Method for calibrating and converting CMYK colors

Abstract

A method for calibrating and converting colors of a digital image into printer specific colorant control values is disclosed. This method comprises the steps of calibrating a specific printer of a printer type according to a printer type specific procedure, wherein for each colorant separately a one-dimensional relationship between a printer tone value and a colorant control value is established; profiling the printer type to establish an output profile that defines a relationship between color values in a profile connecting color space and the printer tone values; deriving from the input profile and the output profile a one-dimensional relationship between a black channel and the black printer tone value of the printer; determining if an image color value is to be converted either by using the profiles or by using the one-dimensional relation; converting the image color value and subsequently using the printer calibration relations to obtain the colorant control values. This procedure enables the use of G7 colors in a printer that is limited in its amount of colorant that can be applied locally.

Claims

1. A method for calibrating and converting colors of a digital image into printer specific colorant control values, the method comprising the steps of: a) calibrating a specific printer of a printer type according to a printer type specific procedure, wherein for each colorant separately a one-dimensional relationship between a printer tone value and a colorant control value is established; b) profiling the printer type to establish a profile combination that defines a relationship between color values in an image color space and the printer tone values; c) deriving from the profile combination a one-dimensional relationship between a single color component in the image color value and a corresponding component in the printer tone value; d) determining if an image color value is to be converted either with the profile combination or with the one-dimensional relation, and e) converting the image color value as determined in step d) and subsequently with the calibration relations established in step a) to obtain the colorant control values, wherein the single color component is a black component, and wherein the one-dimensional relationship is derived by the steps of: c1) determining a lightness corresponding to an input black channel value; c2) determining an equivalent G7 channel value for this lightness in dependence on the no-coverage lightness and the 100%-coverage lightness on the input side; c3) determining an output lightness for the G7 channel value in dependence on the no-coverage lightness and the 100%-coverage lightness on the output side; and c4) determining a black printer tone value from the output lightness.

2. The method according to claim 1, wherein the step of determining how an image color value is to be converted (step d) comprises a step of determining if all but the black component in the image color value are lower than a predefined threshold.

3. A non-transitory computer readable medium storing computer executable instructions for executing a method according to claim 1.

4. A printer comprising a printer controller and a print engine, the printer controller including a CPU that is programmed to: a) calibrate the print engine according to a printer type specific procedure, wherein for each colorant separately a one-dimensional relationship between a printer tone value and a colorant control value is established; b) profile the print engine to establish a profile combination that defines a relationship between color values in an image color space and the printer tone values; c) derive from the profile combination a one-dimensional relationship between a single color component in the image color value and a corresponding component in the printer tone value; d) determine if an image color value is to be converted either with the profile combination or with the one-dimensional relation, and e) convert the image color value as determined in step d) and subsequently with the calibration relations established in step a) to obtain the colorant control values, wherein the single color component is a black component, and wherein CPU is further programmed to determine a lightness corresponding to an input black channel value; determine an equivalent G7 channel value for this lightness in dependence on the no-coverage lightness and the 100%-coverage lightness on the input side; determine an output lightness for the G7 channel value in dependence on the no-coverage lightness and the 100%-coverage lightness on the output side; and determine a black printer tone value from the output lightness, in order to derive said one-dimensional relationship.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:

(2) FIG. 1 is a conversion scheme of a printer only applying a calibration according to a G7 specification;

(3) FIG. 2 is a conversion scheme according to the present invention, and

(4) FIG. 3 is a graphical outline of the way to find the bypass relation.

DETAILED DESCRIPTION OF EMBODIMENTS

(5) The present invention will now be described with reference to the accompanying drawings, wherein the same or similar elements are identified with the same reference numeral.

