Abstract:
A computerized printer utilizes a color separation and printing process that adds brightness, saturation and ink density to traditional CMYK printing processes and optionally eliminates the need for Black (K) color separation data and Black ink in the printer. The process includes the creation of Red, Green, and Blue (R′G′B′) color separation data channels for use by a computer processor to complement the dynamic range of traditional Cyan, Magenta, and Yellow (CMY) color separation data channels. Accordingly, the process gives printers the capacity to utilize a six-color separation process with Cyan, Magenta, Yellow, Red, Green, and Blue (CMYR′G′B′) color separation data channels. The standard Black (K) separation data can be combined within the R′G′B′ separation data to optically simulate Black ink in print. The process can be applied to printing processes including: digital, flexography, inkjet, lithography, rotary gravure, rotary letterpress and screen-printing.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Provisional U.S. Application Ser. No. 60/939,071, filed May 20, 2007, the entire disclosure of which is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to the art of color separation and reproduction in print. In particular, a 6-color (CMYR′G′B′) separation process that can be applied to a printing process, namely, Digital, Flexography, Inkjet, Lithography, Rotary Gravure, Rotary Letterpress, and screen-printing. 
     BACKGROUND OF THE INVENTION 
     U.S. Pat. No. 5,751,326, (Bernasconi), issued to the present applicant, describes a method of color separation for a printing process wherein an image source is scanned one or more times to produce a plurality of data channels each of which provides a representation of one color separation of the image source, the data provided by each channel being restricted to represent a printable tone density range, and the channels being separated into two groups, a first providing separation data representing Cyan, Magenta, Yellow and Black (CMYK) separations, the second providing Red, Green and Blue (R′G′B′) separation data representing a saturation image. 
     U.S. Pat. No. 5,751,326 describes a method of scanning an image to produce CMYK data and then adjusting the datum while scanning an image again to generate a second R′G′B′ data set. This R′G′B′ color separation data was derived directly from the original image. 
     As described in U.S. Pat. No. 5,751,326, the CMYK printing process can only reproduce a limited tonal range, commonly referred to at the CMYK “color gamut”. Saturated Red, Green and Blue hues reproduced with Cyan, Magenta and Yellow (CMY) inks alone will often lack saturation and high ink density due to the limitations of the printable dynamic range. Halftone screening technologies can only reproduce 100 gray levels per primary color. Tone compression from the CMY ink densities also restricts the printable color gamut. 
     In recent years conventional drum or flatbed scanners that can color separate photographic originals, namely panchromatic emulsions, utilizing Red, Green and Blue (RGB) filters, have become virtually redundant in the graphic arts industry. Conventional drum or flatbed scanners have rapidly been replaced by digital cameras, where no panchromatic emulsion exists in the immediate creation of a color image. The data captured by a digital camera has already been digitally separated into three (3) gray scale images commonly known in the graphic arts and printing industries as a Red, Green and Blue (RGB) digital image. 
     A number of different color models have been developed to define color spaces, such as RGB, CIELAB (L*a*b*), L*C*h°, CMYK, etc. The International Color Consortium (ICC) has developed a specification defining “profiles” relating to the characteristics of the various groups. ICC profiles are used to describe the color attributes of a particular device or viewing requirement by defining a mapping between the source or target color space and a Profile Connection Space (PCS). This PCS can be either L*a*b* or CIE XYZ color space. Mappings may be done using tables, to which interpolation is applied, or through a series of algorithms or parameters for transformations. To convert from RGB to CMYK, the first step is to obtain the two ICC profiles concerned. To perform the conversion, each RGB triplet R, G, B is first converted to the PCS using the RGB profile. If necessary, the PCS is converted between L*a*b* and CIE XYZ, a well-defined transformation. Then the PCS is converted to the four values of C, M, Y, K required. 
     Every device that captures or displays color will have its own ICC profile. Some manufacturers provide profiles for their products, and there are also several software products available that enable end users to generate their own ICC profiles, typically through the use of a colorimeter or spectrodensitometer. 
     In CMYK process printing, each primary Cyan, Magenta and Yellow (CMY) ink absorbs Red, Green and Blue light respectively. In theory, a region having 300% (C+M+Y) ink coverage should reproduce an achromatic (Black) appearance, subject to the print density of each primary ink. In practice however, overprinting CMY alone results in a dark brown hue. The CMY density attained typically does not exceed 1.50. 
     This is because the combined (trapped) CMY inks fail to absorb all the complementary (RGB) light transmitted. Black (K) ink is traditionally used to overprint CMY, thereby absorbing the residual RGB light. The trapped C+M+Y+K result reproduces and denser, more neutralized Black appearance. The CMYK density attained typically exceeds 1.80. 
     In converting RGB data to printable CMYK data, the RGB data must first be converted to L*a*b* data, and then the L*a*b* data must be converted to CMYK data using appropriate conversion methods such as ICC profiles. Color information on the display monitor may “appear” to be lost during this color conversion procedure. The industry term used for this apparent loss of color information is called “gamut compression”. For example, digital color images must first be converted from RGB (256 gray levels per primary color) to CMYK (100 gray levels per primary color) in order to reproduce them using CMYK inks and halftone screens. This is because no known print process can print continuous shades of gray. This RGB to CMYK color conversion can also be referred to as the “halftone preparation” step. 
     It is important to note that even after the CMYK data had been assigned 100 gray levels per primary color (to ultimately produce a halftone separation), the actual “background” digital data residing in the CPU is still 256 gray levels per primary color (8-bit data). The physical halftone compression step to 100 gray levels per primary color has yet to occur. The CMYK data has simply been “prepared” in readiness to produce a printable halftone range for each color separation: C, M, Y and K. This final compression step is only applied when the CMYK data is processed through a software Raster Imager Processor (R.I.P.) to create a 1-bit TIFF file. A 1-bit TIFF file can only record 100 gray (halftone) levels per primary color. Therefore, every 2.56 CMYK gray levels must be assigned only one (1) output halftone value to record the dynamic range of 2.00 (log 10 100=2.00). The compression ratio is 2.56:1. 
     Attempts to find the optimum formula for conversion between RGB and CMYK values and L*a*b* often founder because RGB and CMYK are not absolute color spaces and so have no precise relation to L*a*b*. To convert between RGB and L*a*b*, for example, it is necessary to determine or assume an absolute color space for the RGB data, such as sRGB or Adobe RGB (1998). For each of these absolute spaces, there are standard techniques for converting to and from the XYZ absolute color space (see for example sRGB color space specification of the transformation) which can be combined with the following transformations to convert them to L*a*b*. 
     The following formulae is commonly known and used by the ICC for transforming XYZ to CIE L*a*b* (CIELAB). 
     The forward transformation:—
 
