Abstract:
A method for processing an input digital image having an x,y array of input pixels having associated input pixel values, to produce a multichannel multitoned output image having two or more colorant channels, wherein at least two of the colorant channels are similar, having substantially the same color but different densities, including determining continuous-tone pixel values for each colorant channel of the input digital image in response to the input pixel values using an ink manifold processor; for each colorant channel, selecting a multitone process from a plurality of multitone processes in response to an input pixel value using a multitone process controller; for each colorant channel, applying the selected multitone process to the continuous-tone pixel value to produce a multitoned output pixel value; and repeating steps a)-c) for each input pixel in the input digital image to produce the multichannel multitoned output image.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    Reference is made to commonly assigned U.S. patent application Ser. No. 09/887,420 filed Jun. 22, 2001 by Spaulding et al., entitled “Method for Halftoning a Multichannel Digital Color Image Having at Least One Group of Similar Color Channels”, the disclosure of which is herein incorporated by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates generally to the field of digital image processing, and more particularly to multitoning digital color images for a color output device having at least one group of similar color channels with substantially the same color but with different densities.  
         BACKGROUND OF THE INVENTION  
         [0003]    Halftoning is a technique that is commonly used in digital imaging to create the appearance of intermediate tones when only two colorant levels (i.e., ink or no ink) are available. Halftoning methods rely on the fact that an observer&#39;s eye will spatially average over some local area of the image so that intermediate tone levels can be created by turning some of the pixels “on” and some of the pixels “off” in some small region. The fraction of the pixels, which are turned on, will determine the apparent gray level. Examples of common halftoning techniques include ordered dither and error diffusion. Multitoning is a simple extension of halftoning for systems that can print more than two colorant levels. For example, consider an inkjet printer that may print no dot, a small dot, or a large dot at each pixel, resulting in three density levels.  
           [0004]    The ordered dither technique is typically implemented by forming a dither matrix having M x xM y  elements that is tiled across the image. The elements of the dither matrix are distributed across the range of expected input tone values so that the full range of tones can be reproduced. A basic flow chart for this approach is shown in FIG. 1. A given pixel in the image has column and row addresses x and y, respectively, and an input pixel value I(x,y). Modulo operators  10  are used to determine indices x d  and y d  that are used to address the dither matrix  12 . The threshold value d(x d ,y d ) stored in the dither matrix at this address is then compared to the input pixel value I(x,y) using a comparator  14 . If the input pixel value is equal to or below the threshold value, the output pixel value O(x,y) is set to “off”; if the value is above the threshold, the output pixel value O(x,y) is set to “on”.  
           [0005]    A second implementation of this basic technique is shown in FIG. 2. In this case the threshold value d(x d ,y d ) is combined with the input pixel value I(x,y) using an adder  24 . A threshold operator  26  is then used to threshold the combined value to determine the output pixel value O(x,y).  
           [0006]    Another implementation of this basic technique is shown in FIG. 3. In this case a set of k dither bitmaps  32  are formed corresponding to the halftone patterns formed when the dither matrix is thresholded at each input level k. The input pixel value I(x,y) is then used to select which of the dither bitmaps should be used for each pixel. Modulo operators  30  are used to determine indices x d  and y d  that are used to address the selected dither bitmap to determine the output pixel value O(x,y). This implementation has the advantage that fewer computations are necessary to process each pixel. It should be noted that any dither pattern that can be implemented using the methods of FIG. 1 and FIG. 2 can also be implemented using the method of FIG. 3. However, it is possible to define a set of dither bitmaps for the method of FIG. 3 that could not be implemented using the other methods. This is because in the dither matrix implementations once a certain pixel is turned on at the input value corresponding to the threshold, it will stay on for all higher input values. With the dither bitmaps approach this constraint is not present. For example, a certain pixel could be “off” for an input value of  150 , “on” for an input value of  151 , and “off” again for an input value of  152 .  
           [0007]    Yet another implementation of this basic technique is shown in FIG. 4. In this case a set of dither look-up tables (LUTs)  36  are used. The dither LUTs  36  are addressed by the input pixel value I(x,y) and store the corresponding output pixel value to be produced at some pixel location. Modulo operators  30  are used to determine indices x d  and y d  that are used to address a dither LUT selector  34  which selects one of the dither LUTs  36 . The output pixel value O(x,y) is determined by addressing the selected dither LUT with the input pixel value I(x,y). This implementation also has the advantage that fewer computations are necessary to process each pixel. It should be noted that any dither pattern that can be implemented using the methods of FIG. 1, and FIG. 2 can also be implemented using the method of FIG. 4. However, as with the method shown in FIG. 3, it is also possible to define a set of dither LUTs  36  that could not be implemented using the other methods.  
