Abstract:
A dither matrix is applied to a high-resolution image to compare the value of each of the pixels that compose it with a threshold value of the matrix and to obtain an output value of the matrix (Dither matrix value) from each comparison. To each pixel value of the image there is applied an algorithm involving simple but displacement operation, namely shifts to the left and shifts to the right. The pixel values of a low-resolution image are output from the applied algorithm.

Description:
PRIORITY CLAIM  
       [0001]     This is a continuation-in-part application which claims priority from PCT/IT2005/000080, published in English, filed Feb. 16, 2005, based on European patent Application No. 04425101.5, filed Feb. 18, 2004, which are incorporated herein by reference. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention relates in general to techniques for processing digital images and, more particularly, to a method of obtaining a low-resolution image from a high-resolution image by means of the technique of “ordered dithering”.  
       BACKGROUND  
       [0003]     The quality of an image presented on a digital image display device is determined by the chromatic resolution and the spatial resolution of the display device. The chromatic resolution is defined by the number of bits needed to memorize an element of the image (pixel) and the spatial resolution is defined by the number of pixels of the display device. A typical display system, known as “true color”, makes it possible to associate with each point of the display device a digital value that directly defines the color to be displayed. For example, the color of a pixel may be represented by a digital value of 24 bits, that is to say, the intensity of each primary color (R, G, B) is represented by 8 bits.  
         [0004]     Certain applications require one to convert a digital image of high chromatic resolution into a digital image of a lower resolution, for example, when one wants to utilize a small-size display device and/or wants to utilize a video memory of reduced dimensions for storing the data that define the image. Various known techniques make it possible to obtain quality images even in these cases, notwithstanding the low chromatic resolution actually available, by exploiting the capacity of the human eye to merge the tonal or chromatic values of adjacent pixels to perceive an intermediate tonal value or color. One of these techniques, known as “ordered dithering”, makes it possible to represent, for each primary color level of the image, the entire range of intensity values by means of image elements having only one of two possible intensity levels. The choice of one or the other intensity level is made by examining the image with a bidimensional matrix of predetermined threshold values [dither matrix]: the value of each image element is compared with the corresponding threshold value of the dither matrix to assume one of the two predetermined values on the basis of the outcome of the comparison. The dither matrix is typically a square matrix of size 4×4 or 8×8 that is repeatedly applied to the image to be processed and in such a way as to cover it completely. The same operation is repeated for all the color levels that make up the image.  
         [0005]     Various methods are known for putting the techniques of ordered dithering into practice, but all call for the use of complex hardware and/or software systems.  
         [0006]     There is thus a strongly felt need for methods that are less demanding as far as hardware and/or software resources are concerned.  
       SUMMARY  
       [0007]     According to one aspect of the present invention, a method processes a starting image having a first chromatic resolution and formed by N×M pixels to generate an output image having a second chromatic resolution that is lower than said first chromatic resolution. The method includes generating first and second dither matrices each including a plurality of threshold values. The threshold values are the same in each matrix but being arranged differently within the matrices. The method further includes repeatedly applying the first and second dither matrices to selected groups of pixels in the starting image to generate corresponding groups of pixels in the output image. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The invention will be more readily understood from the following detailed description of some of its implementations, said description, which is being given by way of example, makes reference to the drawings attached hereto, of which:  
         [0009]      FIG. 1  is a schematic representation that illustrates the configuration of a 2×2 dither matrix;  
         [0010]      FIG. 2  shows how a 4×4 dither matrix is obtained from the 2×2 dither matrix of  FIG. 1 ;  
         [0011]      FIGS. 3, 5  and  6  are block diagrams that illustrate three ways of implementing a method in accordance with embodiments of the present invention,  
         [0012]      FIG. 4  schematically illustrates how the elements of two dither matrices are selected in accordance with the position of the pixels according to an embodiment of the present invention;  
         [0013]      FIG. 7  is a block diagram that illustrates a method in accordance with an embodiment of the present invention; and  
         [0014]      FIG. 8  shows a table of values assigned to the parameters indicated in the block diagram of  FIG. 7  according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0015]     The following discussion is presented to enable a person skilled in the art to make and use the invention. Various modifications to the embodiments will be readily apparent to those skilled in the art, and the generic principles herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.  
         [0016]     As briefly mentioned above, the ordered dithering technique requires each pixel value p of a monochromatic image to be compared with a threshold value t. When p&lt;t, the method puts p=a, otherwise p=b, where a and b are the two available intensity values. The threshold value may be equal for every pixel positions (for example, it could be the arithmetic mean, or an average value of the two values a and b), or could have different values according to the pixel position. In this latter case, i.e. when use is made of a dithering technique depending on the position, the results are better.  
         [0017]     A uniform area of the image is examined by means of a configuration made up of the two values a and b, which is subsequently shifted until the whole area is covered. For example, referring to  FIG. 1 , utilizing a configuration formed by 2×2 elements, which may be only white or black, it is possible to simulate five different tones of gray (GR_TN): the field of the possible gray levels, defined between 0 and 1, is divided into five intervals (INT) as shown in  FIG. 1 . For example, all the gray levels comprised in the interval ⅛-⅜ are represented by the tone ¼ defined by the second configuration from the left. This technique can be put into practice by utilizing a matrix of threshold values that contains a regular configuration of the upper limits of the intervals and defines a threshold for every pixel position. In general, the intervals are equal, so that it is sufficient to define the sequence of the pixel values and arrange them into a dither matrix. As far as the example of  FIG. 1  is concerned, the dither matrix is the 2×2 matrix shown in  FIG. 2 . As a general rule, given an n×n matrix, the values k of the matrix belong to the set of the whole numbers comprised between 0 and n 2 −1 and the threshold values associated with every value k are obtained from the relationship (2k+1)/2n 2 . To generate larger dither matrices a recursive method can be used, as shown in  FIG. 2 , starting from the matrix according to the example of  FIG. 1 .  
         [0018]     Let us consider the implementation form of a method in accordance with an embodiment of the present invention illustrated by  FIG. 3 . Let us suppose that the image is constituted by three color levels or channels, indicated by RGB (red, green, blue), and by a luminosity (or brightness) level or channel, indicated by A. For each level there is executed the algorithm represented inside the box relating to level R. This algorithm implements the ordered dithering of the image by converting 8-bit intensity input values, indicated by in_value_r, into 4-bit intensity values, indicated by Out_r.  
         [0019]     The same dither matrix of n×n elements is utilized for processing the channels RBA. A different dither matrix of n×n elements is used for processing the channel G (green) in order to take account of the different sensitivity of the human eye to the color green. Preferably, n is chosen between 2 and 8.  
         [0020]     By way of example, four dither matrices that can be used according to embodiments of the present invention are presented below in tabular form. In the tables: 
        range=2 bit     —     in     —     bit     —     out   −1 (where bit _in and bit_out represent, respectively, the number of input bits and the number of output bits,     elements=dim_Mtx 2  (where dim_Mtx represents the dimension of the dither matrix),     round represents an operator who rounds off the expression in parentheses to the nearest whole number,     and to every pair of position values (Xvalue, Yvalue) there corresponds an element of the matrix RBA and an element of the matrix G.        
 