 L*= 116 f ( Y/Y   n )−16
 
 a*= 500 [f ( X/X   n )− f ( Y/Y   n )]
 
 b*= 200 [f ( Y/Y   n ) f ( Z/Z   n )]
         where f(t)=t 1/3  for t&gt;0.008865,
           f(t)=7.787t+16/116 otherwise.   
               

     Here X n , Y n  and Z n  are the CIE XYZ tristimulus values of the reference white point. 
     The division of the f(t) function into two domains was done to prevent an infinite slope at t=0. f(t) was assumed to be linear below some t=t0, and was assumed to match the t 1/3  part of the function at t 0  in both value and slope. In other words:
 
 t   0   1/3   =at   0   +b (match in value)
 
1/(3 t   0   2/3 )= a (match in slope)
 
     The value of b was chosen to be 16/116. The above two equations can be solved for a and t 0 . 
     a=1/(3δ 2 )=7.787037 . . . 
     t 0 =δ 3 =0.008856 . . . 
     where δ=6/29. Note that 16/116=2δ/3. 
     The International Color Consortium defines the format precisely but do not define algorithms or processing details. This means there is room for variation between different applications and systems that work with ICC profiles. 
     BRIEF SUMMARY OF THE INVENTION 
     It is desirable to provide a method for use via a computer processor in a printer ( 50 ) generating printed matter ( 60 ), the method including appropriate steps for obtaining color separation data thus improving on known processes and/or ameliorating one or more of the problems of the known processes. 
     The process of the invention derives second color separation data from first color separation data, the first data and the second data being available for use in a printing process. 
     The first data can be compressed color separation data, and the second data can be complementary color separation data. 
     The compressed data can be CMYK data and the complementary data can be adapted to at least partially compensate for the dynamic range lost from the compressed CMYK data as a result of tone compression to a printable dynamic range. 
     In one embodiment, a color separation process is provided wherein standard Cyan, Magenta &amp; Yellow (CMY) color separations are complemented with Red, Green and Blue (R′G′B′) color separations to reproduce a color gamut in a print process. The R′G′B′ color separations are derived from the cross-coupled CMY data channels. 
     The standard Black (K) separation data can be combined within the R′G′B′ color separations to optically simulate Black ink in print. 
     The invention also provides a method for converting grayscale data to substitute color separation data. 
     The substitute data can be combined with the complementary data. 
     The invention also provides a digital color separation process to create color separation data which represents a combined saturation and neutral gray tonal range. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A preferred embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which: 
         FIG. 1  illustrates exemplary L*C*h° and absolute density values for Cyan, Magenta, Yellow, Black, Red, Green and Blue printing inks; 
         FIG. 2A  illustrates CMY ink overprints for a print process; 
         FIG. 2B  illustrates R′G′B′ ink overprints for a print process; 
         FIG. 2C  illustrates the R′G′B′ overprints combined with the CMY overprints; 
         FIG. 2D  illustrates the hue angle as represented on a typical color wheel; 
         FIG. 3  illustrates a prior art process for generating color separations; 
         FIG. 4  illustrates a process for generating color separations according to a first embodiment of the invention; 
         FIG. 5  illustrates a process for generating a complementary data set according to an embodiment of the invention; 
         FIG. 6A  illustrates a process for substituting the grayscale data according to an embodiment of the invention; 
         FIG. 6B  illustrates an expanded version of  FIG. 2A ; 
         FIG. 7A  illustrates a multi-stage color separation data conversion and flowchart of a process according to an embodiment of the invention; 
         FIG. 7B  represents a modification of  FIG. 7A ; 
         FIG. 8  illustrates a table for conversion of Black (K) color separation data according to an embodiment of the invention; 
         FIG. 9  illustrates a chart for conversion of Black (K) color separation data according to an embodiment of the invention; 
         FIG. 10  illustrates grey scale multiplication; 
         FIG. 11  is a chart illustrating CMY to RGB conversion; 
         FIG. 12  illustrates CMY conversion using gamma=1; 
         FIG. 13  illustrates CMY conversion using gamma=2; 
         FIG. 14  illustrates a Red color correction mask; 
         FIG. 15  illustrates a Green color correction mask; 
         FIG. 16  illustrates a Blue color correction mask; 
         FIG. 17  illustrates compression of Black (K) data; 
         FIG. 18  is a flow diagram of an alternative implementation of the invention. 
         FIG. 19  is a schematic showing the connections of the computerized components implementing the method described herein. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will be described with reference to the embodiments shown in the drawings. Without limiting the scope of the invention, the computerized method disclosed herein is applicable, at a minimum, to printing processes controlled, at least in part, by a computer processor. The computer processor may be any type of software driven device known in the art, including but not limited to raster image processors. 