           [0008]    A variety of different dither matrices have been used in the past which create resulting images with different visual characteristics. Common types include random dither matrices, clustered-dot dither matrices, and Bayer (dispersed dot) dither matrices. Sullivan and Miller (U.S. Pat. No. 4,920,501) have introduced a technique to design dither bitmaps, which have minimum visual modulation. These patterns are sometimes called “blue noise” patterns. They are attractive in that the patterns that are produced have minimum visibility to human observers, and as a result the images produced are visually pleasing relative to images produced using other patterns. The basic procedure involved in the creation of these patterns is to use an optimization technique, such as stochastic annealing, to determine the arrangement of the matrix elements which minimizes some cost function based upon weighting the frequency components of the halftone patterns with a human visual sensitivity function. Sullivan and Ray have also disclosed an enhancement to this method, which insures that the bitmaps for each of the gray levels are correlated in such a way that they can be implemented using a dither matrix (U.S. Pat. No. 5,214,517). Other extensions to this basic approach include a method for simultaneously optimizing all of the gray levels (U.S. Pat. No. 5,754,311), a method for designing blue noise dither matrices for use with multilevel output devices (U.S. Pat. No. 5,586,203), and a method for creating partially correlated dither matrices (U.S. Pat. No. 5,946,452).  
           [0009]    Parker and Mitsa (U.S. Pat. No. 5,111,310) and Lin (U.S. Pat. Nos. 5,317,418 and 5,469,515) have also introduced techniques for the design of dither patterns. Although the actual optimization methods that they have used are somewhat different, the resulting patterns are quite similar to those created using the methods described in the Sullivan et al. patents.  
           [0010]    When producing color images on binary output devices, it is necessary to halftone each of the color channels. Typically the color planes are cyan, magenta and yellow (CMY), or cyan, magenta, yellow and black (CMYK). If the same halftoning pattern is used for each of the color channels, this is known as “dot-on-dot” printing. This approach is rarely used in practice because it results in a higher level of luminance modulation, and additionally the color reproduction characteristics will be more sensitive to registration errors. The higher level of luminance modulation will cause the halftone patterns to be more visible than one of the color channels printed alone. The increased sensitivity to registration errors may cause the reproduced color to drift page to page, or even within a page, as the registration characteristics of printing apparatus change. In the graphic arts field the preferred solution has typically been to decorrelate the halftone patterns by rotating the halftone patterns to different “screen angles.” For conventional graphic arts halftone methods, this rotation can either be accomplished optically or digitally.  
           [0011]    A number of techniques have been suggested for decorrelating the halftone patterns used for ordered dither (for example, see U.S. Pat. No. 5,341,228). These include using independently derived dither matrices for each color channel, spatially shifting the dither matrix patterns for each color plane relative to a reference dither matrix, and using an inverse dither matrix for one or more of the color channels. Each of these techniques has advantage that the halftone patterns for the different color channels will be decorrelated, and therefore the resulting halftone image should produce lower amounts of luminance modulation and sensitivity to registration errors relative to the nominal dot-on-dot printing case. However, none of these techniques will produce images that exhibit optimal appearance, particularly with respect to the visibility of the halftone patterns.  
           [0012]    In U.S. Pat. No. 5,822,451, Spaulding et al. have disclosed a method for halftoning a multichannel digital color image wherein dither matrices for the different color channels of the color image are designed jointly to minimize a visual cost function. One embodiment of this method is shown in FIG. 5. It can be seen that a conventional dither halftoning method like that shown in FIG. 1 is applied to each channel of a multichannel color image, wherein the dither matrices used for each color channel are jointly optimized. In particular, a multichannel input color image having a cyan input image  40 A, a magenta input image  40 B, and a yellow input image  40 C is processed to form a multichannel output color image having a corresponding cyan output image  48 A, magenta output image  48 B, and yellow output image  48 C. Modulo operators  42 A,  42 B, and  42 C are used to determine the dither matrix column and row addresses x d  and y d , respectively, for an input pixel having image column and row addresses x and y, respectively. The dither matrix column and row addresses are used to address jointly optimized dither matrices  44 A,  44 B, and  44 C which are jointly optimized for the cyan, magenta, and yellow color channels to determine cyan, magenta, and yellow dither values d c (x d ,y d ), d m (x d ,y d ), and d y (x d ,y d ), respectively. The addressed dither values are then compared to the input pixel values, I c (x,y), I m (x,y), and I y (x,y) for the corresponding color channels using comparators  46 A,  46 B, and  46 C to determine the output pixel values O c (x,y), O m (x,y), and O y (x,y). If the input pixel value is larger than the addressed dither value, the comparator will turn the output pixel value to “on.” Conversely, if the input pixel value is equal to or smaller than the addressed dither value, the comparator will turn the output pixel value to “off.” Any other variety of dither algorithms could also be used, such as the methods shown in FIG. 2, FIG. 3 or FIG. 4.  
