The present invention relates to computerized color graphics, color reproduction, and electronic printing systems. In particular, the present invention relates to a method and apparatus for characterizing colorants such as inks and predicting the color appearance of placing one or more colorant layers on top of a substrate such as paper or film as occurs, for example, in printing a page and photography.
Computerized color graphics systems and electronic printing systems are known in the art. Typically, they enable a user to produce a color image (any visual two-dimensional pattern including text, graphic line art, continuous tone images, etc.), and from that image to produce a picture which can be printed via a color reproduction system, for example, by producing color separation plates for offset printing. There has been much effort in the past to develop ways to accurately predict or simulate the appearance of such images when printed on a substrate, for example paper or film, using a number of colorants, such as inks, the prediction carried out without actually printing the images.
Printing may be carried out using halftoning, also called screening, which is the process of creating the illusion of a continuous tone (xe2x80x9cCT,xe2x80x9d xe2x80x9ccontonexe2x80x9d) image using an output (e.g., printing) device capable only of binary output (ink deposited or not deposited at any location on a substrate). For color printing, several images (xe2x80x9cseparationsxe2x80x9d) are produced in the primary colorants (typically inks) used to print in color, and printed over each other in a press. For typical four color printing, four images are produced in cyan (xe2x80x9cCxe2x80x9d), magenta (xe2x80x9cMxe2x80x9d), yellow (xe2x80x9cYxe2x80x9d) and black (xe2x80x9cKxe2x80x9d), and each of these images are halftoned. Usually, digital halftoning is used together with an imagesetter, laser printer, ink jet printer, digital film recorder, or other recorder output device.
It is desirable to be able to calculate accurately how a picture will look when one prints an image, including a halftoned continuous tone color image, with a certain technique on a certain substrate using a certain set of colorants (e.g., inks). A method of predicting the color appearance can be used for example to display a simulation of the color appearance on a computer display, or to print a simulation of the color appearance on a more easily accessible and cheaper printer as a proof of what is to be printed finally in production.
It is desirable to do this for both reflection printing on an opaque substrate, and transmission imaging on a transparent substrate.
By accurately calculating we mean a maximum deviation between predicted color and actual color of the order of 5 CIELAB Delta E units, and an average deviation of about 2 CIELAB Delta E units.
Various methods are known for calculating the color resulting from superimposing a set of colorant layers on a substrate. These methods can be divided in two groups. The first group are characterized by requiring printing and measuring a relatively large number of overprints of the colorants. That is, this group includes methods that for a particular set of colorants, a particular printing technique (e.g., offset printing on a particular imagesetter), a particular substrate type (e.g., paper, or film for a photographic transparency or print, textile, sheet of plastic, etc.), a particular substrate color (e.g., the color of the paper, or of the transparent film in the case of a transparency, or of the textile, or of the plastic sheet, etc.) and a particular order of printing the colorants, involve printing a relatively large number of overprints of the colorants in the set, for example as patches. These patches are measured with a spectrophotometer or calorimeter and the measurements are used to calculate any overprint of colorants in the set using mathematical techniques, for example, interpolation. If carried out well and carefully, these methods can lead to accurate results. Modem color management techniques such as COLORSYNC(trademark) (Apple Computer, Inc., Cupertino, Calif.) and the methods promoted by the International Color Consortium (ICC, see http://www.color.org) use such techniques. While these techniques can produce accurate results, and also work for halftone images, there are several drawbacks with such methods. One is that a large number of color patches of overprints need to be made. For example, the IT8.7.3 chart (American National Standards Institute [ANSI] Committee IT8 for Digital Data Exchange Standards) contains nearly a thousand patches for a four color output. Hence it is very difficult to characterize sets of more than four colorants, for example printing with six or seven colors (xe2x80x9cHI-FIxe2x80x9d printing including PANTONE(copyright) Hexachrome from Pantone, Inc., Carlstadt, N.J.). There also are applications where inks other than cyan, magenta, yellow and black need to be used. Another drawback is that a set of patches will only be useful for accurately calculating overprints using the particular colorants and the particular colorant printing order used in the patches. Changing one colorant in the colorant set or changing the order of overprinting typically requires redoing the whole job of printing the set of patches.
An additional problem occurs when using such characterizations with more than four inks in a typical modem color management workflow, such as the ICC workflow. The ICC standard uses look up tables and interpolation to predict the CIELAB values of a particular output device. The number of nodes in these lookup tables increases exponentially with the number of inks, and the amount of computer resources becomes too high to be feasible.
