Patent Publication Number: US-6714180-B1

Title: Automatic control of gray scaling algorithms

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to video display systems. More particularly, the present invention relates to apparatus and methods for creating gray scale pixels on computer displays. 
     2. The Prior Art 
     Display controller devices, such as flat panel controllers use several different methods to develop gray scales on a display. A gray scale represents the different intensities of each of the Red, Green, and Blue pixels in the display. The gray scales for flat panel displays are typically created using a combination of analog techniques in the panel drivers. Known analog techniques include frame rate (temporal) gray scaling, and spatial gray scaling (dithering). Those of ordinary skill in the art understand that the word “gray” is used even though it may relate to shades of Red, Green, or Blue. 
     Frame rate or temporal gray scaling has been described in the art. This technique trades off video frames for gray scales. It does this by modulating the pixel signal on and off on a per-frame basis. If a display has only two display colors (i.e., black and white), then a pixel may be turned on in every other frame to simulate a 50% gray value. The actual algorithm used in real-world applications is more complex due to the requirements to remove artifacts. This method for creating gray scale works quite well on computer interfaces since the data does not change much, and only contains a few colors in a typical application. 
     Spatial gray scaling is normally performed by trading pixel resolution on the screen for perceived shades of gray. If it is desired to generate 4 more shades on a display, then a two by two matrix is often used. Spatial gray scaling looks best on continuous tone images. These tend to have a limited bandwidth, but are much more sensitive to color. Spatial gray scaling tends to blur text, and is not recommended for computer graphics. 
     A typical prior-art display system is called upon to display both computer graphics and continuous tone images. Such a display system achieves colors by both spatial and temporal gray scaling. The result is typically a compromise between requirements for graphics images and requirements for continuous tone images. Gray scale implementations using combinations of both spatial and temporal gray scaling are preselected and fixed into display systems. 
     BRIEF DESCRIPTION OF THE INVENTION 
     A method for developing gray scale pixels on a computer display according to the present invention comprises examining a run of pixels in the image content data for display on at least a portion of a computer display, determining whether the image content comprises computer graphics or continuous tone images, and choosing from among spatial and temporal gray scaling to apply to the image content data in response to whether the image content comprises computer graphics or continuous tone images. 
     According to one embodiment of the present invention, the number of unique colors in a selected run of pixels on the screen are counted to determine whether the image is a computer graphics image or a continuous tone image, and the gray scaling method is adjusted to best match the displayed image. 
     According to another embodiment of the present invention, the number of times that the same color appears in a selected run of pixels is counted. If the same color appears more than a threshold number of times, the image subject matter is probably computer graphics rather than a continuous tone image. 
     According to yet another embodiment of the present invention, the frequency content of a selected run of pixels may be determined. If the frequency content of the selected run of pixels is high (e.g., above about 5 MHz) the content is probably computer graphics text. On the other hand, if the frequency content of the selected run of pixels is low (e.g., below about 2 MHz) the content is probably a continuous tone image. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a flow diagram of the steps comprising a generic method according to the present invention. 
     FIG. 2 is a flow diagram of illustrative steps comprising a first method according to the present invention. 
     FIG. 3 is a block diagram of a circuit according to a presently preferred embodiment of the present invention according to the method illustrated in the flow diagram of FIG.  2 . 
     FIG. 4 is a flow diagram of illustrative steps which may be inserted in place of step  32  of FIG.  2 . 
     FIG. 5 is a flow diagram of illustrative steps comprising a second method according to the present invention. 
     FIG. 6 is a block diagram of a circuit according to a second embodiment of the present invention according to the method illustrated in the flow diagram of FIG.  5 . 
     FIG. 7 is a flow diagram of illustrative steps comprising a third method according to the present invention. 
     FIG. 8 is a block diagram of a circuit according to a third embodiment of the present invention according to the method illustrated in the flow diagram of FIG.  6 . 
     FIGS. 9 a  and  9   b , respectively, are graphs illustrating typical distributions of instantaneous slope values of color differences between adjacent pixels for computer graphics images and continuous tone images, respectively. 
     FIG. 10 is a block diagram of an illustrative overall architecture of a system for on-the-fly switching between two or more gray scale algorithms in the pixel pipeline according to the present invention. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     Those of ordinary skill in the art will realize that the following description of the present invention is illustrative only and not in any way limiting. Other embodiments of the invention will readily suggest themselves to such skilled persons. 