           [0013]    When multiple inks are used for a particular printer color channel, there are many ways to determine the color values for each of the individual colorants. For example, the color values for the individual colorants may be determined by splitting a corresponding input color channel (e.g., a cyan input color channel can be split into a light cyan color channel and a dark cyan color channel by using the cyan color value to address a look-up table for the light cyan color channel, and a second look-up table for the dark cyan color channel as taught by Couwenhoven et al. in U.S. Pat. No. 6,312,101.) Alternatively, the color values for the individual colorants can be determined using more complicated color modeling techniques. In that case, a multi-dimensional look-up table, or some other complex function may be necessary to compute the color values.  
           [0014]    While the halftoning method of Spaulding et al. taught in U.S. Pat. No. 5,822,451 can, in general, be applied to determine dither matrices for a set of color channels, including each of the similar colorant color channels, there are several limitations to this approach. First of all, the computational complexity of the optimization process increases substantially with every additional color channel, which can make determination of the dither matrices impractical in some cases. Secondly, the definition of the visual cost function values can become quite ambiguous due to the complex relationship that may exist between the amounts of the individual colorants that are used in different parts of color space. Additionally, it is desirable, in many cases, to minimize the overlap of the different density colorants in the color channels that have multiple colorants, in order to minimize the amount of white paper that results when there is partial coverage. The prior art makes no provision for meeting this criterion while maintaining the desirable characteristics of the jointly optimized dither patterns. For example, if a given color calls for 50% coverage of light cyan ink and 50% coverage of dark cyan ink, the method of Spaulding et al. may produce dither patterns where some pixels have neither light ink or dark ink, and other pixels have both light ink and dark ink. Similarly, U.S. Pat. Nos. 4,635,078 and 5,729,259 describe printer systems with light and dark inks, but do not provide for jointly optimized dither patterns.  
           [0015]    The aforementioned problems will become more severe in printers that have more than two similar colorants in one or more of the color channels. For example, a printer that uses three different densities of black ink will exhibit undesirable artifacts if the prior art halftoning methods are used. U.S. Pat. No. 5,142,374 discloses a method of printing an image using three black colorants of different densities, but neglects the tone levels where two or more of the colorants are mixed in various percentages, resulting in undesirable grainy artifacts at these tone levels.  
           [0016]    U.S. Pat. No. 6,164,747 discloses a method of printing an image using three (or more) black colorants wherein an input digital image is multitoned before the image is split into separate ink channels. This architecture does not provide for full control over the halftone patterns for each ink that are result near the tone levels where transitions between the different density inks occur, resulting in grainy artifacts at those tone levels.  
         SUMMARY OF THE INVENTION  
         [0017]    It is an object of the present invention to provide an improved way for processing a digital image so that the processed digital image can be applied to a multichannel multicolorant printer wherein optimal image quality is maintained throughout the tonescale.  
           [0018]    It is an object of the present invention to produce a multichannel multitone output image from an input digital image which has at least two color channels having substantially the same color but different densities.  
           [0019]    These objects are achieved by a method for processing an input digital image having an x,y array of input pixels having associated input pixel values, to produce a multichannel multitoned output image having two or more colorant channels, wherein at least two of the colorant channels are similar, having substantially the same color but different densities, comprising the steps of:  
           [0020]    a) determining continuous-tone pixel values for each colorant channel of the input digital image in response to the input pixel values using an ink manifold processor;  
           [0021]    b) for each colorant channel, selecting a multitone process from a plurality of multitone processes in response to an input pixel value using a multitone process controller;  
           [0022]    c) for each colorant channel, applying the selected multitone process to the continuous-tone pixel value to produce a multitoned output pixel value; and  
           [0023]    d) repeating steps a)-c) for each input pixel in the input digital image to produce the multichannel multitoned output image.  
         ADVANTAGES  
         [0024]    The present invention provides a way to process an input digital image to produce a multichannel multitoned output image that can be applied to a digital printer to produce high quality prints when the input digital image has at least two colorant channels having substantially the same color but different densities.  
           [0025]    The present invention has the further advantage that a set of dither matrices that have been optimized for use with a conventional color printer can be simply modified for use in a printer where one or more of the color channels is printed using multiple colorants of similar color but different densities.  
           [0026]    The present invention has the additional advantage that it provides a computationally efficient means for producing optimized dither matrices for color printers where one or more of the color channels is printed using multiple similar colorants having substantially the same color but different densities.  
