EP0916491B1 - Method for the determination of colorimetric gradients - Google Patents

Description

The invention relates to a method for determining the color value gradients Image element of a printed image when the layer thickness changes on the print participating printing inks according to the preamble of the independent claim.

The regulation of the coloring in modern printing machines, especially in Offset printing, is advantageously carried out with color distance control. A typical one Color distance-controlled control method is for example in EP-B2-0 228 347 and described in DE 195 15 499 C2. In this procedure, one with the Printing press printed sheets in a number of test areas related to of a selected color coordinate system measured colorimetrically. From the the color coordinates obtained in the process become the color distance vectors towards the same Color coordinate system related target color coordinates are calculated. This Color distance vectors are calculated with the help of color value gradients Layer thickness change vectors converted, and the control of the color of the Printing machine is converted on the basis of the color difference vectors Layer thickness change vectors made. The fields are the test areas of color control strips printed with the actual printed image.

In the meantime, i.a. scanning devices known as scanners have become known, which allow a large number of the entire image content of a printed sheet of relatively small picture elements with reasonable effort and in a very short time measured colorimetrically or spectrophotometrically. These scanners offer the basic metrological requirements for the regulation of Ink guide of a printing machine not only to use test strips printed with it, but the color information from all picture elements of the whole actual To use the printed image for this purpose. A difficulty with this as a so-called Measurement in the picture, however, is by the in the Given the four-color printing problem of the black component, to which As is well known, not only the printing ink black itself, but also that superimposed colored colors contribute. A reliable determination of the for the calculation of the input variables required for the color control Color value gradients for all occurring in a printed image, very Different printing situations are not possible using the usual methods. Another difficulty arises from the enormously high required Computational effort and the associated unacceptably long in practice Computing times.

EP 143 744 A1 describes a method for regulating the color guidance in an offset printing press in which the remissions in four on each picture element spectral ranges can be measured separately. The remission values are in Area coverage converted. From a target-actual comparison of remission values control signals for controlling the color guidance are derived.

Starting from this prior art, it is an object of the present Invention to improve a method of the generic type in that it can also be used for the so-called measurement in the image. In particular, the Invention the determination of color value gradients on any picture elements Enables print image and thereby the influences of all involved printing inks, especially the printing ink black, can be safely separated. A Another object of the invention is to determine the color value gradients enable with practically reasonable effort and high speed and are the prerequisites for the computational feasibility of the To create press control based on measurements in the printed image.

The solution to this problem underlying the invention results from the im characterizing part of the independent claim 1 features described. Particularly advantageous refinements and developments are the subject of dependent claims.

The invention is explained in more detail below with reference to the drawing, the only one of which Figure shows a schematic diagram of the control or regulation of a printing press.

According to the illustration, a printing press 1, in particular a multi-color offset printing press, Print sheets 3, which the desired print image and possibly additionally have pressure control elements. The sheets 3 are the ongoing printing process and a spectrophotometric Scanning device 2 supplied. This scans the printed sheets 3 essentially the entire surface from pixel to pixel. The size of each Picture elements 4 is typically about 2.5 mm x 2.5 mm, corresponding to around 130,000 Image elements 4 with a printing sheet 3 of common dimensions. The one from the Sampling device 2 generates samples, typically spectral Remission values are essentially made up of a computer Evaluation device 5 is analyzed and input variables for one of the Control unit 9 assigned to printing press 1, which in turn processes the Coloring elements of the printing press 1 in accordance with these input parameters controls. The input variables are, at least in the case of an offset printing press 1, typically around zonal layer thickness changes for the individual inks involved in printing. The determination of the above Input variables or changes in layer thickness are made by comparing the Sampled values or quantities derived therefrom, in particular color measurement values (Color locations or color vectors) of a so-called OK sheet 3 with the corresponding sizes of one taken from the current printing process Printing sheet 3 in the sense that the by the input sizes or Changes in layer thickness caused changes in the settings of the Coloring organs of the printing press 1 the best possible adjustment of the color impression of the printed sheets 3 to the OK sheet 3 Consequence. For comparison, another OK sheet 3 can also be used Reference can be used, for example corresponding default values or corresponding values obtained from prepress.

The arrangement outlined corresponds so far essentially to conventional, e.g. in DE-A 44 15 486 arrangements and methods described in detail for Coloring control of printing presses 1 and is therefore necessary for the person skilled in the art no further explanation.