Also included in this first group of known methods are those that use the well known Neugebauer equations to predict the color of an overprint of colorants, and their derivative based on Yule-Nielsen and spectral Neugebauer equations (See H. E. J. Neugebauer: xe2x80x9cDie theoretischen Grundlagen des Mehrfarbenbuchdrucksxe2x80x9d, Zeitschrift fur wissenschaftliche Photographie, Photophysik und Photochemie, Band 36, Heft 4, April 1937; J. S. Arney, C. D. Arney:xe2x80x9d Modeling the Yule-Nielsen Halftone effect,xe2x80x9d Journal of imaging science and technology, vol. 40, No. 3, pp. 233-238, June 1996; R. Rolleston and R. Balasubramanian: xe2x80x9cAccuracy of Various Types of Neugebauer Modelxe2x80x9d, ISandT and SID""s Color Imaging Conference: Transforms and Transportability of Color, pp. 32-37, 1993). These Neugebauer-like models still need the knowledge of the color of the overprints of the primary inks, so still requires measurements overprint combinations of the colorants and the gradation steps of the colorants. Also, Neugebauer equations-based methods are known not to produce accurate results.
A second group of prior art methods are those that determine spectral characterizations of individual colorants that can be used for predicting the color of overprints of so-characterized colorants. These methods, for a fixed substrate and printing technique, involve making one or more printouts of each colorant on one or more substrates, measuring the prints, and out of this data extracting a set of one or more parameters for each colorant that can be used to calculate an overprint of each colorant. These methods thus have the advantage of not requiring producing a large set of overprints. One such prior art method uses the two-parameter Kubelka-Munk method which describes an ink wither by one spectral parameter, (K/S)(xcex), or by two spectral parameters, scattering S(xcex) and absorption xcex1(xcex), where xcex is the wavelength. See James H. Nobbs: xe2x80x9cKubelka-Munk Theory and the Prediction of Reflectancexe2x80x9d, Rev. Prog. Coloration, Vol. 15, pp. 66-75, 1985.
Determining the two colorant parameters involves measuring the spectrum on a bare substrate and on a black substrate, and solving the resulting equations. There also exist in the literature refinements on the two-parameter Kubelka-Munk theory that incorporate internal reflection, anisotropic scattering and other second order effects. However, these methods are applicable only for full (i.e., 100%) coverage of a layer of ink of a particular thickness, and/or are not applicable for variable ink coverages, for example, halftoning at less than 100% dot percentage. In addition, the colorant parameters they determine are neither substantially independent of the lowest substrate color nor are capable of incorporating dependencies on other ink layers. Thus these parameters are not easily applied to predicting the appearance of sequentially applied inks at less than 100% coverage wherein the substrate with the previously deposited inks may be regarded as a new substrate, with characterizable influence from layer to layer.
In order to so predict the appearance of overprints, it is advantageous to spectrally characterizing colorants by parameters that are substantially invariant for all substrates of the same type, and independent of the substrate color, in that, when there is no interaction between colorant layers, one can treat a substrate with one or more colorant overprints on it as a raw colored substrate of the same type but a different color, that different color dependent on the one or more colorant overprints.
In summary, prior art techniques that are capable of accurately calculating overprints of inks, including rasterized (i.e., halftoned) inks, require making a number of overprints of the colorants, so do include determining spectral parameters of a colorant that spectrally characterize the colorant. Furthermore, prior art techniques that do spectrally characterize the colorants are not applicable to less than 100% coverage, do not produce parameters that are substantially independent of substrate color, and do not produce parameters that can take into account interactions between the colorant layers.
Thus there is a need in the art for a method of characterizing colorants capable of accurately predicting the color of an overprint of the colorants without requiring measurement of overprints of the colorants and capable of predicting the color of overprints of halftones.
The Parent Invention describes such a method for determining and using colorant parameters that are substantially independent on the substrate color. The use of these parameters is described when there is no interaction between the layers. At least two parameters are needed for non-scattering parameters and at least three parameters are needed for scattering colorants. Such colorant parameters also may be used when there is some interaction between the layers of colorants. Parameters in such a case also may be determined that are capable of characterizing some interdependency of the colorants deposited, so that even when there is interaction between layers, one can treat a substrate with one or more colorant overprints on it as a raw colored substrate of the same type. Needed is a more detailed description of how the Parent Invention is applicable to this case of the colorant layers influenced each other. That is, the effect of any colorant being influenced by one or more of the inks printed before or after in an overprint.