     The improving speed of computers and the development of International Color Consortium (ICC) profiles according to the International Organization for Standardization (ISO) standard—ISO 15076, enables a process according to an embodiment of the present invention to be implemented by applying a series of ICC profiles and suitable color data conversion techniques to RGB, L*a*b* and CMYK color separation data to selectively or simultaneously create Red, Green and Blue (R′G′B′) color separation data channels representing a saturation image which can be used to complement the dynamic range of the CMY data to compensate for the loss of dynamic range from the CMYK data set as a result of tone compression to a printable dynamic range. 
     A further embodiment of the present invention involves the optional elimination of the process Black (K) color separation data and the optional elimination of the Black (K) ink within any printing process. The conventional process Black (K) color separation data and Black printing ink can be substituted, when required, with Red, Green and Blue color separation data and Red, Green and Blue printing ink respectively to simulate a neutral or gray tonal range in print. According to an embodiment of the invention, neutral or gray tone is defined under the L*C*h° color space wherein the Chroma (C*) value is three (3) or less, where the L*C*h° color space is measured according to the ISO standard—ISO 13655. 
     The addition of Red, Green and Blue color separations printed in Red, Green and Blue inks respectively will add superior brightness, saturation and density when combined with the Cyan, Magenta and Yellow separations printed in Cyan, Magenta and Yellow inks respectively. For example, a three (3) color overprint combination of Red, Magenta and Yellow ink will reproduce a Red hue with much stronger saturation and density of approximately 1.80 compared to a two (2) color Magenta and Yellow overprint density of approximately 1.40. A three (3) color overprint combination of Green, Cyan and Yellow ink will produce a Green hue with much stronger saturation and density of approximately 1.80 when compared to a two (2) color Cyan and Yellow overprint density of approximately 1.40. A three (3) color overprint combination of Blue, Cyan and Magenta ink will produce a Blue hue with much stronger saturation and density of approximately 1.80 when compared to a two (2) color Cyan and Magenta overprint density of approximately 1.40. The combined Red, Green, and Blue print densities of 1.80, achieved by a combination of Red, Magenta and Yellow color separation data or Green, Cyan and Yellow or Blue color separation data, Cyan and Magenta color separation data, is due to the additional Red, Green and Blue color separation data being recorded via a second halftone dynamic range. 
     The Black (K) ink can be further substituted by overprinting equal parts of Red, Green and Blue ink to reproduce an achromatic density of approximately 1.80. The CMYK dynamic range of 100 gray levels per primary color (log 10 100=2.00) is automatically extended when the second R′G′B′ dynamic range of 100 gray levels per primary color overprints the first CMYK dynamic range. The total dynamic range is therefore extended to 200 gray levels per primary color separation. This equates to an effective printable dynamic range of 2.30 d (log 10 200=2.30). 
     In one embodiment, the present invention provides a method of color separation for a print process wherein a primary L*a*b* data source is digitally converted by applying a series of conversion steps to produce secondary and tertiary color separation data sets, the secondary color separation data set representing a printable dynamic range provided from the primary color separation data set, the tertiary color separation data set representing a printable dynamic range and compensating for color saturation data lost from the secondary color separation data set as a result of tone compression to a printable dynamic range; transferring gray scale data provided from the secondary color separation data set to an intermediate color separation data set; combining the intermediate color separation data set with the tertiary color separation data set; removing the gray scale data from the secondary color separation data set. The secondary color separation data set representing Cyan, Magenta, Yellow and Black (CMYK) separations. The tertiary color separation data representing Red, Green and Blue (R′G′B′) color separations comprising a combined saturation and neutral gray tonal range and substantially comprising a representation of the dynamic range lost from the secondary CMYK color separation data set as a result of tone compression to a printable dynamic range. The traditional process Black (K) color separation and Black printing ink will be replaced by overprinting equal parts of Red, Green and Blue ink to simulate the neutral gray tonal range in print. 
     The achromatic tonal range for the Red, Green and Blue color separations can be extrapolated from the process Black (K) color separation data channel, the data from the Black (K) color separation data channel being duplicated and combined into the Red, Green and Blue color separation data channels. The data within the Black (K) color separation data channel being deleted from the CMYK data set. 
     An industry standard spectrodensitometer can be used to measure L*C*h° data for the Cyan, Magenta, Yellow, Black (K), Red, Green and Blue printing inks to be printed utilizing a printing process. The spectrodensitometer should be programmed to measure D50/2 and STATUS T absolute density. Densitometric measurements are made in accordance with ANSI CGATS.4-1993 (reaffirmed 1998). 
       FIG. 1  illustrates the density and L*C*h° values for the Cyan, Magenta, Yellow, Black, Red, Green and Blue printing inks. 