           [0027]    The present invention has the further advantage of minimizing the amount of dot overlap for the different density colorants, thereby minimizing the amount of white dots in areas where only one ink color (e.g., black) is printed. This results in reducing the visibility of the halftone patterns for these image regions.  
           [0028]    The present invention has the further advantage of minimizing the visibility of the halftone patterns generated by mixing of the similar colorants within a color channel over the entire tonescale, even when more than two similar colorants are used for a given color channel. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]    FIGS.  1 - 4  are schematic block diagrams showing prior art ordered dither techniques for halftoning;  
         [0030]    [0030]FIG. 5 is a schematic block diagram showing a prior art ordered dither technique including jointly optimized dither matrices;  
         [0031]    [0031]FIG. 6 is a schematic block diagram showing a method for halftoning an image in accordance with the present invention;  
         [0032]    [0032]FIG. 7 is a schematic block diagram showing an alternative method for halftoning an image in accordance with the present invention;  
         [0033]    [0033]FIG. 8 is a schematic block diagram illustrating a multilevel halftoning method in accordance with the present invention;  
         [0034]    [0034]FIG. 9 is a schematic block diagram illustrating a multilevel halftoning method in accordance with the present invention;  
         [0035]    [0035]FIG. 10 is a graph of an ink manifold function for a printer that uses three different density black colorants;  
         [0036]    [0036]FIG. 11 is a table of dither matrix values;  
         [0037]    [0037]FIG. 12 is a table of inverted dither matrix values;  
         [0038]    [0038]FIG. 13 is a graph showing dot patterns formed by mixing a light and medium density black ink using prior art methods;  
         [0039]    [0039]FIG. 14 is a graph showing dot patterns formed by mixing a light and medium density black ink in accordance with the present invention;  
         [0040]    [0040]FIG. 15 is a graph showing dot patterns formed by mixing a light, medium, and dark density black ink;  
         [0041]    [0041]FIG. 16 is a graph showing dot patterns formed by mixing a light, medium, and dark density black ink according to the present invention; and  
         [0042]    [0042]FIG. 17 is a graph showing the output of the multitone process controller of FIG. 9 in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0043]    The present invention comprises a method for multitoning a digital image for printing on a device having at least one color channel that uses multiple colorants of similar color but different densities, so as to minimize the visibility of halftone patterns generated using a set of dither matrices. Methods for halftoning color image data for output on such printers are described commonly assigned U.S. patent application Ser. No. 09/887,420 filed Jun. 22, 2001 by Spaulding et al., entitled “Method for Halftoning a Multichannel Digital Color Image Having at Least One Group of Similar Color Channels”, the disclosure of which is herein incorporated by reference. Such methods are respectively shown in FIG. 6, FIG. 7, and FIG. 8.  
         [0044]    [0044]FIG. 6 shows an implementation using a set of comparators  56 A- 56  in which a multi-channel input color image has a dark cyan input image  50 A, a light cyan input image  50 B, a dark magenta input image  50 C, a light magenta input image  50 D, and a yellow input image  50 E. The input images for each color channel are processed using a conventional dither halftoning method to form corresponding output images  58 A,  58 B,  58 C,  58 D and  58 E. Modulo operators  52 A,  52 B,  52 C,  52 D and  52 E are used to determine the dither matrix column and row addresses x d  and y d , respectively for an input pixel having image column and row addresses x and y, respectively. The dither matrix column and row addresses are used to address dither matrices  54 A,  59 B,  54 C,  59 D and  54 E for the dark cyan, light cyan, dark magenta, light magenta and yellow color channels, respectively, to determine addressed dither values d c (x d ,y d ), d c (x d ,y d ), d M (x d ,y d ), d m (x d ,y d ), and d y (x d ,y d ).  
         [0045]    [0045]FIG. 7 shows an implementation using a set of adders  70 A- 70 E and threshold operators  72 A-E. FIG. 8 shows an implementation in a multilevel system using a set of quantizers  92 A-E. The dither halftoning method of FIG. 8 is very similar to that shown in FIG. 7 except that the threshold operators  72 A-E have been replaced by quantizers  92 A-E, the dither matrices  54 A,  54 C and  54 E have been replaced by multi-toning dither matrices  94 A,  94 C and  94 E, and the inverted dither matrices  59 B and  59 D have been replaced by inverted multi-toning dither matrices  99 B and  99 D. The quantizers map modified input values within specified quantization intervals to corresponding output values.  
         [0046]    These methods provide for optimal image quality in printers using two similar colorants, but do not contain sufficient flexibility to provide for optimal image quality in printers having three or more similar colorants. The term “similar colorants” refers to colorants having substantially the same color but different densities. The present invention provides for optimal image quality in a printer using any number of similar colorants. Assuming in the following example, that the printing device uses three black colorants with light, medium, and dark optical density, but it should be noted that this technique can easily be generalized to other types of imaging systems having multiple similar colorants as well, including printers that use multiple densities of cyan, magenta, or yellow ink.  