         [0025]     2×2 dither matrix  
                                                           Xvalue   Yvalue   RBA   G                           0   0   0   round(range * 2/                       elements + 0.5)           1   0   round(range * 2/   round(range * 3/                   elements + 0.5)   elements + 0.5)           0   1   round(range * 3/   round(range * 1/                   elements + 0.5)   elements + 0.5)           1   1   round(range * 1/   0                   elements + 0.5)                      
 
         [0026]     3×3 dither matrix  
                                                           Xvalue   Yvalue   RBA   G                           0   0   round(range * 7/   round(range * 2/                   elements + 0.5)   elements + 0.5)           1   0   round(range * 2/   round(range * 3/                   elements + 0.5)   elements + 0.5)           2   0   round(range * 3/   round(range * 0/                   elements + 0.5)   elements + 0.5)           0   1   round(range * 0/   round(range * 4/                   elements + 0.5)   elements + 0.5)           1   1   round(range * 4/   round(range * 8/                   elements + 0.5)   elements + 0.5)           2   1   round(range * 8/   round(range * 5/                   elements + 0.5)   elements + 0.5)           0   2   round(range * 5/   round(range * 6/                   elements + 0.5)   elements + 0.5)           1   2   round(range * 6/   round(range * 1/                   elements + 0.5)   elements + 0.5)           2   2   round(range * 1/   round(range * 7/                   elements + 0.5)   elements + 0.5)                      
 