     Referring again to  FIG. 1 , at a solid ink density of 1.25, the Cyan ink should measure L*=57, C*=60, h°=235. At a solid ink density of 1.25, the Magenta ink should measure L*=50, C*=72, h°=355. At a solid ink density of 0.95, the Yellow ink should measure L*=90, C*=91, h°=94. At a solid ink density of 1.15, the Red ink should measure L*=60, C*=79, h°=37. At a solid ink density of 1.15, the Green ink should measure L*=63, C*=75, h°=158. At a solid ink density of 1.25, the Blue ink should measure L*=41, C*=56, h°=274. The Black ink should measure a solid print density of 1.30. 
     The L*C*h° values quoted relate to printing the Cyan, Magenta, Yellow, Red, Green and Blue (CMYR′G′B′) inks onto a substrate with a reference Chroma (C*) value of less than two (2) and a reference density of 0.08 (Dmin) or less. The CMYR′G′B′ inks should be printed within a ΔE (delta E) of five (5) or less relative to the L*C*h° values quoted. A Red, Green and Blue overprint that contains equal halftone percentages of Red, Green and Blue ink should have an achromatic appearance wherein the Chroma (C*) value measured is three (3) or less. 
       FIGS. 1 &amp; 2  illustrate aspects of color definition schemes which will be referred to in the following description. 
       FIG. 1  illustrates L*C*h° values for CMYK and R′G′B′ where L* is the lightness (luminance), C* is chroma, h° is hue and D is absolute density. 
     As shown at  102 , for Cyan, L*=57, C*=60, h°=235 and D=1.25. 
     At  112 , the values for Magenta are shown as, L*=50, C*=72, h°=355° and D=1.25. 
     At  122 , the values for Yellow are shown as, L*=90, C*=91, h°=94° and D=0.95. 
     At  132 , the values for Black are shown as, L*=26, C*=1, h°=n/a and D=1.25. 
     At  142 , the values for Red are shown as, L*=60, C*=79, h°=37° and D=1.15. 
     At  152 , the values for Green are shown as, L*=63, C*=75, h°=158° and D=1.15. 
     At  162 , the values for Blue are shown as, L*=41, C*=56, h°=274° and D=1.25. 
     The formula for calculating the optimum solid print density for a process ink is: (L*+C)/2=bv. Where: L=Lightness, C=Chroma, bv=Brightness Value. *Rule: L&lt;C. This formula need only be applied to the Blue and Cyan inks because they have the lowest brightness values relative to process Magenta: 61 bv, Green: 69 bv, Red: 69.5 bv and Yellow: 90.5 bv. 
     For example, Blue printed at 1.25 density: (41+56)/2=48 bv. As you increase Blue ink above 1.30 density, the bv will actually decrease. Therefore, the optimum ink film thickness (density) is directly correlated to the highest bv. The Red and Green process inks are then printed in gray balance relative to the Blue density (correlated to the highest bv). 
     Cyan printed at 1.25 density: (57+60)/2=58.5 bv. The Magenta and Yellow process inks are printed in gray balance relative to the Cyan density (correlated to highest bv). 
     The solid ink density values at  102 ,  112 ,  122 ,  132 ,  142 ,  152  &amp;  162  must also be reproduced within the halftone dynamic range (2.00 d) for each color separation. For example, Cyan ink should be calibrated to print at halftone density values of; 100%=1.25 (Dmax), 75%=0.85, 50%=0.52, 25%=0.29, 5%=0.12, 0%=0.08 (Dmin). Magenta ink should be calibrated to print at; 100%=1.25 (Dmax), 75%=0.85, 50%=0.52, 25%=0.29, 5%=0.12, 0%=0.08 (Dmin). Yellow ink should be calibrated to print at; 100%=0.95 (Dmax), 75%=0.72, 50%=0.48, 25%=0.29, 5%=0.12, 0%=0.08 (Dmin). Black ink (optional) should be calibrated to print at; 100%=1.30 (Dmax), 75%=0.85, 50%=0.52, 25%=0.29, 5%=0.12, 0%=0.08 (Dmin). Red ink should be calibrated to print at; 100%=1.15 (Dmax), 75%=0.82, 50%=0.52, 25%=0.29, 5%=0.12, 0%=0.08 (Dmin). Green ink should be calibrated to print at; 100%=1.15 (Dmax), 75%=0.82, 50%=0.52, 25%=0.29, 5%=0.12, 0%=0.08 (Dmin). Blue ink should be calibrated to print at; 100%=1.25 (Dmax), 75%=0.85, 50%=0.52, 25%=0.29, 5%=0.12, 0%=0.08 (Dmin). 
       FIG. 2A  illustrates the CMY inks as three overlapping circles where  202  is the Yellow circle,  204  is the Magenta circle, and  206  is the cyan circle. The circles are arranged around a set of orthogonal axes  250 ,  252 . The Yellow circle  202  and the Magenta circle  204  overlap (see cusp  214 ) to form a blended region  203  which appears as varying hues of red, depending on the proportions of Magenta and Yellow ink which are graphically indicated by the hue angle-h° illustrated in  FIG. 2D . 
     Similarly, the Magenta and Cyan circles  204 ,  206  overlap at  205  to provide a range of Blue hues as determined by the hue angle-h°, and the Yellow and Cyan circles  202 ,  206  overlap at  207  to provide a range of Green hues as determined by the angle hue-h°. A tricuspid central region  226  represents the overlap of the C, M, and Y circles. 
       FIG. 2B  illustrates illustrates the R′G′B′ inks as three overlapping circles, with circle  208  being the Blue circle,  210  being the Green circle, and  212  being the Red circle. Again there are regions of overlap of the Red and Green circles (see cusp  222 ), which can be combined to produce a dark brown region. Similarly Red and Blue (see cusp  224 ), and Green and Blue (see cusp  220 ). A tricuspid region  228  is defined by the ink overlap of Red, Green, and Blue that can combine to produce a gray/black or achromatic region. 
       FIG. 2C  illustrates the superposition of  FIGS. 2A and 2B . As can be seen by way of example, the line  232  passes through the cusp  214  of the Magenta circle  204  and the Yellow circle  202  and effectively bisects the Red circle  212 . 
     The three-sided area  226 , together with the central six-sided area  230  indicates the overlap of R, G, and B. Similarly, the three-sided area  228  indicates the overlap of C, M, and Y. The six-sided central area  230  indicates the overlap of R′G′B′ together with CMY. The overlap areas of overlap  226  and  228  are notionally gray/black. However, when CMY is used to reproduce black, the result is often unsatisfactory. 