         [0047]    An input digital image has an x,y array of input pixels for each color channel, where each input pixel is represented by a digital code value, typically in the range 0-4095. The input digital image may contain any number of color channels, and typically 1, 3, or 4 channel images corresponding to grayscale, red-green-blue (RGB), or cyan-magenta-yellow-black (CMYK) colors respectively are encountered. A printing device may have a corresponding set of colorants that are used to reproduce the image, or it may have a different set of colorants, in which case a conversion process is required to accurately reproduce the image. In describing a preferred embodiment of the present invention, consider a printer with three black colorants is used to reproduce an input digital image having a single grayscale color channel. The three black colorants have different optical densities referred to as K 1  (light density), K 2  (medium density), and K 3  (dark density). It will be obvious to one skilled in the art that the invention is not limited to this scenario, and may be applied to any single color channel of a multichannel input digital image, or may be applied to a printer that uses colorants other than black, or more than three black colorants.  
         [0048]    With reference to FIG. 9, one embodiment of the present invention is shown. In this embodiment, an ink manifold processor  100  is used to split a single channel grayscale input digital image into three separate colorant channels corresponding to the K 1 , K 2 , and K 3  inks. The details of the ink manifold processor  100  are not fundamental to the invention, and a method for performing the splitting performed by the ink manifold processor is specified in U.S. Pat. No. 6,312,101 to Couwenhoven et al. According to this method, a look-up table for each colorant channel is indexed by the input digital code value to produce a continuous-tone pixel value for each colorant that corresponds to the amount of colorant that gets printed at the pixel location on the page. The set of look-up tables for the colorant channels that specifies the splitting process is called the manifold LUT, an example of which is shown in FIG. 10, and will be discussed in more detail shortly.  
         [0049]    Still referring to FIG. 9, after the ink manifold processor  100  performs the splitting of the input digital code value into continuous-tone pixel values for each colorant channel, the continuous-tone pixel values are subsequently multitoned using multitone processors  130 ,  140 ,  150 . The multitone processors  130 ,  140 ,  150  operate on the K 1 , K 2 , and K 3  colorant channels respectively, and perform the function of reducing the number of gray levels in the continuous-tone pixel values to produce a multichannel multitoned output image having multitoned output pixel values which have fewer gray levels. The number of gray levels in the multitoned output pixel values matches the number of gray levels available in an inkjet printer  120 . The multitoning process is required because the inkjet printer  120  can typically print many fewer density levels than are specified by the continuous-tone pixel values. As described above, there are many different implementations of the multitone processors  130 ,  140 ,  150  that may be applied to the present invention, and the particular implementation is not fundamental to the invention. In this example, the multitone processors  130 ,  140 ,  150  implement a blue noise dither process using a matrix of threshold values designed to minimize a visual cost function as set forth in U.S. Pat. No. 5,822,451 to Spaulding et al., and according to the implementation described by FIG. 1, although any of the dither implementations could be equally used. One such matrix of dither values is shown in FIG. 11. This matrix is an 8×8 array of threshold values on the range (1, 64). As understood by one skilled in the art, the thresholds in the matrix should be scaled to match the range of the continuous-tone pixel values (which in case of the current example is (0,4095)) when used by the multitone processors  130 ,  140 ,  150 . For clarity of explanation, the threshold values in the dither matrix are shown in the range (1, 64) so as to aid in understanding the order in which pixels in the 8×8 pattern will be turned on. As shown, the pixels in the pattern will be turned on in sequence starting at the location occupied by 1 and ending at the location occupied by 64.  
         [0050]    Referring again to FIG. 9, the multitoned output pixel values are received by the inkjet printer  120 , which places the desired amount of each of the K 1 , K 2 , and K 3  inks on the page accordingly. A multitone process controller  110  is used to control the behavior of the multitone processors  130 ,  140 ,  150  responsive to the input pixel value. In a preferred embodiment, the multitone process controller  110  selects a multitone process to be used in each of the multitone processors  130 ,  140 ,  150 . In the case of a matrix based dither process, the multitone process controller  110  would adaptively select the matrix of dither values that is used for each colorant channel as a function of the input pixel value. The functioning of the multitone process controller  110  is an important aspect of the invention, which is used advantageously to solve many of the aforementioned problems, and will be described in detail hereinbelow.  