         [0027]     4×4 dither matrix  
                                                           Xvalue   Yvalue   RBA   G                           0   0   0   round(range * 10/                       elements + 0.5)           1   0   round(range * 8/   round(range * 12/                   elements + 0.5)   elements + 0.5)           2   0   round(range * 2/   round(range * 4/                   elements + 0.5)   elements + 0.5)           3   0   round(range * 10/   round(range * 14/                   elements + 0.5)   elements + 0.5)           0   1   round(range * 12/   round(range * 6/                   elements + 0.5)   elements + 0.5)           1   1   round(range * 4/   round(range * 3/                   elements + 0.5)   elements + 0.5)           2   1   round(range * 14/   round(range * 11/                   elements + 0.5)   elements + 0.5)           3   1   round(range * 6/   round(range * 1/                   elements + 0.5)   elements + 0.5)           0   2   round(range * 3/   round(range * 9/                   elements + 0.5)   elements + 0.5)           1   2   round(range * 11/   round(range * 15/                   elements + 0.5)   elements + 0.5)           2   2   round(range * 1/   round(range * 7/                   elements + 0.5)   elements + 0.5)           3   2   round(range * 9/   round(range * 13/                   elements + 0.5)   elements + 0.5)           0   3   round(range * 15/   round(range * 5/                   elements + 0.5)   elements + 0.5)           1   3   round(range * 7/   0                   elements + 0.5)           2   3   round(range * 13/   round(range * 8/                   elements + 0.5)   elements + 0.5)           3   3   round(range * 5/   round(range * 2/                   elements + 0.5)   elements + 0.5)                      
 