Examples of when the effect of a colorant (e.g., ink) may be so dependent on inks earlier printed on which the new ink is to be printed, include phenomena such as xe2x80x9cink trappingxe2x80x9d in which an ink is xe2x80x9ctaken upxe2x80x9d less by the substrate when another ink has first been printed onto this substrate. Such a phenomenon could occur, for example, in offset printing. Another case is when the parameters of one or more inks already deposited onto a substrate are influenced on a new ink being deposited on top of these previously deposited inks. A lower ink may xe2x80x9cspreadxe2x80x9d as a result of additional inks deposited on top, so that the area the lower ink covers is increased. This is called xe2x80x9cspreadingxe2x80x9d herein, and might occur, for example, with ink-jet printing. Other examples when one colorant may be dependent on one or more overlying or underlying colorant layers include an ink-re-transfer-like phenomenon which could occur, for example, in dye sublimation printing, and mixing of the inks, which can occur in many printing techniques.
In the cases of xe2x80x9ctrappingxe2x80x9d and xe2x80x9cspreadingxe2x80x9d, the perceived ink quantity (halftone coverage percentage) also may change from the amounts laid down because of such interdependencies.
Thus there is a need in the art for a method for characterizing the spectral properties of colorants overprinted at a coverage percentage which may be less than 100%, on a reflective or transmittive substrate, the characterizing being by a small number of colorant parameters that are a function of the coverage percentage and that are substantially invariant to the substrate color for all substrates of a particular substrate type and for a particular printing technique, such colorant parameters capable of substantially incorporate the dependency on the colorants deposited before and after. Also, there still is a need in the art for a method and apparatus for accurately predicting the color of printing a set of colorants at any coverage percentages on a reflective or transmittive substrate, the method not requiring making and measuring overprints of the colorants, and applicable to overprinting where each colorant is laid at less than 100% coverage, for example using halftoning.
An object of this invention is a method for characterizing the spectral properties of colorants such as inks when printed on a substrate of a particular type by a small number of parameters which are independent of the base substrate color and that can incorporate interaction between other colorants printed on the substrate. A further object of this invention is a method and apparatus which use such characterization for predicting the color of printing a set of colorants such as inks on a substrate. Other objects would be clear from the description below.
These and other objects of the invention are provided for in a method and apparatus for determining a set of colorant parameters that spectrally characterize a colorant in order to predict the color spectrum of an overprint of the colorant deposited using a printing technique at a coverage percentage on a substrate of a substrate type, the set of colorant parameters substantially independent of the base substrate color and capable of incorporating mutual dependencies of the colorant with other colorants deposited on the substrate. For the example of offset printing using halftoning, these colorant parameters are a function of the dot percentage and wavelength.
At least two parameters are determined for characterizing a non scattering colorant, and at least three are determined for a colorant that has scattering properties. When mutual interaction of colorant layers in an overprint is possible, at least six parameters are determined for characterizing a non scattering colorant, and at least eight are determined for a colorant that has scattering properties.
In one aspect of the invention, the method includes making a number of sets of prints of the colorant at a range of coverage percentages on different background colors, for example, different dot percentages for the case of offset printing. To incorporate mutual interaction, more parameters need to be determined, so additional set of prints are made. For example, three sets of prints are made on three backgrounds for non scattering colorants and on four backgrounds for scattering colorants and in addition, if necessary to obtain a sufficient number of sets of prints on different backgrounds to determine the parameters, one or more similar sets of printouts are overprinted with one or more overprinting colorants.
The spectra of the prints, for example, the reflection spectra in the case of offset printing, are measured, and equations in the set of parameters are formed and numerically solved for the parameters. The backgrounds are, in one embodiment, two lightly colored backgrounds, a medium colored background, and, for the case of four sets of prints, a dark colored background and any overprinting is done with a lightly colored colorant. In one implementation for offset printing, the four backgrounds are formed by pre-printing the backgrounds, for example, in the case of the lightly colored background using 25% halftone dots, for the medium dark background, using 50% halftone dots with black ink, and in the case of the dark background used for the case of a colorant that has a scattering component, using 100% black ink, the overprinting happens for instance with a 100% colorant in a second set of prints. For coverage percentages for which prints are not made, interpolation is used to determine the parameters for such colorant coverages.
Some colorants, called derived colorants, can be defined by a recipe of concentrations of basic inks. In another aspect of the invention, the set of colorant parameters of a derived colorant is determined by making measurements on a set of dilutions of each of the basic colorants on several background colors on one or more substrates of the same substrate type, a set of dilutions in this sense possibly including the undiluted basic colorant. The prints that are made of each of the dilutions at different colorant coverages are the same as would be made to determine the colorant parameters of each of the dilutions of the basic colorants. Measurements of the resulting spectra are made. From these, coefficients, for example Kubelka-Munk coefficients are determined for the dilutions at the coverage percentages, and, using interpolation, Kubelka-Munk coefficients are determined for the concentrations specified in the recipe of the derived colorant. Then, using a formula for mixtures of colorants using the coefficients, for example, the Kubelka-Munk theory for mixtures modified for all the printed coverage percentages, the spectra that would result by printing the derived colorant on the different backgrounds at the different coverage percentages are calculated without actually having to print the derived colorant. From these calculated spectra, using steps analogous to the case of determining the spectral parameters directly, the spectral parameters of the derived colorant are determined.