     Preferably, the Cyan, Magenta and Yellow inks are mono pigmented. However, the Red, Green and Blue inks should be made up of two (2) pigments maximum. 
     L*a*b* data is another three-parameter color notation which is commonly used. L is luminance, and a* &amp; b* are chrominance values ranging from Red to Green and Blue to Yellow respectively. In generating CMYK color separations, an initial set of RGB color separations can be derived from an image. These can then be converted to L*a*b* data using ICC profiles. The L*a*b* data can then be converted to CMYK using ICC profiles. 
     The CMYK data (i.e. pixel data in the CPU) has a dynamic range of 256 gray-levels (2.41 d). The invention enables the R′G′B′ data to be derived from the secondary CMYK (256 gray-level) data. U.S. Pat. No. 5,751,326 refers to the CMYK data set as a result of tone compression to a printable density range. The printable density range quoted is 100 gray-levels (or 2.00 d) per primary color after the CMYK (256 gray-levels) is compressed to 100 gray-levels using halftone screening. 
       FIG. 3  illustrates the process of U.S. Pat. No. 5,751,326. The original image (1000 gray levels (3.00 d)) is scanned at  304  to produce the CMYK data at 256 levels (2.41 d). This is subjected to a 2.56:1 compression at  308  to produce the compressed CMYK data having 100 levels at  308 . 
     A second scan is carried out at  310  to produce the R′G′B′ data having 256 levels. This is in turn subjected to halftone compression of 2.56:1 at  712  to produce R′G′B′ data having 100 halftone levels at  314 . Thus the R′G′B′ data is derived from the source image scanned at  310 . 
     The inventive color separation method can derive complementary R′G′B′ data from CMYK data as shown in  FIG. 4 . Master data, for example L*a*b* data, having a dynamic range of 256 gray levels (2.41 d) is provided at  402 . The L*a*b* data in turn can be derived from initial RGB data from an original image. The L*a*b* data is then converted to CMYK data having 256 gray levels (2.41 d) at  404  using a standard ICC profile. The CMYK data is then subjected to halftone compression of 2.56:1 at  406  to provide the CMYK halftone separation data with 100 gray levels (2.00 d) at  408 . 
     As shown in  FIG. 4 , the 256 level R′G′B′ data at  410  is derived from the 256 level CMYK data derived at  404 . Thus the CMYK data from which the R′G′B′ data is derived at  410  has not undergone the dynamic range loss which is incurred in the final CMYK halftone compression step  406 . 
     The 256 gray level data at  404  is also manipulated to provide the complementary R′G′B′ data having 256 gray levels at  410  using, for example, an algorithm or lookup table. The 256 gray level R′G′B′ data is then subjected to halftone compression at  412  to produce R′G′B′ halftone separation data with 100 gray levels at  414  for use in a printing process together with the CMYK halftone separations at  408 . 
       FIG. 5  illustrates a generalized process for generating a complementary data set according to an embodiment of the invention. 
     A first color separation data set, for example a CMYK data set is shown at  502 . The CMYK data set  102  includes four data fields,  504 ,  506 ,  508 ,  510  for the C, M, Y, and K data respectively for each pixel of an image. This data may have been derived from L*a*b* data using a standard ICC profile. 
     At step  512 , the M and Y data is subjected to a first process, such as an algorithm or look up table, to produce (-GB) data. 
     At step  514 , the C and Y data is subjected to a second process to produce (-RB) data. 
     At step  516 , the C and M data is subjected to a third process to produce (-RG) data. 
     The (-GB) data from step  512  is then used to provide R′ data at step  518 , the (-RB) data from step  514  provides G′ data at step  520 , and the (-RG) data provides B′ data at  522 . 
     The CMYK data  502 ,  504 ,  506 ,  508  can have 256 gray levels, and this is converted to R′G′B′ data having 256 gray levels at  512 ,  514 ,  516 ,  518 ,  520 ,  122 . 
     Both the CMYK data  504 ,  506 ,  508 , and the R′G′B′ data  518 ,  120 ,  522  can then be subjected to halftone compression to 100 gray level data for use to control a printing process. In this arrangement, a seven (7) ink reproduction of the original image can be reproduced. 
     Preferably, the data is stored as a data set, and can be utilized subsequently. However, in some cases, the data may be generated as streaming data and processed “on the fly”. 
       FIG. 6A  illustrates a second embodiment of the invention in which the Black data from the CMYK data set is replaced by R′G′B′ data. The R′G′B′ data representing the Black data from the CMYK data can be combined with the complementary R′G′B′ data derived from the CMYK data. 
     In  FIG. 6A , the CMYK data set  602  contains the fields  604 ,  606 ,  608 ,  610  representing the C data, the M data, the Y data, and the K (Black) data respectively. As in the process illustrated in  FIG. 5 , the (-GB) data, the (-RB) data and the (-RG) data are derived from the CMYK data at steps  612 ,  614 , and  616 . However, the K data is also converted to R′G′B′ data at  611  to produce a Red component Kr, a Green component Kg, and a Blue component Kb, and these components are also combined with the associated (-GB), (-RB), and (-RG) data to produce the combined complementary data and substitute data at  618 ,  620 , and  622 . 