     Computer displays may comprise text, graphics, or computer generated images. Depending on the content of any given display image, a different gray scaling algorithm may be used, so that the image is displayed in the best form. 
     A selection of pixels being used to make the determination of the primary content of the display image may comprise a complete frame, a scan line or selected group of scan lines, or an otherwise defined screen region. The gray scaling method is then adjusted according to the determination. 
     Where the image content determination may be made on the basis of a selected screen region other than a line or a frame, numerous criteria for selecting screen regions may be employed. For example, where multiple windows are displayed in an image frame, the contents of each window may be individually evaluated. Other similar variations of this embodiment are included within the scope of the present invention. 
     A presently preferred criteria is that a pixel run with a color count of fewer than 30 colors comprises a computer graphics image, and a pixel run with a color count equal to or greater than 30 colors comprises a continuous tone (i.e. photographic) image to a second embodiment of the present invention, the number of unique colors in a selected run of pixels on the screen are counted to determine whether the image is a computer graphics image, a computer generated image, or a continuous tone image, and the gray scaling method is adjusted to best match the displayed image. A presently preferred criteria is that a pixel run with a color count of fewer than 30 colors comprises a computer graphics image, and a pixel run with a color count of greater than 100 colors comprises a continuous tone (i.e. photographic) image. Pixel runs having between 30 and 100 colors are treated as computer-generated images such as those used in games. 
     Referring first to FIG. 1, a flow diagram illustrates the steps comprising a generic method according to the present invention. First, a pixel run is defined in block  10 . A pixel run is a group of pixels which are to be examined to determine the nature of the image they comprise. According to the present invention, a pixel run may be defined in numerous ways. For example, a pixel run may comprise one or more lines of a display or an entire frame of the display. The pixel run may also comprise a portion of a display such as a window in the display or another region of the display. According to the present invention, more than one run of pixels may be defined. For example, a display comprising a document page of text overlayed by a window containing a photograph (or vise versa) may be contemplated. In the first instance, two runs of pixels may be defined, one comprising the document page of text another comprising the window superimposed on the field. In the second instance, two runs of pixels may also be defined, one comprising the photograph another comprising the window of text superimposed on the photograph. 
     At block  12  a selected parameter of the pixels in the defined run of pixels is examined as will be disclosed herein. By examining the selected parameter of the pixels in the defined run of pixels, a determination may be made as to whether the image comprising the pixels is a computer graphics image or a continuous tone image. This determination is made at block  14 . As will be appreciated by those of ordinary skill in the art, the pixels may be examined while in the pixel pipeline at an early enough stage to enable a selected gray scale algorithm to be applied to them prior to their arrival at the display. 
     If the result of the inquiry in block  14  is that the image is a computer graphics image, a gray scale algorithm most well suited for computer graphics images is applied at block  16  prior to displaying the pixels. If, however, the result of the inquiry in block  14  is that the image is a continuous tone image, a gray scale algorithm for continuous tone images is applied at block  18  prior to displaying the pixels. 
     As a result of performing the method illustrated in FIG. 1, switching between more than one gray scale algorithm may be performed “on the fly” during a single display frame so that the individual component image portions of a single frame may be displayed using a gray scale algorithm best suited to the image content. 
     According to the present invention, the pixels may be examined using one of several criteria. Exemplary pixel examination methods and apparatus are illustrated in FIG.  2  through FIG.  8 . 
     FIGS. 2 and 3 together illustrate a first method and apparatus for examining the run of pixels in block  12  of FIG.  1  and determining the nature of the image in block  14  of FIG. 1 are shown. FIG. 2 is a flow diagram of the blocks comprising a first method according to the present invention for examining a run of pixels wherein the number of colors in a selected run of pixels in a display frame are counted to determine whether the image is a computer graphics image or a continuous tone image, and the gray scaling method is adjusted to match the displayed image. FIG. 3 is a block diagram of a circuit according to a presently preferred embodiment of the present invention for counting the number of colors in a selected run of pixels in a display frame to determine whether the image is a computer graphics image or a continuous tone image according to the method illustrated in the flow diagram of FIG.  2 . From a description of FIGS. 2 and 3 herein, persons of ordinary skill in the art will appreciate that software or a state machine may be employed to direct the functions of the hardware elements to carry out the process described with respect thereto. 