         [0051]    Consider, as shown in FIG. 10, that the manifold LUT specifies that the K 1 , K 2 , and K 3  inks are used in the following way. Starting with white paper, as more density is required, the K 1  ink is first added until nearly every pixel printed in a local region on the page contains a dot of K 1  ink. Then, K 2  ink is added to further increase the density until nearly every pixel contains an overlap of both K 1  and K 2  ink. Then, as we increase towards maximum density, the K 3  ink is added as the K 1  ink is removed. Thus, the maximum density is achieved when every pixel on the page contains an overlap of K 2  and K 3  ink. It is important to note that the form of the manifold LUT is not fundamental to the invention, and the invention may be applied to a system using any particular manifold LUT. To understand the motivation for the use of the multitone process controller  110  of FIG. 9, two tone levels in the tonescale of the printer of the current example will be examined. The first of these two tone levels roughly corresponds to the point in the tonescale at which the K 1  ink is nearing full coverage and the K 2  ink is just starting to be used. This is called the “first overlap region”, which is shown in FIG. 10. The second of the two tone levels roughly corresponds to the point in the tonescale at which the K 2  ink is nearing full coverage, the K 3  ink is just starting to be used, and the K 1  ink is at full coverage and just starting to be removed. This is called the “second overlap region”, which is also shown in FIG. 10.  
         [0052]    In the first overlap region, the K 1  continuous-tone pixel value will be large, and the K 2  continuous-tone pixel value will be small. After the multitoning step, this means that most of the pixels will contain a K 1  dot, and a few of them will contain K 2  dots. Referring to FIG. 13, a small (8×8 pixel) region of a printed image is shown which represents a typical halftone pattern that might be produced in the current example for tone levels near the first overlap region. In FIG. 13, an 8×8 region  200  of K 1  dots  210  results from multitoning the K 1  colorant channel using the matrix of dither values M shown in FIG. 11. The corresponding 8×8 region  220  of K 2  dots  230  results from multitoning the K 2  colorant channel using the dither matrix M. The superposition of these two bitmaps on the printed page produces an 8×8 pattern  240 , in which some pixels have no dots  260 , some pixels have only K 1  dots  270 , and some pixels have both K 1  and K 2  dots  250 . This pattern is undesirable because it produces a “grainy” appearance when viewed by the human eye due to the fact that some pixels are white and some are very dark. The ideal pattern would appear smooth and continuous to the human observer, and would be optimally achieved if each pixel in the pattern was the same density.  
         [0053]    As taught in commonly assigned U.S. patent application Ser. No. 09/887,420 filed Jun. 22, 2001 by Spaulding et al., entitled “Method for Halftoning a Multichannel Digital Color Image Having at Least One Group of Similar Color Channels”, the disclosure of which is herein incorporated by reference, the grainy appearance of tone levels near the first overlap region may be reduced by modifying the multitone processor  140  that is used for the K 2  channel. One simple modification that is taught in commonly assigned U.S. patent application Ser. No. 09/887,420 filed Jun. 22, 2001 by Spaulding et al., entitled “Method for Halftoning a Multichannel Digital Color Image Having at Least One Group of Similar Color Channels”, the disclosure of which is herein incorporated by reference, is to use an “inverted” matrix of dither values for one colorant channel. An inverted dither matrix is simply computed by subtracting a dither matrix from a predetermined maximum value, which is typically the value of the largest threshold in the matrix. The inverted dither matrix, M −1 , of the dither matrix M is shown in FIG. 12. Simply stated, the first pixels to turn “on” in an inverted dither matrix will be at the same locations as the last pixels that remain “off” in the non-inverted dither matrix. This is demonstrated by turning to FIG. 14, in which the inverted dither matrix M −1  is now used to multitone the K 2  channel, and the (non-inverted) dither matrix M is still used to multitone the K 1  channel. Using this arrangement, the 8×8 pixel region  320  now contains K 2  dots  330  that overlap the white spaces in the K 1  bitmap  300  where no K 1  dots  310  are printed. Thus, the superposition of the K 1  and K 2  bitmaps on the page results in an 8×8 pixel region  340  in which only K 1  dots  350  or K 2  dots  360  are printed. There are no pixels that contain overprints of K 1  and K 2 , and none are left white. This pattern will be much more pleasing to the human eye, as it will appear more smooth and continuous. The multitone process controller  110  of FIG. 9 is used to specify that dither matrix M −1  is used for channel K 2 , and dither matrix M is used for K 1 , thereby producing the optimal pattern shown in FIG. 14 for tone levels near the first overlap region. However, the benefits of the multitone process controller  110  become even greater upon examining the behavior near the second overlap region, as will be discussed next.  