       FIG. 6B  illustrates the process of generating first R′G′B′ data  617 ,  619 ,  621  from CMYK data and then adding the second R′G′B′ values  611  from the K data to the first R′G′B′ data to produce the combined R′G′B′ data at  618 ,  620 ,  622 . 
     In the arrangement of  FIG. 6A , there are two (2) data sets. The first data set incorporates the CMY data, while the second data set incorporates both the complementary R′G′B′ data and the substitute date Kr, Kg, Kb. Again, this data can be compressed from 256 to 100 gray levels (2.56:1) for a printing process. Thus this enables the image to be reproduced using only six (6) process inks, CMY and R′G′B′. 
     In one embodiment of the invention, a software program can be written to perform the Red, Green and Blue color separation method. Its functionality should include: 
     A) a dynamic range compression function able to compress pixel information from 0-255 gray levels to 0-0 gray levels; 
     B) a selective color correction function able to identify Red (Magenta and Yellow), Green (Cyan and Yellow) and Blue (Cyan and Magenta) pixel information in the CMYK data set in the dynamic range 0-255 gray levels; 
     C) a multiply pixel function able to multiply pixel information in the dynamic range 0-255 gray levels; 
     D) a duplicate pixel function able to duplicate pixel information within a dynamic range 0-255 gray levels. 
     The software programming functions A, B, C, and D are commonly known in the printing and graphic arts industry to perform the art of CMYK color separation. 
     As a preferred embodiment, a software R.I.P. could be programmed to execute steps A, B, C, D. 
     The programming steps to create Red, Green and Blue color separations for a print process can include the following: 
     Step 1: A gamma curve (ratio 1:2=gamma 2.0) is applied to restrict the CMYK input dynamic range of 0-255 gray levels to an output dynamic range of 0-127 gray levels. In addition, the Black channel pixel information can be further edited by applying the dynamic range compression function: 0-255 to 0-0 as illustrated in  FIGS. 12 &amp; 13  illustrate gamma values of 1.0 and 2.0 respectively.  FIG. 17  is included for the sake of completeness to illustrate the compression of the K data form 0-255 to 0-0. 
     Step 2: After applying Step 1, a selective color correction algorithm is applied to record Red hues in the CMYK data wherein the Magenta and Yellow pixels intersect in the dynamic range 0-127. The Black data channel can be used to extrapolate the Red (Magenta and Yellow) data by applying a selective color correction function-Black in Reds with 100% opacity. The Red selective color correction mask will identify the intersecting Magenta and Yellow pixels in the dynamic range 127 m/0y&gt;0y/0m&gt;127y/0m. The cross-coupled Magenta and Yellow pixel data recorded within the Black channel can now be exported to a separate Alfa spot color channel. The Alfa spot color channel containing the Magenta and Yellow pixel data is renamed “Red” color separation data for a printing process. The software program recalls the original CMYK data thereafter, as illustrated in  FIG. 14 . 
     Step 3: After applying Step 1, a selective color correction algorithm is applied to record Green hues in the CMYK data wherein the Cyan and Yellow pixels intersect in the dynamic range 0-127. The Black data channel can be used to extrapolate the Green (Cyan and Yellow) data by applying a selective color correction function-Black in Greens with 100% opacity. The Green selective color correction mask will identify the intersecting Cyan and Yellow pixels in the dynamic range 127c/0y&gt;0c/0y&gt;127y/0c. The cross-coupled Cyan and Yellow pixel data recorded within the Black channel can now be exported to a separate Alfa spot color channel. The Alfa spot color channel containing the Cyan and Yellow pixel data is renamed “Green” color separation data for a printing process. The software program recalls the original CMYK data thereafter, as illustrated in  FIG. 15 . 
     Step 4: After applying Step 1, a selective color correction algorithm is applied to record Blue hues in the CMYK data wherein the Cyan and Magenta pixels intersect in the dynamic range 0-127. The Black data channel can be used to extrapolate the Blue (Cyan and Magenta) data by applying a selective color correction function-Black in Blues with 100% opacity. The Blue selective color correction mask will identify the intersecting Cyan and Magenta pixels in the dynamic range 127m/0c&gt;0c/0m&gt;127c/0m. The cross-coupled Cyan and Magenta pixel data recorded within the Black channel can now be exported to a separate Alfa spot color channel. The Alfa spot color channel containing the Cyan and Magenta pixel data is named “Blue” color separation data for a printing process. The software program recalls the original CMYK data thereafter, as illustrated in  FIG. 16 . 
     Step 5: The original Black channel pixel information is combined with the Red, Green and Blue channel pixel information by applying a tonal transfer to the Black channel pixel information as per the Black converter L.U.T. as illustrated in  FIG. 8 . The Black channel pixel information is then duplicated and combined, via the multiply pixel algorithm, into all three (3) Red, Green and Blue color separation data channels. The multiply pixel algorithm is set to 100% opacity. 
     The chart of  FIG. 10  illustrates the process of multiplying two (2) sets of data  1010 ,  1012 , the product being the curve  1014 . 
     The Black channel pixel information is deleted from the CMYK data set by applying a dynamic range compression function: 0-255&gt;0-0, as described with reference to  FIG. 17 . 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 CMY-R′G′B′ Look up Table (Halftone %) 
               