     According to the method depicted in FIG. 2, the process begins at block  20  by using the value of the current color as an address for a random access memory. Next, at block  22 , the contents of the memory location pointed to by the current color is incremented. 
     A determination is then made, in block  24 , as to whether the pixel just processed was the last pixel in the run of pixels to be processed. If not, process control returns to block  20  to process another pixel. If it is determined that the last pixel processed was the last pixel in the run of pixels, the process proceeds to block  26 , in which the number of colors which appear greater than a threshold number of times is determined. 
     Next, in block  28 , the number of colors which appeared a number of times greater than the first threshold is compared with a second threshold value. As presently preferred, this second threshold may be in a range of from about 30 to about 50 colors, and preferably about 40. 
     If the number of colors is less than a second threshold value, it is determined, at block  30 , that the pixel run comprises a computer graphics image, and processing proceeds to block  16  of FIG. 1 to apply a gray scale algorithm suitable for computer graphics images. 
     If the number of colors is greater than a second threshold value, it is determined, at block  32 , that the pixel run comprises a continuous tone image and processing proceeds to block  18  of FIG. 1 to apply a gray scale algorithm suitable for continuous tone images. 
     Referring now to FIG. 3, one embodiment of an apparatus employing the blocks of the method of FIG. 2 is described. In FIG. 3, the value of the color is used as an address for Random Access Memory (RAM)  34 . The contents of the memory location pointed to by the current color is incremented by incrementor circuit  36 . 
     The number of colors which appear greater than a threshold number of times by causing multiplexer  38  to pass the output of counter  40  to the address inputs of RAM  34  rather than the current color value which was previously passed to the address inputs of RAM  34 . 
     The value of counter  40  ranges from 0 to the total number of color values possible in the system. Therefore, for an 8-bit color scheme, the output of counter  40  will range from 0 to 255, since, in an eight bit color scheme, 256 different colors are possible. The output of counter  40  is used to address RAM  34  to access each of the possible color value locations, each color location containing the number of times the color value represented by that memory location appeared in the range of pixels. 
     For each value of counter  40 , the output of RAM  34  is presented to comparator  42 , which compares the contents of each address in RAM  34  with a first threshold value, which may be contained, for example, in a register  44 . In each instance where the contents of an address in RAM  34  is equal to or greater than the first threshold value, counter  46  is incremented. As presently preferred, this first threshold may be in a range of from about 2% to about 20% of the total number of pixels in the run of pixels, and preferably about 10%. Thus, if you are examining 200 pixels, the first threshold would preferably be a value between about 4 and about 40. 
     Next, comparator  48  compares the output of counter  46  with a second threshold value, which may be contained, for example, in a register  50 . As presently preferred, this second threshold may be in a range of from about 30 to about 50 colors, and preferably about 40. 
     If the output of comparator  48  is low, indicating that the contents of counter  46  is less than the threshold value contained in register  50 , a gray scale algorithm suitable for computer graphics images is applied to the image data. If the output of comparator  48  is high, indicating that the contents of counter  46  is equal to or more than the threshold value contained in register  50 , a gray scale algorithm suitable for continuous tone images is applied to the image data. 
     One method of determining the type of image being analyzed is to count the number of unique colors, and make a decision based only upon that count. Those of ordinary skill in the art will readily recognize that many different ways exist with which to count the number of unique colors. One method utilizing the embodiment of FIG. 3 is to set the first threshold to 1, thus allowing counter  46  to count the number of unique colors present in the image. Threshold  2  would then represent the decision point. Thus, a number of unique colors lower than threshold  2  would represent a computer graphics image, and a higher number of unique colors would represent a continuous tone image. 
     FIG. 4 is a flow diagram of illustrative blocks which may be inserted in place of block  32  of FIG. 2 to determine whether an image is a continuous tone image, or a computer generated image. 
     Referring to FIG. 4, instead of block  32  deciding that the image being analyzed comprises a continuous tone image, block  32  is modified to comprise additional blocks which provide for choosing between continuous tone images and computer generated images. 
     Recall that block  32  is performed when the number of colors appearing more than a threshold number of times in a run of pixels exceeds a predetermined value. Therefore, at this point, it has been determined that the image being analyzed is not a graphics image. Using the method of FIG. 4, it is possible to further determine whether the image is either of a continuous tone image, or a computer generated image. 