(6) In FIG. 1 an existing conversion scheme is shown. This conversion, following a G7 specification (ANSI CGATS/IDEAlliance TR015-2015) comprises four single colorant transfer curves 2, indicated by LUT.sub.G7, wherein a CMYK color as given in an input image 1 is component by component converted into printer control values. These input colors have four components, C.sub.i, M.sub.i, Y.sub.i, and K.sub.i, which is considered to be a set S.sub.i. The color signal that is sent to the printer 3 are the colorant control values C.sub.p, M.sub.p, Y.sub.p, and K.sub.p, or in short S.sub.p. Since S.sub.p is different for each individual printer 3, a full, complex calibration is needed to determine the mutual dependent transfer curves. In particular, it is not possible to follow this calibration procedure if a TAC limit is set on the print process of the printer 3.

(7) In FIG. 2 a color conversion scheme according to the present invention is shown. A color of the input image 1 with color components C.sub.i, M.sub.i, Y.sub.i, and K.sub.i, is examined for its optimal conversion path in decision box 4. If the color is to be rendered with black colorant only (4, path Y), in this case by checking if C.sub.i=M.sub.i=Y.sub.i=0, the output values C.sub.o, M.sub.o, Y.sub.o, are set to zero in bypass box 8 and only a K.sub.o output value is passed to the printer calibration box 9. Otherwise (4, N) the input image color is passed to an appropriate input Color Matching Profile (CMP.sub.i) 6 and output Color Matching Profile (CMP.sub.o) 7, wherein the CMYK value is interpreted as a physical color in a Profile Matching Color Space (PMS), in this case L, a, b, and converted to an output color value for the CMYK printer 3. The output color value is appropriate for any printer 3 of the same type, as long as the printer is calibrated according to a fixed procedure. The selected profile may be one of a set of profiles, taking into consideration a rendering intent and a media family to which the receiving substrate in the printer belongs. The calibration box 9 comprises four transfer curves (LUT.sub.C etc.), which are one-dimensional relations between an input and output value, that are established by a calibration procedure that takes one colorant at a time into consideration. This differs from the procedure as needed for the conversion scheme of FIG. 1, in that a low TAC can be observed. No compound colors are necessary in this calibration. The relation between K.sub.i and K.sub.0 in bypass box 8 is established along a line of steps, graphically shown in FIG. 3.

(8) FIG. 3 comprises four graphs, labelled A to D. Graph A indicates a relation 20 between an input value K.sub.i on a horizontal axis 11, running from 0 to 1, and a corresponding lightness L.sub.i on a vertical axis 10, running from 0 to 100. This relation is obtained from the input profile CMP.sub.i and defines a lightness value W.sub.i associated with an input medium without colorant and a lightness B.sub.i associated with 100%, or a maximum amount of black. Graph B indicates a relation 21 between a black tone value on the horizontal axis and a lightness on the vertical axis according to the G7 specification, given the values W.sub.i and B.sub.i as found from the profile CMP.sub.i, following the dotted lines 12 and 13 The tone value K.sub.G7 is found for the particular value L.sub.i. Graph C shows the relation 22 between the tone value and the lightness according to the G7 specification using the values W.sub.o and B.sub.o that are obtained from the output profile CMP.sub.o for 0% and 100% black tone value on the output side. For the specific value K.sub.G7 an output lightness L.sub.o is found, which is used in graph D, showing the output relation 23 between the black tone value and the lightness as represented in the output profile CMP.sub.o, to obtain an output tone value K.sub.o. Thus, a relation between an input value K.sub.i and an output value K.sub.o can be obtained that is used in the bypass box 8 as LUT.sub.bp. It is understood that the shown relations are exaggerated to demonstrate the workings of the individual steps. If a G7 verification test is conducted and a G7 compatible input profile is used, the printer 3 will respond as if calibrated according to the G7 specification.

(9) Thus, a color conversion path is defined that prevents the printer from color recipes that exceed a TAC limit on the one hand and obeys a predefined behaviour, such as G7 behaviour, on the other hand. It is noted that a number of conversions as indicated in this scheme separately may be combined to provide a faster processing. This does not change the content of the described processing steps.

(10) The skilled person will recognise that other embodiments are possible within the scope of the appended claims.