obtained using spatial shading for color generation. It will be noted that this method of color generation yielded only a small color variation with changes in viewing angle (which is desirable), but was not able to achieve target color  226  (which is undesirable). This is because with spatial shading, the number of colors that can be produced is much reduced. Where the target color happens to be among those achievable by spatial shading, then this is a desirable approach in terms of viewing angle color independence, but where some of the colors that must be displayed are outside the range of those achievable using spatial shading, this approach is not attractive.  
         [0034]     Bracket  230  in  FIG. 8  and line  256  in Table  250  of  FIG. 9  are the result of combining both gray level mixing and spatial shading according to the present invention, as has been already described in connection with  FIGS. 2-7 . It will be noted that the invented approach is able to achieve target color  226 , which is not possible with spatial shading alone, and also has an angular color shift that is 40% less than that obtained with gray level mixing alone. While the angular color shift is larger than with spatial shading alone, the fact that spatial shading was not able to produce the target color rules it out as a viable approach in this situation. Thus, the invented approach of using both gray level mixing and spatial shading at the same time, in the manner described herein, provides a significant overall improvement over the prior art.  
         [0035]     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the appended claims and the legal equivalents thereof.