     At block  52 , a register (such as register  46  seen in FIG. 3) may be loaded to contain a third threshold value which represents the value at which continuous tone images may be separated from computer generated images. The third threshold will be most likely be in a range of 80 to 120 colors, preferably about 100. 
     At block  54 , the number of colors appearing greater than a threshold number of times is compared to the third threshold value to determine if the number of colors is less than the third threshold amount. If yes, the number of colors is between 30 and 100, and the method proceeds with block  56  where the proper gray scale algorithm for a continuous tone image is applied to the image data. If no, the method proceeds with block  58  where the proper gray scale algorithm for a computer generated image is applied to the image data. 
     FIGS. 5 and 6 depict a second illustrative method and apparatus for examining the run of pixels in block  12  of FIG.  1  and determining the nature of the image in block  14  of FIG. 1 are shown. FIG. 5 is a flow diagram of the blocks comprising a second method according to the present invention for examining the run of pixels wherein the number of times that a selected color appears in a selected run of pixels in a display frame is counted, and the gray scaling method is adjusted to best match the displayed image. FIG. 6 is a block diagram of a circuit according to a second embodiment of the present invention for counting the number of times that a selected color appears in a selected run of pixels in a display frame according to the method illustrated in the flow diagram of FIG.  5 . As with the other embodiments described herein, persons of ordinary skill in the art will appreciate that software or a state machine may be employed to carry out the process described with respect thereto. From a description of FIGS. 5 and 6 herein, persons of ordinary skill in the art will appreciate that software or a state machine may be employed to direct the functions of the hardware elements to carry out the process described with respect thereto. 
     The method of FIG. 5 begins at block  60 , where a counter which will ultimately count the number of occurrances of colors is reset to a zero value. Next, at block  62 , the current color value is compared with the previous color value. If the current color value and the previous color value are equal, at block  64  the counter is enabled and incremented. If the current color value is not equal to the previous color value, at block  66  the counter is disabled and not incremented. 
     Next, at block  68 , a determination is made whether the last pixel evaluated was the last pixel in the run of pixels being examined. If not, process control is returned to block  62  to evaluate the next incoming pixel. If, at block  68 , it is determined that the last pixel evaluated was the last pixel in the run of pixels being examined, the process proceeds to block  70 , where the counter value is optionally latched. 
     At block  72 , it is determined whether the final counter value is equal to or greater than a preselected threshold. If no, it is determined, in block  74 , that the pixel run comprises a computer graphics image, and processing proceeds to block  16  of FIG. 1 to apply a gray scale algorithm suitable for computer graphics images. If yes, it is determined, at block  76 , that the pixel run comprises a continuous tone image and processing proceeds to block  18  of FIG. 1 to apply a gray scale algorithm suitable for continuous tone images. 
     Using the apparatus of FIG. 6, the one embodiment employing the method of FIG. 5 is represented, the apparatus operation beginning when counter  78  is reset to a zero value. Next, the current color is compared in comparator  80  with the contents of register  82 . As may be seen from an examination of FIG. 6, register  82  is loaded with the value of the current color via a load clock from the output of inverter  70  whenever comparator  80  determines that the current color is not the same as the previous color. 
     The output of comparator  80  is also connected to the enable input of counter  78  and will therefore either enable or disable counter  78 , depending on the results of the comparison between the current color and the stored color in register  82 . Counter  78  is clocked by the system dot clock. Those of ordinary skill in the art are readily aware that a dot clock is a clock which controls the timing of the display of information on a video screen, thus synchronizing the retrieval of information from video memory, and the actual display of that information. Thus the frequency of a dot clock will be higher for higher display resolutions, and lower for lower display resolutions. 
     If comparator  80  has determined that the current color is the same as the previous color, counter  78  is enabled and is incremented. If comparator  80  has determined that the current color is not the same as the previous color, counter  78  is disabled and is not incremented and register  82  is loaded with the value of the current color. 
     If the pixel just examined is the last pixel in the region being examined, the value of counter  78  is latched into register  86 . 
     Next, comparator  88  determines whether the latched count stored in register  86  is equal to or greater than a preselected threshold value stored in register  90 . 