         [0028]     8×8 dither matrix  
                                                           Xvalue   Yvalue   RBA   G                           0   0   round(range * 0/   round(range * 32/                   elements + 0.5)   elements + 0.5)           1   0   round(range * 32/   round(range * 8/                   elements + 0.5)   elements + 0.5)           2   0   round(range * 8/   round(range * 40/                   elements + 0.5)   elements + 0.5)           3   0   round(range * 40/   round(range * 2/                   elements + 0.5)   elements + 0.5)           4   0   round(range * 2/   round(range * 44/                   elements + 0.5)   elements + 0.5)           5   0   round(range * 44/   round(range * 10/                   elements + 0.5)   elements + 0.5)           6   0   round(range * 10/   round(range * 42/                   elements + 0.5)   elements + 0.5)           7   0   round(range * 42/   round(range * 48/                   elements + 0.5)   elements + 0.5)           0   1   round(range * 48/   round(range * 16/                   elements + 0.5)   elements + 0.5)           1   1   round(range * 16/   round(range * 56/                   elements + 0.5)   elements + 0.5)           2   1   round(range * 56/   round(range * 24/                   elements + 0.5)   elements + 0.5)           3   1   round(range * 24/   round(range * 50/                   elements + 0.5)   elements + 0.5)           4   1   round(range * 50/   round(range * 18/                   elements + 0.5)   elements + 0.5)           5   1   round(range * 18/   round(range * 58/                   elements + 0.5)   elements + 0.5)           6   1   round(range * 58/   round(range * 26/                   elements + 0.5)   elements + 0.5)           7   1   round(range * 26/   round(range * 12/                   elements + 0.5)   elements + 0.5)           0   2   round(range * 12/   round(range * 44/                   elements + 0.5)   elements + 0.5)           1   2   round(range * 44/   round(range * 4/                   elements + 0.5)   elements + 0.5)           2   2   round(range * 4/   round(range * 46/                   elements + 0.5)   elements + 0.5)           3   2   round(range * 46/   round(range * 14/                   elements + 0.5)   elements + 0.5)           4   2   round(range * 14/   round(range * 46/                   elements + 0.5)   elements + 0.5)           5   2   round(range * 46/   round(range * 6/                   elements + 0.5)   elements + 0.5)           6   2   round(range * 6/   round(range * 38/                   elements + 0.5)   elements + 0.5)           7   2   round(range * 38/   round(range * 60/                   elements + 0.5)   elements + 0.5)           0   3   round(range * 60/   round(range * 28/                   elements + 0.5)   elements + 0.5)           1   3   round(range * 28/   round(range * 52/                   elements + 0.5)   elements + 0.5)           2   3   round(range * 52/   round(range * 20/                   elements + 0.5)   elements + 0.5)           3   3   round(range * 20/   round(range * 62/                   elements + 0.5)   elements + 0.5)           4   3   round(range * 62/   round(range * 30/                   elements + 0.5)   elements + 0.5)           5   3   round(range * 30/   round(range * 54/                   elements + 0.5)   elements + 0.5)           6   3   round(range * 54/   round(range * 22/                   elements + 0.5)   elements + 0.5)           7   3   round(range * 22/   round(range * 3/                   elements + 0.5)   elements + 0.5)           0   4   round(range * 3/   round(range * 35/                   elements + 0.5)   elements + 0.5)           1   4   round(range * 35/   round(range * 11/                   elements + 0.5)   elements + 0.5)           2   4   round(range * 11/   round(range * 43/                   elements + 0.5)   elements + 0.5)           3   4   round(range * 43/   round(range * 1/                   elements + 0.5)   elements + 0.5)           4   4   round(range * 1/   round(range * 33/                   elements + 0.5)   elements + 0.5)           5   4   round(range * 33/   round(range * 9/                   elements + 0.5)   elements + 0.5)           6   4   round(range * 9/   round(range * 41/                   elements + 0.5)   elements + 0.5)           7   4   round(range * 41/   round(range * 51/                   elements + 0.5)   elements + 0.5)           0   5   round(range * 51/   round(range * 19/                   elements + 0.5)   elements + 0.5)           1   5   round(range * 19/   round(range * 59/                   elements + 0.5)   elements + 0.5)           2   5   round(range * 59/   round(range * 27/                   elements + 0.5)   elements + 0.5)           3   5   round(range * 27/   round(range * 49/                   elements + 0.5)   elements + 0.5)           4   5   round(range * 49/   round(range * 17/                   elements + 0.5)   elements + 0.5)           5   5   round(range * 17/   round(range * 57/                   elements + 0.5)   elements + 0.5)           6   5   round(range * 57/   round(range * 25/                   elements + 0.5)   elements + 0.5)           7   5   round(range * 25/   round(range * 15/                   elements + 0.5)   elements + 0.5)           0   6   round(range * 15/   round(range * 47/                   elements + 0.5)   elements + 0.5)           1   6   round(range * 47/   round(range * 7/                   elements + 0.5)   elements + 0.5)           2   6   round(range * 7/   round(range * 39/                   elements + 0.5)   elements + 0.5)           3   6   round(range * 39/   round(range * 13/                   elements + 0.5)   elements + 0.5)           4   6   round(range * 13/   round(range * 45/                   elements + 0.5)   elements + 0.5)           5   6   round(range * 45/   round(range * 5/                   elements + 0.5)   elements + 0.5)           6   6   round(range * 5/   round(range * 37/                   elements + 0.5)   elements + 0.5)           7   6   round(range * 37/   round(range * 63/                   elements + 0.5)   elements + 0.5)           0   7   round(range * 63/   round(range * 31/                   elements + 0.5)   elements + 0.5)           1   7   round(range * 31/   round(range * 55/                   elements + 0.5)   elements + 0.5)           2   7   round(range * 55/   round(range * 23/                   elements + 0.5)   elements + 0.5)           3   7   round(range * 23/   round(range * 61/                   elements + 0.5)   elements + 0.5)           4   7   round(range * 61/   round(range * 29/                   elements + 0.5)   elements + 0.5)           5   7   round(range * 29/   round(range * 53/                   elements + 0.5)   elements + 0.5)           6   7   round(range * 53/   round(range * 21/                   elements + 0.5)   elements + 0.5)           7   7   round(range * 21/   round(range * 0/                   elements + 0.5)   elements + 0.5)                      
 