             
          
           
               
                   
                 INPUT % 
                 OUTPUT % 
               
               
                   
                   
               
               
                   
                 CMY (x2) 
                 R′G′B′ (y1) 
               
               
                   
                 100 
                 100 
               
               
                   
                 90 
                 80 
               
               
                   
                 80 
                 60 
               
               
                   
                 70 
                 40 
               
               
                   
                 60 
                 20 
               
               
                   
                 50 
                 0 
               
               
                   
                   
               
               
                   
                 R′G′B′ output gamma curve: ratio 1:2 = 2.0 
               
               
                   
                 x1 = Highest C, M or Y value. 
               
               
                   
                 x2 = 2nd highest C, M or Y value at or higher than 50% (gray level 127). 
               
               
                   
                 y1 = R′G′B′ saturation value. 
               
             
          
         
       
     
     The chart of  FIG. 11  illustrates the corresponding graph for Table 1. 
     The R′G′B′ saturation value for any given Red, Green or Blue hue is determined from the 2nd highest (x2) C, M or Y pixel value at or higher than 50% (gray level 127). 
     Below are several examples of R′G′B′ output data derived from CMY input data: 
     x1=100c 
     x2=80m&gt;y1=60(Blue) 
     x1=80y 
     x2=70c&gt;y1=40(Green) 
     x1=100m 
     x2=100y&gt;y1=100(Red) 
     x1=50m 
     x2=30y&gt;y1=0(x2 is below 50%) 
     R′G′B′ Desaturation Point 
     When the 3rd highest C, M or Y (x3) pixel value reaches 50% or above (gray level 127), the R′G′B′ (y1) saturation value is reduced as the Red, Green or Blue hue becomes desaturated (neutralized). 
     x1=Highest C, M or Y value 
     x2=2nd highest C, M or Y value (above 50%) 
     x3=3rd highest C, M or Y value (above 50%) 
     y2=R′G′B′ value (calculated from x2) 
     y3=R′G′B′ value (calculated from x3) 
     y2−y3=y1 
     For Example: 
     x1=100c 
     x2=80y&gt;y2=60 
     x3=70m&gt;y3=40 (y2)−(y3)=20(Green) 
     x1=100c 
     x2=100 m&gt;y2=100 
     x3=80y&gt;y3=60 (y2)−(y3)=40(Blue) 
     x1=80c 
     x2=70m&gt;y2=40 
     x3=70y&gt;y3=40 (y2)−(y3)=0(Neutral) 
     The conventional Cyan, Magenta and Yellow color separation data channels will print Cyan, Magenta and Yellow ink respectively. The Red, Green and Blue color separation data channels will print a combined saturation and neutral tone density image with Red, Green and Blue inks respectively. The Black (K) color separation data channel and Black printing ink may still be used in any printing process where the Black substitution is not performed. 
     A computer with a Central Processing Unit (C.P.U.) and the required software R.I.P. can be programmed to perform the fore mentioned steps to create Red, Green and Blue color separations for a printing process. 
       FIG. 7A  illustrates an end-to-end process for producing color separation data from an original RGB separation data derived from an original image. The process provides a method of producing secondary and tertiary color separation data sets from a primary color separation data set, the tertiary color separation data being derived from the secondary color separation data set. The secondary and tertiary data sets are available to control color reproduction in a printing process. 
     At step  702 , RGB data is obtained from a digital color image of a selected color image, and this is then be converted to L*a*b* data at step  704  using a first algorithm. A standard ICC algorithm can be used in this conversion. The L*a*b* data serves as a primary data set from which the secondary data set is derived. 