     If the output of comparator  88  is low, indicating that the contents of register  88  are less than the threshold value contained in register  90 , a gray scale algorithm suitable for computer graphics images is applied to the video data. If the output of comparator  88  is high, indicating that the contents of register  86  is equal to or more than the threshold value contained in register  90 , a gray scale algorithm suitable for continuous tone images is applied to the video data. 
     The threshold set in register  90  is preferably selected to be between about 20% and about 70% of the total number of pixels in the run of pixels being evaluated, but preferably about 50%. Persons of ordinary skill in the art will appreciate that, over a selected pixel run, the contents of counter  78  will be much higher for computer graphics images, which contain long runs of a solid background color. Continuous tone images ostensibly having large same-color background areas should inevitably contain a certain amount of noise which will cause the background color to vary. Noise-induced variations, even as small as one color shade, will affect the final count of counter  78 . 
     Referring now to FIGS. 7 and 8 taken together, a third illustrative method and apparatus for examining the run of pixels in block  12  of FIG.  1  and determining the nature of the image in block  14  of FIG. 1 are shown. FIG. 7 is a flow diagram of illustrative blocks comprising a third method according to the present invention for examining the run of pixels wherein the frequency content of a selected run of pixels in a display frame is determined, and the gray scaling method is adjusted to best match the displayed image. FIG. 8 is a block diagram of a circuit according to a third embodiment of the present invention for evaluating the frequency content of a selected run of pixels in a display frame according to the method illustrated in the flow diagram of FIG.  7 . From a description of FIGS. 7 and 8 herein, persons of ordinary skill in the art will appreciate that software or a state machine may be employed to direct the functions of the hardware elements to carry out the process described with respect thereto. 
     The method begins at block  92 , where the absolute value of the difference between the color of the current pixel and the color of the previous pixel is determined. The method for computing an absolute value is well known to those of ordinary skill in the art. 
     Next, at block  94 , the absolute value calculated in block  92  is compared with the previous maximum absolute value determined during the current pixel run. If this is the first absolute value computed for the current pixel run, the current value is necessarily greater than the previous maximum. 
     If the current absolute value is greater than the previous maximum absolute value, the new maximum value is stored, as in block  96 . After storing the new maximum value, or if the current absolute value is less than or equal to the previous maximum value the process proceeds with block  98 , where it is determined if the most recently evaluated pixel was the last pixel in the run of pixels to be evaluated. 
     If the current pixel was not the last pixel in the run of pixels, the process returns to block  92  to evaluate the next incoming pixel in the run of pixels being examined. If the current pixel was the last pixel in the run of pixels, the process proceeds to block  100 , where the maximum absolute value determined during the pixel run is evaluated. 
     There are several methods which may be employed to evaluate the maximum absolute value to determine whether the pixel run contain computer graphics image or a continuous tone image. The first method comprises a simple evaluation of the magnitude of the maximum absolute value of the difference between adjacent pixels in the run of pixels. 
     According to one method of the present invention for performing the process of block  100 , the instantaneous slope values are examined. Computer graphics images are expected to contain large numbers of zero values and small numbers of high values which occur at pixel color transitions at the edges of characters. On the other hand, continuous tone images are expected to contain large numbers of small to medium values and very few large values. 
     Referring now to FIG. 8, one embodiment apparatus for performing the method of FIG. 7 is described. 
     Subtractor circuit  102  determines the absolute value of the difference between the color of the current pixel and the color of the previous pixel. One input to subtractor circuit  102  is the current color value, and the other input is the previous color value. The previous color value is held in register  104 , clocked by the system dot clock. Absolute value circuit  106  strips the subtraction result of any minus sign as is well known in the art. 
     Next, comparator  108  compares the output of absolute value circuit  106  with the previous maximum absolute value determined during the current pixel run stored in register  110 . 
     The output of comparator  108  is used as a clock input to register  110 . This arrangement assures that register  110  will only be clocked if the output of absolute value circuit  106  is greater that the current contents of register  110 . Thus, if the output of absolute value circuit  106  is greater than the current value contained within register  104 , register  104  is clocked. If the output of absolute value circuit  106  is less than or equal to the current value contained within register  110 . 
     If the current pixel was not the last pixel in the run of pixels, the process repeats with new pixel data. If the current pixel was the last pixel in the run of pixels, the maximum absolute value determined during the current pixel run stored in register  110  is evaluated. 