In another aspect of the invention, a method for using the parameters for determining a single overprint of a colorant on a substrate also is disclosed. Using the result that the colorant parameters are substantially independent of substrate color and can incorporate the effects of interdependency with other colorants, a method is disclosed for determining the spectrum of several overprints of several colorants by sequentially applying the method for one colorant. One first determines the coverage percentages xe2x80x98belowxe2x80x99 and xe2x80x98abovexe2x80x99 each colorant, i.e., for a particular colorant, those on which the colorant is deposited and those deposited over the colorant, and from these percentages, one modifies the colorant parameters and then determines the color of the colorants when deposited sequentially on the background in much the same way as if the colorant parameters were substantially independent of each other (see the Parent Invention). the reflection spectrum of an overprint of the first colorant on the raw substrate. One then treats the resulting print as a new substrate of the same type but a different color (the color given by the first obtained reflection spectrum) and in the same way determines the reflection spectrum of overprinting a second colorant on top of the first. One continues this process until one has determined the reflection spectrum of all the overprints of the several colorants.
Also disclosed is an apparatus for using the colorant parameters to determine the spectrum of overprints on a substrate of a set of colorants at a set of coverage percentages. The apparatus comprises a first memory for storing the spectrum of the substrate, a logic unit with inputs specifying the coverage percentages and a set of outputs which are the values of the colorant parameters (as modified by interaction between the colorant layers) at the coverage percentages. When inter-layer interaction may occur, some dot gain values also are input for a range of coverage percentages. The outputs of the logic unit are coupled to a combiner unit, which also has an input coupled to the first memory. The combiner unit determines the color spectrum of the overprints on the substrate of the set of colorants at the set of coverage percentages. One embodiment of the apparatus described comprises the logic unit being implemented as one or more lookup tables and an arithmetic unit. Another embodiment described comprises the logic unit being implemented as one or more interpolators for determining by interpolation the colorant parameters at the required coverage percentages for each colorant from values of the parameters at a set of fixed values of coverage percentages and an arithmetic unit that incorporates the calculation of the average coverage percentage below and above each colorant and the modification of the colorant parameters by these coverage percentages. The combiner unit is implemented as one or more arithmetic units, each arithmetic unit having an associated background spectrum as an input and a set of colorant parameters at a coverage percentage as another input and determining the spectrum of the overprint of that colorant on a substrate having the associated background spectrum. A parallel implementation is described in which the number of sets of interpolators in the logic unit is the number of colorants and the number of arithmetic units in the combiner unit also is the number of colorants. A serial implementation also is disclosed which is applicable to the case of no interaction between th layers and in which the logic unit comprises a single set of interpolators for a single colorant and the combiner unit comprises a single arithmetic unit for determining a single overprint. The operation of such a serial implementation is that the logic unit and arithmetic unit determine one overprint at a time in sequence, starting with an overprint on the raw substrate, and continuing with the second overprint being on the raw overprint with the first colorant, etc., until all overprints have been determined. After a number of cycles equal to the number of colorant overprints, the output of the arithmetic unit is the spectrum of all the overprints.
Also disclosed is a method and apparatus for simulating the appearance of overprints of a set of colorants on a display using colorant parameters determined according to one or more embodiments of the method of the invention for such determination. One embodiment of the method includes the steps of determining the spectrum of the overprints according to one or more of the above embodiments for determining the spectrum of overprints, and from the spectrum, determining the CIE-XYZ values of the overprint using CIE illuminant-observer weightings, and converting these XYZ values to RGB values to drive a CRT (or LCD, etc.) monitor using matrix multiplication and, in some embodiments, also one-dimensional lookup tables. Another aspect of the method includes using a matrix multiplier, one dimensional lookup tables, a multidimensional lookup table and interpolation for converting the XYZ values to the device dependent color values needed to drive a proof printer. One embodiment of the apparatus includes an apparatus for determining the spectrum of the overprints according to one or more of the above embodiments, and multiplier adders for determining the CIE-XYZ values of the overprint using CIE illuminant-observer weightings, and a matrix multiplier and, in some embodiments, also one dimensional lookup tables to produce RGB values to drive a CRT monitor. Another aspect of the apparatus includes a matrix multiplier, one dimensional lookup tables, a multidimensional lookup table and a multidimensional interpolator for converting the XYZ values to the device dependent color values needed to drive a proof printer.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments with the accompanying drawings.