         [0029]     From the observation of the above-reported tables one can deduce that the n×n dither matrix for processing of the G channel has the same elements, i.e. the same threshold values, of the respective n×n dither matrix used for processing the RBA channels, however the elements of the dither matrix used for processing the G channel have, within said matrix, a different spatial distribution with respect the spatial distribution of said elements in the dither matrix used for processing the RBA channels.  
         [0030]     It is also to be noticed that, in the above reported tables, all the elements of the dither matrices are expressed in parametric form, each of said elements (except for the element having the value equal to zero) being obtainable as a function of the following parameters: dimension of the dither matrix, number of the bits used for the input digital value (i.e. the number of bits which corresponds to the chromatic resolution of the digital image to be processed by means of the dithering) and number of the bits used for the output digital value (i.e. the number of bits which corresponds to the chromatic resolution of the digital image provided and output from the dithering).  
         [0031]     The selection of the threshold values, i.e. the values of the dither matrix with which the pixel intensity values of the various channels that make up the image are to be compared, is made in the manner explained hereinbelow with reference to  FIG. 4 .  
         [0032]     The coordinates that define the position of a pixel of the image are indicated by X and Y. A threshold value of the matrix is selected by utilizing a binary number made up of the m least significant bits of the binary number that represents the position, where m depends on the dimensions of the employed dither matrix. For example, when 
        X=110100     Y=110001     m=3 (8×8 dither matrix),     the last three bits of each coordinate,     x=100     y=001,     represent the coordinates of the threshold value of matrices DithermatrixRBA and DithermatrixG. The threshold values in this example are represented by a binary number (Dither matrix value for RBA, Dither matrix value for G) made up of seven bits at the very most.        
 
         [0040]     When a 2×2 dither matrix is used, one bit is sufficient for the X-coordinates and one bit for the Y-coordinates to address the threshold value. For 3×3 and 4×4 dither matrices, on the other hand, two bits will be needed for the X-coordinates and two bits for the Y-coordinates.  
         [0041]     Coming back to the block diagram of  FIG. 3 , the symbols there used have the following meanings: 
        &gt;&gt;i: displacement of i bits towards the right, i.e. removal of the least significant i bits     &lt;&lt;i: displacement of i bits towards the left, i.e. addition of i least significant bits equal to 0     &gt;=: greater or equal to     !=: different from     DIFF: difference     COMP: comparison     MUX: multiplexing     4, 8, 12 on the connecting lines indicate the number of bits of the binary numbers transmitted on the lines     a and b: binary numbers constituting the inputs of the various functional blocks (in general different for each block).        
 
         [0051]     For the purposes of better understanding the algorithm shown in  FIG. 3  it may be useful to consider the various operations on the basis of the following example, in which in_value_r=10101110 and “Dither matrix value for RBA”=0111. 
        1. Displacement by four bits towards the left of the input value In_value_r=10101110     b=101011100000     2. Calculation b−a in DIFF1:     a=10101110     b=101011100000     b−a=101000110010     3. Displacement of b−a (result of operation 2) by eight bits towards the right: 1010.     4. Displacement by four bits towards the left of the result of equation 3:     b=10100000     and calculation of a−b (10101110-10100000) in DIFF2: new a at output: 1110.     5. Calculation a−b (1110-1010) in DIFF3:     new a at output from DIFF3: 0100.     6. Comparison between a at output from DIFF3 (0100) and “Dithermatrix value for RBA” (=0111) if greater or equal:     output from COMP=1     otherwise:     output from COMP=0     in this example: output from COMP=0     7. When the result of operation 3 (1010) is different from 1111 (equivalent to 15 in decimal notation):     result equal to 1     otherwise: result equal to 0.        
 
         [0072]     This result controls the multiplexer MUX 1  with the following logic: 
        when it is equal to 1, the result of operation 6 is chosen,     when it is equal to 0, the value 0 is chosen.        
 