         [0054]    For tone levels near the second overlap region, the K 2  ink is at nearly full coverage, the K 3  ink is just starting to be used, and the K 1  ink is at nearly full coverage and just starting to be removed (refer to the ink manifold curves shown in FIG. 10). For the same reasons stated above, it would be desirable to have the dither matrices used for the K 2  and K 3  channels be inverses of each other, so as to prevent the grainy appearance that can arise from having the K 2  and K 3  dots overlap. Since the dither matrix M −1  is used for the K 2  channel, then the dither matrix M must be used for the K 3  channel. However, this can also lead to undesirable grainy patterns because the K 1  and K 3  channels are being multitoned with the same matrix, as will now be shown.  
         [0055]    Turning to FIG. 15, the 8×8 pixel regions corresponding to the K 1 , K 2 , and K 3  channels for a tone level near the second overlap region are shown as patterns  400 ,  420 , and  440  respectively. The K 1  pattern  400  contains K 1  dots  410  and white holes where no K 1  dot is placed, the K 2  pattern  420  contains K 2  dots  430  and white holes where no K 2  dot is placed, and the K 3  pattern  440  contains K 3  dots  450  and white holes where no K 3  dot is placed. Note that the K 3  dots  450  overlap the white holes in the K 2  pattern  420  as desired, but they do not overlap the white holes in the K 1  pattern  400 . Thus, the superposition of the three patterns  460  is shown in which some pixels  480  have K 2  dots only, some pixels  470  have K 1  and K 3  dots, and other pixels  490  have K 1  and K 2  dots. The K 2  dots  480  will be significantly lighter that the rest of the pixels, and the K 1 +K 3  dots  470  will be significantly darker, resulting again in a grainy appearance to the region. Thus, a dilemma results in which three conflicting conditions need to be simultaneously satisfied: (a) the K 1  and K 2  dither matrices must be inverses; (b) the K 2  and K 3  dither matrices must be inverses; and (c) the K 1  and K 3  dither matrices must be inverses.  
         [0056]    This set of conflicting constraints is solved using the multitone process controller  110  to modify the multitone processors  130 ,  140 ,  150  as a function of the input pixel value. This represents a significant departure from the prior art, and solves the conflicting set of conditions stated above by recognizing that these conditions do not simultaneously exist over the entire range of input pixel values. This will now be described in detail. Turning to FIG. 16, it can be seen that for tone levels near the second overlap region, if the dither matrix that is used for the K 1  channel is switched to be M −1 , then the desired behavior is achieved. The 8×8 pixel region  500  of K 1  dots  510  now has white holes that correspond with the 8×8 pixel region  520  of K 2  dots  530 . Furthermore, the occupied pixels of the 8×8 pixel region  540  of K 3  dots  550  will fill in the white holes in the K 1  and K 2  bitmaps. Thus, the superposition of the K 1 , K 2 , and K 3  bitmaps results in a pattern  560  which contains pixels  580  that have K 1  and K 2  dots, or pixels  570  that have only K 3  dots. These pixels will be relatively close in density, resulting in a smoother, more uniform pattern. The key to achieving these optimal patterns is the ability of the multitone process controller  110  to change the dither matrix that is used for any channel as a function of the input pixel value. Also, a key realization is that once the continuous-tone pixel value for a channel reaches the maximum value, then all dither matrices will produce the same pattern, namely, every pixel will contain a dot. Thus, once the manifold LUT of FIG. 10 drives the continuous-tone pixel value for K 1  to its maximum value (in between the first and second overlap regions), the multitone process controller  110  switches the dither matrix that is used for the K 1  channel from M to M −1 . In a preferred embodiment, a threshold input pixel value is chosen which corresponds roughly to the center of the region where the K 1  ink is at its maximum value. If the input pixel value is above this threshold, then the dither matrix used to process the K 1  channel is switched to be M −1 , otherwise dither matrix M is used. This is graphically shown in FIG. 17, which shows the dither matrix that the multitone process controller  110  selects for each colorant channel as a function of the input pixel value. This arrangement provides for optimal image quality in both the first and second overlap regions by providing for patterns that have minimized graininess.  
         [0057]    Switching between a set of dither matrices is just one way that the multitone process controller  110  can modify the multitone processors  130 ,  140 ,  150 . As mentioned above, the multitone process controller  110  can also modify a set of dither bitmaps, dither LUTs or error diffusion parameters based on the input pixel value as needed to achieve optimal image quality. Also, the example presented above was described with respect to a particular manifold LUT, but those skilled in the art will recognize that other manifold LUTs exists, and the invention will apply to those equally well. The invention can also be applied to a color printer in which one or more of the color channels uses multiple similar colorants. For example, a printer having three cyan colorants, three magenta colorants, a yellow, and a black colorant may use the present invention advantageously to control the processing of the cyan and magenta channels.  