     At step  706 , the secondary data set, a CMYK data set in this example, is derived from the L*a*b* data set again using a standard ICC profile. This results in the individual C, M, Y, and K data fields  708 ,  710 ,  712 ,  714 . As in the previous embodiments, the M and Y data is combined to produce (-GB) data, the C and Y data is combined to produce the (-RB) data, and the C and M data is combined to produce the (-RG) data at  724 ,  726 , and  728 . At  724 , the M and Y data are combined to produce a (-GB) data value which is then converted to an equivalent Red value R′ for the tertiary data set at  730 . This value can be used to partially compensate for the data lost from the compressed halftone CMYK data. Similarly, at  726 , C and Y are combined to produce a (-RB) value which is converted to a G value at  732  for the tertiary data set. Again, this value is used to partially compensate for lost data from the secondary compressed halftone CMYK data set. At  728 , the C and M values are combined to produce a (-RG) value which is converted to a B value for the tertiary data set at  734 , and this value also is used to compensate for information lost from the from the compressed halftone CMYK data set. 
     At the same time, the Black data K from  714  is converted to its constituent R′G′B′ components Kr, Kg, Kb, as intermediate data  718 ,  720 ,  722  to substitute for the K data, and the intermediate data components are combined with the resultant (-GB), (-RB) and (-RG) data at  724 ,  726 ,  728  to produce the combined complementary and substitute data output  730 ,  732 ,  734 . This method produces six (6) color data similar to that produced by the process of  FIG. 2 , being the CMY data at  308 ,  310 ,  312 , and the R′G′B′ data at  324 ,  326 ,  328  which can be subjected to halftone compression and used to produce a reproduction of the initial RGB image. 
       FIG. 7B  illustrates a process similar to that of  FIG. 7A  in which the intermediate steps of converting RGB to L*a*b* and converting L*a*b* to CMYK are omitted. Thus the RGB at  702  is converted directly to CMYK  705 . 
       FIG. 8  shows a table illustrating digital conversion of the Black (K) color separation data into Red, Green and Blue color separation data via a Look Up Table (L.U.T.)  802  according to an embodiment of the invention. A graph representative of the L.U.T. is shown in  FIG. 9  at  904  with the K values represented on the abscissa and the R value shown on the ordinate. G and B values correspond to the R value as shown in the table. 
     The grayscale value for the specific Black information of the corresponding pixel is matched to the corresponding R′G′B′ values from the look up table. This Red, Green and Blue color separation data is then digitally merged into the Red, Green and Blue color separation data channels as per  FIG. 6A ,  6 B or  7 A &amp;  7 B. The Black (K) color separation data is deleted after it has been converted via a L.U.T. into Red, Green and Blue color separation data. 
       FIG. 18  illustrates an alternative method of deriving the complementary R′G′B′ data according to an embodiment of the invention. At  1804 , CMYK data at  256  gray levels is generated from the L*a*b* data at  1802 . and subjected to halftone compression to 100 gray level data at  1806 ,  1808 . CMYK difference data is generated at  1822  by determining the difference between the data at  1804  and the data at  1808 , and this CMYK difference data is then converted to R′G′B′ data at  1810  and then compressed to halftone data at  1812 ,  1814  for use in the printing process. 
     It will be apparent that changes in, and modifications to the invention may be made without departing from the spirit and scope thereof. 
     While the term “data set” is used in the specification, the term is to be understood as being capable of encompassing ephemeral, real time data as well as static or stored data. Similarly, a reference to “data” encompasses static or stored data, such as a data set, unless the context requires otherwise. 
     Where ever it is used, the word “comprising” is to be understood in its “open” sense, that is, in the sense of “including”, and thus not limited to its “closed” sense, that is the sense of “consisting only of”. A corresponding meaning is to be attributed to the corresponding words “comprise”, “comprised” and “comprises” where they appear. 
     It will be understood that the invention disclosed and defined herein extends to all alternative combinations of two (2) or more of the individual features mentioned or evident from the text. All of these different combinations constitute various alternative aspects of the invention. 
     While particular embodiments of this invention have been described, it will be evident to those skilled in the art that the present invention may be embodied in other specific forms without departing from the essential characteristics thereof. The present embodiments and examples are therefore to be considered in all respects as illustrative and not restrictive, and all modifications which would be obvious to those skilled in the art are therefore intended to be embraced therein. The invention is further set forth in the following claims.