         [0075]     In this example the result is equal to 0.  
         [0076]     The output of MUX 1  controls the multiplexer MUX 2 . 
        8. Logic of MUX 2 :     incoming values:     selection 0: value resulting from operation 3 (1010)     selection 1: value resulting from operation 3 increas-ed by one unit (1011);     the choice between these two values is made on the basis of the result of operation 7 (in this example equal to 0), so that at the output from MUX 2  we have the value 1010     9. Logic of MUX 3      incoming values:     selection 0: in_value_r displaced by four bits towards the right (1010)     selection 1: result of operation 8 (1010)     the choice between these two values is made on the basis of the value of Enable (example: Enable=1, Out_r=1010).        
 
         [0087]     The “Dither matrix value for G” extracted from the dither matrix G is used for the G channel.  
         [0088]     Let us now consider a second form of implementation of a method in accordance with an embodiment of the present invention illustrated by  FIG. 5 . In this case, once again, the image is constituted by four channels (RGBA) and the algorithm set out within the box in relation with channel R is executed for each of the channels. This time the algorithm carries out the ordered dithering of the image by converting the eight-bit intensity values, again indicated by in_value_r into five-bit intensity values, again indicated by Out_r.  
         [0089]     One and the same dither matrix of n×n elements is used for processing the channels RBA, while another dither matrix of n×n elements is used for the channel G.  
         [0090]     The notations and symbols indicated in  FIG. 3  are re-employed with the same meanings in  FIG. 5 .  FIG. 5  also contains the notation ADD, which indicates a sum.  
         [0091]     In this case, once again, it may be helpful to consider the various operations in the form of a specific example in which in_value_r=10101110 and “Dither matrix value for RBA”=0111. 
        1. Calculation b−a     a=101011100 (obtained by displacing In_value_r by one bit towards the left)     b=101011100000000 (obtained by displacing In_value_r by seven bits towards the left)     b−a=101010110100100 at the output of DIFF1.     2. Displacement of the output of DIFF1 by 10 bits towards the right:     result: 10101     3. Displacement by two bits to the left, displacement by eight bits to the left and sum in ADD:     10101+1010100+1010100000000=1010101101001.     4. Displacement of the output of ADD by five bits towards the right and calculation of a−b in DIFF2:     a=10101110     b=10101011     a−b= 11      5. Comparison. When the output of DIFF2 is greater or equal to “Dither matrix value for RBA”: result equal to 1,     otherwise: result equal to 0.        
 
         [0106]     In this example, output DIFF2=11 and “Dither matrix value for RBA”=0111, so that the result is equal to 0. 
        6. When the result of operation 2 is different from 11111 (equivalent to 31 in decimal notation):     result equal to 1, otherwise: result equal to 0.        
 
         [0109]     In this example the result is equal to 1.  
         [0110]     This result is used to control the multiplexer MUX 1 .  
         [0111]     Logic of MUX 1 : 
        in the case in which the result of operation 6 is equal to one, the result of operation 5 is chosen.        
 
         [0113]     In the case in which the result of operation 6 is equal to zero, the value 0 is chosen.  
         [0114]     In this example the result of operation 6 is equal to 1, and therefore  0  will be chosen.  
         [0115]     The output of MUX 1  is used to control the multiplexer MUX 2 . 
        7. Logic of MUX 2 .     incoming values:     selection 0: result of operation 2 (10101)     selection 1: result of operation 2 increased by one unit (10110).        
 
         [0120]     The choice between these two values is made on the basis of the result of operation 6 (in this example equal to 0), so that at the output from MUX 2  we have the value 10101). 
        9. Logic of MUX 3      incoming values:     selection 0: in_value_r displaced by three bits towards the right (10101)     selection 1: result of operation 7 (10101).        
 