         [0058]    A computer program product may include one or more storage medium, for example; magnetic storage media such as magnetic disk (such as a floppy disk) or magnetic tape; optical storage media such as optical disk, optical tape, or machine readable bar code; solid-state electronic storage devices such as random access memory (RAM), or read-only memory (ROM); or any other physical device or media employed to store a computer program having instructions for controlling one or more computers to practice the method according to the present invention.  
         [0059]    The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.  
         [0060]    Parts List  
         [0061]    [0061] 10  modulo operators  
         [0062]    [0062] 12  dither matrix  
         [0063]    [0063] 14  comparator  
         [0064]    [0064] 24  adder  
         [0065]    [0065] 26  threshold operator  
         [0066]    [0066] 30  modulo operators  
         [0067]    [0067] 32  dither bitmaps  
         [0068]    [0068] 34  dither LUT selector  
         [0069]    [0069] 36  dither LUTs  
         [0070]    [0070] 40 A-C input images  
         [0071]    [0071] 42 A-C modulo operators  
         [0072]    [0072] 44 A-C dither matrices  
         [0073]    [0073] 46 A-C comparators  
         [0074]    [0074] 48 A-C output images  
         [0075]    [0075] 50 A-E input images  
         [0076]    [0076] 52 A-E modulo operators  
         [0077]    [0077] 54 A optimized cyan dither matrix  
         [0078]    [0078] 54 C optimized magenta dither matrix  
         [0079]    [0079] 54 E optimized yellow dither matrix  
         [0080]    [0080] 56 A-E comparators  
         [0081]    [0081] 58 A-E output images  
         [0082]    [0082] 59 B inverted cyan dither matrix  
         [0083]    [0083] 59 D inverted magenta dither matrix  
         [0084]    [0084] 70 A-E adders  
         [0085]    [0085] 72 A-E threshold operators  
         [0086]    [0086] 90 A-E look-up tables  
         [0087]    [0087] 90 A-E quantizers  
         [0088]    [0088] 94 A optimized cyan dither matrix  
         [0089]    [0089] 94 C optimized magenta dither matrix  
         [0090]    Parts List (con&#39;t)  
         [0091]    [0091] 94 E optimized yellow dither matrix  
         [0092]    [0092] 99 B inverted cyan dither matrix  
         [0093]    [0093] 99 D inverted magenta dither matrix  
         [0094]    [0094] 100  ink manifold processor  
         [0095]    [0095] 110  multitone process controller  
         [0096]    [0096] 120  inkjet printer  
         [0097]    [0097] 130  multitone processor  
         [0098]    [0098] 140  multitone processor  
         [0099]    [0099] 150  multitone processor  
         [0100]    [0100] 200  8×8 pixel region of printed dots  
         [0101]    [0101] 210  printed ink dot  
         [0102]    [0102] 220  8×8 pixel region of printed dots  
         [0103]    [0103] 230  printed ink dot  
         [0104]    [0104] 240  8×8 pixel region of printed dots  
         [0105]    [0105] 250  printed ink dot  
         [0106]    [0106] 260  white space  
         [0107]    [0107] 270  printed ink dot  
         [0108]    [0108] 300  8×8 pixel region of printed dots  
         [0109]    [0109] 310  printed ink dot  
         [0110]    [0110] 320  8×8 pixel region of printed dots  
         [0111]    [0111] 330  printed ink dot  
         [0112]    [0112] 340  8×8 pixel region of printed dots  
         [0113]    [0113] 350  printed ink dot  
         [0114]    [0114] 360  printed ink dot  
         [0115]    [0115] 400  8×8 pixel region of printed dots  
         [0116]    [0116] 410  printed ink dot  
         [0117]    [0117] 420  8×8 pixel region of printed dots  
         [0118]    [0118] 430  printed ink dot  
         [0119]    [0119] 440  8×8 pixel region of printed dots  
         [0120]    Parts List (con&#39;t)  
         [0121]    [0121] 450  printed ink dot  
         [0122]    [0122] 460  8×8 pixel region of printed dots  
         [0123]    [0123] 470  printed ink dot  
         [0124]    [0124] 480  printed ink dot  
         [0125]    [0125] 490  printed ink dot  
         [0126]    [0126] 500  8×8 pixel region of printed dots  
         [0127]    [0127] 510  printed ink dot  
         [0128]    [0128] 520  8×8 pixel region of printed dots  
         [0129]    [0129] 520  printed ink dot  
         [0130]    [0130] 540  8×8 pixel region of printed dots  
         [0131]    [0131] 550  printed ink dot  
         [0132]    [0132] 560  8×8 pixel region of printed dots  
         [0133]    [0133] 570  printed ink dot  
         [0134]    [0134] 580  printed ink dot