worin I_i die im betreffenden Bildelement 4 gemessene Infrarot-Remission und I_in die an einer unbedruckten Stelle des Druckbogens 43 gemessene Infrarot-Remission bedeuten. Der Infrarot-Wert I kann daher wie der Helligkeitswert L nur Werte von 0-100 annehmen. Die Berechnung der Farbwerte L,a,b und des Infrarot-Werts I aus den spektralen Remissionswerten erfolgt in der Auswerteeinrichtung 5. Der Vollständigkeit halber sei erwähnt, dass die Ermittlung der Farbwerte L,a,b (oder entsprechender Werte eines anderen Farbraums) auch ohne spektrale Abtastung mittels geeigneter Farbmessgeräte erfolgen könnte.A first aspect of the present invention is the inclusion of the printing ink black in the determination of the color value gradients and the input variables calculated with the aid thereof for the press control. For this purpose, the printing sheets 3 are not only measured in the visible spectral range (approx. 400-700 nm), but also at at least one point in the near infrared, where only the printing ink black has a significant absorption. This makes it possible to selectively record the influence of the printing ink black on the color impression. The reflectance spectra of the individual picture elements 4 thus consist of reflectance values in the visible spectral range, typically 16 reflectance values at intervals of 20 nm each, and a reflectance value in the near infrared range. Color values (color coordinates, color vectors, color locations) relating to a selected color space are calculated from the reflectance values of the visible spectral range. It is preferable to choose a color space that is equally spaced in terms of perception, typically the so-called L, a, b color space according to CIE (Commission Internationale de l'Eclairage). The calculation of the color values L, a, b from the spectral reflectance values of the visible spectral range is standardized by CIE and therefore requires no explanation. The reflectance value in the near infrared is converted into an infrared value I, which corresponds qualitatively to the brightness value L of the color space. This is done analogously to the calculation formula for L according to the relationship:

wherein I _i measured in the respective picture element 4 infrared reflectance and I measured _in the unprinted at a location of the printing sheet 43 infrared reflectance mean. The infrared value I can therefore only assume values from 0-100 like the brightness value L. The calculation of the color values L, a, b and the infrared value I from the spectral reflectance values takes place in the evaluation device 5. For the sake of completeness, it should also be mentioned that the determination of the color values L, a, b (or corresponding values of another color space) also could be done without spectral scanning using suitable color measurement devices.

The color and infrared values L, a, b and I present after the scanning of a printing sheet 3 for each individual picture element 4 form the starting point for the calculation of the color value gradients and with the aid of the input variables for the printing press control device 9. These calculations are also carried out in the evaluation device 5. For the following description, the value quadruple comprising the three color values L, a, b (or the corresponding values of another color system) and the infrared value I for each picture element 4 is simplified as a (four-dimensional) color vector F of the picture element 4 in question, ie: F = (L, a, b, I) The term "color locus" in the four-dimensional color space is understood to mean a point in the color space, the four coordinates of which are the four components of the color vector. The color difference between a picture element 4 and a reference picture element 4 or with the corresponding picture element 4 of a reference, typically an OK sheet 3, is referred to as the color distance vector ΔF, which depends on the relationship ΔF = (ΔL, Δa, Δb, ΔI) = F i -F r = (L i -L r , a i -a r , b i -b r , I i -I r ) results in which the values provided with the index i are those of the picture element 4 under consideration and the values provided with the index r are the components of the color vector of the reference picture element 4 or of the corresponding picture element 4 of the OK sheet 3. The color vectors of the picture elements of the OK sheet or another reference are often also referred to as target color vectors. The color distance ΔE between two picture elements 4 or a picture element 4 and the corresponding picture element 4 of the OK sheet 3 is understood to mean the absolute value of the relevant color distance vector ΔF ΔE = | ΔF | = {(L i - L r ) 2 + (a i - a r ) 2 + (b i - b r ) 2 + (I i - I r ) 2 } 0.5 where the indices i and r in turn have the meaning given. The computer of the evaluation device 5 calculates the color distance vector ΔF for each picture element 4 of the current printed sheet 3 from the color vectors F determined on this and the OK sheet 3.

The input variables to be determined for the printing press control device 9, that is to say the zonal relative changes in layer thickness for the individual printing inks involved in the printing, are also shown vectorially for the following and are referred to collectively as the layer thickness change vector ΔD: ΔD = (ΔD c , ΔD G , ΔD m , ΔD s ) The indices c, g, m and s stand for the printing inks cyan, yellow, magenta and black, the correspondingly indexed components of the vector are the relative changes in layer thickness for the printing ink indicated by the index. The current layer thicknesses themselves can be represented as layer thickness vector D: D = (D c , D G , D m , D s ) where the indices have the same meaning.

According to the teaching of, for example, EP-B2 0 228 347 mentioned at the outset, the relative changes in layer thickness ΔD of the individual printing inks required for the compensation of a color deviation to the reference (OK sheet 3) can be determined from the color spacing vectors ΔF determined on a current printing sheet 3 for reference ( OK sheet 3) according to the equation ΔF = S * ΔD calculate, where S is a so-called sensitivity matrix, which contains as coefficients the partial derivatives of the four components L, a, b, I of the color vector F according to the four components D _c , D _g , D _m , D _{s of} the layer thickness vector D. :

The coefficients of the sensitivity matrix S are usually called Color value gradients. In the explanations below, 16 Color value gradients each represent the summary term sensitivity matrix used.

The sensitivity matrix S is a linear replacement model for the relationship between the changes in the layer thickness of the printing inks involved in the printing and the resulting changes in the color impression of the with the changed Layer thickness values of printed picture element 4. The sensitivity matrix S is not the same for all color locations in the color space, but strictly speaking only applies in the immediate vicinity of a color location, i.e. for each measured color vector F the individual picture elements 4 in the equation ΔF = S * ