BACKGROUND OF THE INVENTION
1. Field of the invention
The invention relates to a method for converting CMYK colours of a digital image into printer specific colorant control values, comprising steps of calibrating a specific printer and profiling a printer type.
2. Description of the Related Art
Digital printing offers a number of advantages over more traditional forms of printing, but also comes with a number of challenges. One of them is the creation of a reliable colour conversion method to convert colours in a digital image into a proper colorant control value that controls a printer to apply an appropriate amount of colorant. A reference frame for this conversion is defined by the International Colour Consortium (ICC, www.color.org), which is already a couple of decades old. Still there is still ample confusion around, which prompts the need to involve colour experts. It has also led to a certification practice for indicating whether a print provider keeps to an agreed standard.
In the ICC framework a colour profile for each colour processing device, such as input devices and output devices, is made and a Profile Connecting Space (PCS) is used to relate different devices. A printer is profiled as an output device by printing a number of single colorant and compound colorant test patches and measuring the colours of these test patches, using physical units, such as CIE XYZ and CIE Lab. In practice, a profile is often made for a printer type in dependence of an output media or media family, such that the profiling procedure does not have to be executed for each individual printer of this printer type. In that case it is only necessary for each individual printer to be properly calibrated in order to redress fabrication tolerances and obtain a defined printer output. Many calibration procedures will do the job, as long as a reproducible relation is obtained between a colorant control value that is used in a printer to control the application of a colorant, and a printer tone value that represents an optical effect of the applied amount of colorant. Such a relation is one-dimensional and is also known as a transfer curve. It is commonly implemented as a look-up table (LUT). In contrast, a profile is a relation between the three dimensions of a physical colour characterization, such as CIE Lab, and the printer tone values.
A standard calibration procedure has been devised for CMYK-printers. This calibration is known as G7 and is described in specification ANSI CGATS/IDEAlliance TR015-2015. This calibration aims at making all CMYK-printers respond similarly by adjusting the transfer curves for the individual colorants in such a way that a simultaneous application of predefined, almost equal amounts of cyan, magenta, and yellow in compound colours result in a neutral gray colour. A black transfer curve is also specified and a certification of a print provider complying with the G7 standard exists, involving a G7 verification.
A disadvantage of the G7 calibration is that it is a rather laborious procedure. A further, even larger disadvantage is that for some print processes, such as ink jet print processes, it may be impossible to apply simultaneously the required amounts of colorant during calibration, as the receiving media may be unable to digest the large colorant amounts. Since the neutral gray colours require both cyan, magenta, and yellow colorant, the sum of these colorant amounts may be 300%, thus easily passing the total area coverage (TAC), which is a limit for the total amount of colorant that may be applied. In that case it is not possible to perform the G7 calibration procedure. In order to make such a printer still respond as G7 calibrated and pass the G7 verification test, a single colorant calibration procedure is applied together with a profiling procedure, establishing individual colorant transfer curves and an output profile. Combining this output profile with a G7 compatible input profile, a system response is obtained that satisfies the requirements of the above-mentioned specification. Such a single colorant calibration is e.g. a so-called Distance-to-White (DTW) calibration, wherein a printer tone value for a single colorant varies linearly with the colour distance in CIE Lab coordinates between a produced colour and the no-coverage colour, which is the colour of a medium without colorant, most often a white colour. This method of calibrating and profiling is a convenient way to make a printer on the one hand respond as a G7 calibrated printer and on the other hand apply single colorant calibration.
After finishing the calibration and the profiling procedure, digital image colours are converted first by an input profile, that converts image colours into PCS colours, then by an output profile, that converts PCS colours into printer tone values, and subsequently by the calibration transfer curves, that converts the printer tone values into specific colorant control values. For digital image colours in CMYK, it is convenient to solve an ambiguity in the rendering of neutral colours as described in US patent 7,995,237
Herein a black preservation method is described that has the ability to preserve black-only content, i.e. the data within an image composed solely of black, including purely neutral gray levels between white and black. This method prevents the occurrence of combinations of C, M, and Y components to represent black-only content, which could lead to loss of fine detail, colour edges due to small misregistration or colour cast in neutral areas.
Combining this black preservation with the indicated calibration and profiling method leads to a problem in the gradation of black colorant, since the black-only input signals are not processed by the profile combination. It is an object of the present invention to provide a method to overcome this problem, thus preparing a printer to pass the G7 verification test.
SUMMARY OF THE INVENTION
In order to achieve this object, the method according to the invention comprises 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 a printer type to establish a profile combination that defines a relationship between colour values in an image colour space and the printer tone values;
- c) deriving from the profile combination a one-dimensional relationship between a single colour component in the image colour value and a corresponding component in the printer tone value;
- d) determining if an image colour value is to be converted either with the profile combination or with the one-dimensional relation;
- e) converting the image colour value as determined in step d) and subsequently with the calibration relations established in step a) to obtain the colorant control values.
Steps a) and b) are well-known in the field of colour matching. In fact, these are the conventional steps for all ICC based workflows. A printer type defines a number of printers that use a similar print engine, the same colorants and the same media to print on. Each specific print engine needs to be calibrated to compensate for individual, manufacturing differences. It is noted that a calibration procedure for individual colorants is not likely to exceed beyond a TAC limit, in contrast to procedures that determine the one-dimensional transfer curves in mutual dependency. After the calibration of an individual printer, a single profiling procedure for any printer of this type may be used for one or several media that are suitable for the printer type, the media collected in what is sometimes called a media family. Thus, the behaviour of colorant compounds in relation to the receiving substrate is contained in an output profile. Combining this output profile with an input profile that defines the colours in an input image prepares the output device for rendering the colours of the image as intended.
From the input and output profiles that are comprised in the profile combination, a one-dimensional relationship, i.e. a single transfer curve, is derived for a colour that may be generated either by a single colorant or by a compound of other colorants. In CMYK printers this is mostly a grey colour that may be generated by black colorant only on a white medium or by compounds of cyan, magenta and yellow colorant. In printers with more colorants, such as red colorant, a red colour may be made by this colorant only or by compounds of magenta and yellow. Such an additional transfer curve is used to bypass the colour management profiles. It has been found by the inventor that this additional transfer curve is to be established based on values already present in the profiles. This provides the advantage that a better continuity between colours processed by the profiles and colours processed by the bypass. All colorant components pass subsequently the individual calibration curves. This way, the printer behaves more consistently.
In an embodiment, the method as outlined is implemented for a CMYK printer, wherein a profile-bypass is used for the black colour channel. Text and lines in an input image defined in black can thus be rendered with black colorant only by using the bypass, whereas gray and black colours in an photographic part of the input image may be rendered by compound colours after passing the colour profiles.
In an embodiment, a distinction between the different gray image elements is made by checking if the colour components of an input colour, except for the black component, are below a threshold value. When the input colour comprises predominantly or only black, the bypass conversion ensures that the output colour still comprises mainly black, whereas the profile conversion might result in a more compound colour with cyan, magenta, and yellow, which is to be avoided.
In a further embodiment, the bypass additional transfer curve 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.
As indicated earlier, the G7 specification prescribes a black tone scale aim (paragraph 5.5) in dependence on the luminous reflectance factor of the dark end of the black scale and the luminous reflectance factor of the substrate. These are readily derived from the lightness that is associated with 100% black and the 0% coverage in the input and output profile. The step sequence above makes the bypass obey this specification.
Further details of the invention are given in the dependent claims. The present invention may also be embodied in a printer controller pertaining to a printer and in a non-transitory computer readable medium storing computer executable instructions.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
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:
Figure 1 is a conversion scheme of a printer only applying a calibration according to a G7 specification;
Figure 2 is a conversion scheme according to the present invention, and
Figure 3 is a graphical outline of the way to find the bypass relation.
DETAILED DESCRIPTION OF EMBODIMENTS
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.
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 LUTG7
, wherein a CMYK colour as given in an input image 1 is component by component converted into printer control values. These input colours have four components, Ci
, and Ki
, which is considered to be a set Si
. The colour signal that is sent to the printer 3 are the colorant control values Cp
, and Kp
, or in short Sp
. Since Sp
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.
In Fig. 2 a colour conversion scheme according to the present invention is shown. A colour of the input image 1 with colour components Ci
, and Ki
, is examined for its optimal conversion path in decision box 4. If the colour is to be rendered with black colorant only (4, path Y), in this case by checking if Ci
= 0, the output values Co
, are set to zero in bypass box 8 and only a Ko
output value is passed to the printer calibration box 9. Otherwise (4, N) the input image colour is passed to an appropriate input Colour Matching Profile (CMPi
) 6 and output Colour Matching Profile (CMPo
) 7, wherein the CMYK value is interpreted as a physical colour in a Profile Matching Colour Space (PMS), in this case L, a, b, and converted to an output colour value for the CMYK printer 3. The output colour 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 (LUTC
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 colours are necessary in this calibration. The relation between Ki
in bypass box 8 is established along a line of steps, graphically shown in Fig. 3.
Fig. 3 comprises four graphs, labelled A to D. Graph A indicates a relation 20 between an input value Ki
on a horizontal axis 11, running from 0 to 1, and a corresponding lightness Li
on a vertical axis 10, running from 0 to 100. This relation is obtained from the input profile CMPi
and defines a lightness value Wi
associated with an input medium without colorant and a lightness Bi
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 Wi
as found from the profile CMPi
, following the dotted lines 12 and 13 The tone value KG7
is found for the particular value Li
. Graph C shows the relation 22 between the tone value and the lightness according to the G7 specification using the values Wo
that are obtained from the output profile CMPo
for 0% and 100% black tone value on the output side. For the specific value KG7
an output lightness Lo
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 CMPo
, to obtain an output tone value Ko
. Thus, a relation between an input value Ki
and an output value Ko
can be obtained that is used in the bypass box 8 as LUTbp
. 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.
Thus, a colour conversion path is defined that prevents the printer from colour 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.
The skilled person will recognise that other embodiments are possible within the scope of the appended claims.
A method for calibrating and converting colours of a digital image (Si
) into printer specific colorant control values (Sp
), 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 (9) between a printer tone value (So) and a colorant control value (Sp) is established;
b) profiling the printer type to establish a profile combination (CMPi, CMPo) that defines a relationship between colour values in an image colour space (Si) and the printer tone values (So);
c) deriving from the profile combination a one-dimensional relationship (LUTbp) between a single colour component in the image colour value (Ki) and a corresponding component in the printer tone value (Ko);
d) determining if an image colour value (Si) is to be converted either with the profile combination (CMPi, CMPo) or with the one-dimensional relation (LUTbp), and
e) converting the image colour value (Si) as determined in step d) and subsequently with the calibration relations (9) established in step a) to obtain the colorant control values (Sp).
2. The method according to claim 1, wherein the single colour component is the black component (K).
3. The method according to claim 2, wherein the step of determining how an image colour value (Si) is to be converted (step d) comprises a step of determining if all but the black component (Ki) in the image colour value (Si) are lower than a predefined threshold.
The method according to claim 2, wherein the one-dimensional relationship (LUTbp
) is derived by the steps of:
c1) determining a lightness (Li) corresponding to an input black channel value (Ki);
c2) determining an equivalent G7 channel value (KG7) for this lightness (Li) in dependence on the no-coverage lightness (Wi) and the 100%-coverage lightness (Bi) on the input side;
c3) determining an output lightness (Lo) for the G7 channel value (KG7) in dependence on the no-coverage lightness (Wo) and the 100%-coverage lightness (Bo) on the output side; and
c4) determining a black printer tone value (Ko) from the output lightness (Lo).
5. A printer comprising a printer controller and a print engine, the printer controller being configured to execute a method according to claim 1.
6. A non-transitory computer readable medium storing computer executable instructions for executing a method according to claim 1.