     To provide additional flexibility in evaluating the run of pixels, a random access memory  112  may be provided to store each absolute value of the difference (instantaneous slope) between adjacent pixels in the run of pixels. Such a RAM  112  may be addressed by a counter  114  which is incremented by the system dot clock. The data is provided from the output of absolute value circuit  106  and may be written into RAM  112  when the incremented address from counter  114  becomes valid. Counter  114  is reset at the beginning of each pixel run. 
     There are several methods which may be employed to evaluate the maximum absolute value to determine whether the pixel run contain computer graphics image or a continuous tone image. The first method comprises a simple evaluation of the magnitude of the maximum absolute value of the difference between adjacent pixels in the run of pixels. 
     FIGS. 9 a  and  9   b , respectively, are graphs illustrating typical distributions of instantaneous slope values for computer graphics images and continuous tone images, respectively. In both FIGS. 9 a  and  9   b , the X-axis represents the instantaneous slope value and the Y-axis represents the number of occurrences of a particular instantaneous slope value. 
     Referring now to FIG. 9 a , the presence of a large number of zero values is observed at reference numeral  116 , indicating no change between adjacent pixels indicative of the presence of large areas of continuous background color, accompanied by a smaller but significant number of a single larger value at reference numeral  118 , indicating high-contrast changes between adjacent pixels indicative of transitions between background color and a single text color. Also shown in FIG. 9 a  at reference numeral  120  is the occurrence of a significant number of occurrences of a second different but also larger value, indicating the presence of two-color text in the pixel run. The difference in positions between the spike at reference numeral  118  and the spike at reference numeral  120  represents the difference in color between the background color and the first text color and the difference in color between the background color and the second text color. 
     Referring now to FIG. 9 b , a more even distribution of instantaneous slope values is shown, indicating that the run of pixels comprises a portion of a continuous tone image. 
     A comparison of the distributions of the instantaneous slopes as seen in FIGS. 9 a  and  9   b  demonstrates the ease with which the two different kinds of images represented therein may be determined. 
     One method according to the present invention employs a random access memory arrangement like the one shown in FIG,  3 , except that the instantaneous slope values are used as addresses to the RAM. The value in each location in the RAM is incremented by an incrementor circuit each time it is addressed. When all of the instantaneous slope values in the RAM  112  of FIG. 8 have been used to address this additional RAM, its contents are evaluated. According to one aspect of the present invention, if the contents of a number of addresses in the RAM are greater than a threshold value contain zero, a decision is made that the pixel run comprised a computer graphics image. According to another aspect of the present invention, the presence of a large number of instantaneous slope values in a small number of single addresses higher than an address representing a threshold contrast value in combination with zero contents at a number of address locations greater than a threshold value, a decision is made that the pixel run comprised a computer graphics image. Other algorithms which operate on this data will be apparent to those of ordinary skill in the art from this disclosure. 
     Once the determination has been made according to the present invention whether the selected pixel run comprises computer graphics or continuous tone image, one of a plurality of gray scale algorithms may be applied to the pixel data while it is still in the pixel pipeline on the way to the display. Thus, switching between two or more gray scale algorithms may be performed on the fly in the pixel pipeline according to the present invention. 
     Referring now to FIG. 10, a block diagram is presented of an illustrative overall architecture of a system  122  according to the present invention for on-the-fly switching between two or more gray scale algorithms in the pixel pipeline. Pixel data conventionally enters a pixel pipeline  124  from a video memory interface (not shown) as is known in the art. Palette  126  converts the pixel data into RGB color components. Multiplexer  128  further processes the pixel data depending on the color space format in which the data is presented. 
     The individual R, G, and B pixel data from multiplexer  128  is presented to gray scale algorithm implementing circuits  130   a ,  130   b , and  130   c , respectively in accordance with the present invention. Unlike prior art schemes for implementing gray scale algorithms which are fixed at the time of manufacture of the video controller or accelerator containing them, circuits  130   a ,  130   b , and  130   c  are capable of implementing either frame rate control (FRC) gray scale algorithms or spatial dithering, both techniques being well known to persons of ordinary skill in the art. 
     According to the present invention, pixel evaluator  132  taps the pixel stream at an appropriate point, such as just prior to palette  126 . 
     While embodiments and applications of this invention have been shown and described, it would be apparent to those skilled in the art that many more modifications than mentioned above are possible without departing from the inventive concepts herein. The invention, therefore, is not to be restricted except in the spirit of the appended claims.