         [0125]     The choice between these two values is made on the basis of the value of Enable (example: Enable=1, Out_r=10101).  
         [0126]     The “Dither matrix value for G” extracted from the dither matrix G is used for the G channel.  
         [0127]     In a third form of implementation of a method in accordance with an embodiment of the present invention illustrated by  FIG. 6  the image is likewise constituted by four channels (RGBA). The ordered dithering algorithm set out in the box relating to the channel R is executed for each channel. In this case the image made up of eight bits per channel is converted into a six-bit image per channel. One and the same dither matrix of n×n elements is used for the three channels RBA, while another dither matrix of n×n elements is used for the channel G.  
         [0128]     The notations and symbols used in  FIG. 6  are the same as those employed in  FIG. 5 .  
         [0129]     The various operations relating to a specific example will now be described with reference to the channel R. Let us consider the case in which the input intensity value in_value_r=10101110 and the Dither-matrix value is equal to 0111. 
        1. Calculation b−a in DIFF1:     a=101011100 (obtained by displacing in_value_r by one bit to the left)     b=1010111000000000 (obtained by displacing in_value_r by eight bits to the left)     b−a=1010110010100100 at the output of DIFF1.     2. Calculation a−b in DIFF2:     a=1010110010100100     b=10101110     a−b=1010101111110110 at the output of DIFF2.     3. Displacement of the output of DIFF2 by ten bits towards the right:     result: 101010     4. Displacement by two bits towards the left and by eight bits towards the left of the result of the previous operation and sum result 1: 10101000     result 2: 10101000000000     sum: 10101010101000 at output of ADD.     5. Displacement of the output of ADD by six bits towards the right and calculation of a−b in DIFF3:     b=10101010     a=10101110     a−b=100 at the output of DIFF3.     6. Comparison     when the output of DIFF  3  is greater or equal to the “Dither matrix value for RBA” (0111):     the result is equal to 1     otherwise the result is equal to 0.        
 
         [0151]     In this example (100 smaller than 0111): result=0. 
        7. When the result of operation 3 is different from 111111 (equivalent to 63 in decimal notation):     the result is equal to 1     otherwise the result is equal to 0     in this example (101010!=11111): result equal to 1.        
 
         [0156]     The result of this operation is used to control the multiplexer MUX  1 .  
         [0157]     Logic of MUX 1 : 
        in the case in which the result of operation 7 is equal to 1, the result of operation 6 is chosen,     in the case in which the result of operation 7 is equal to 0, the value 0 is chosen.        
 
         [0160]     In this example the result of operation 7 is equal to 1, so that the choice falls on the value 0, which is the result of operation 6.  
         [0161]     The output of MUX 1  is then used to control the multiplexer MUX 2 . 
        8. Logic of MUX 2  incoming values:     selection zero: result of operation 3 (101010)     selection 1: result of operation 3 increased by one unit (101011);     the choice between these two values is made on the basis of the result of operation 7 (in this example equal to 0), so that at the output from MUX 2  we have the value 101010.     9. Logic of MUX 3      incoming values:     selection 0: in_value_r displaced by two bits towards the right (101011)     selection 1: result of operation 8 (101010).        
 
         [0170]     The choice between these two values is made on the basis of the value of Enable (example: Enable=1, Out_r=101010).  
         [0171]     The “Dither matrix value for G” extracted from the dither matrix G is used for the G channel.  
         [0172]     The three forms of implementation described above can be represented by a single layout (shown in  FIG. 7 ) in which the various functional blocks are controlled by parameters that can assume different values as indicated in the table of  FIG. 8 . The shown layout refers to the algorithm for processing the channel R, but is also valid for the processing of the other channels, though with the sole variant that in the case of channel G it is used with “Dither matrix value for G” rather than “Dither matrix value for RBA”.  
         [0173]     Varying the parameters as shown in the table of  FIG. 8 , one obtains the algorithms represented in  FIGS. 3, 5  and  6  relating to the conversions of an eight-bit input image into output images of, respectively, four, five and six bits.  
         [0174]     The notations and symbols used are the same as for the implementation forms described above, with sole addition of an operator, indicated by ADD/DIFF, that carries out the function sum or difference in accordance with the value (+ or −) of a parameter (C 3 ).  
         [0175]     As a general rule, the parameters may be chosen by means of heuristic methods by comparing the low-resolution image obtained by applying the algorithm of  FIG. 7  with an equivalent low-resolution image obtained by applying a conventional ordered dithering method taken as a Fsample. In this phase, of course, a complex hardware system and/or a complex software has to be used. Nevertheless, once the parameters have been selected, the algorithm of  FIG. 7  is more advantageous than its conventional counterpart, because it is based on simple operations (“displacements” to the right and to the left) that can be carried out at a very high speed and do not call for complex hardware systems or complex software.  
         [0176]     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention.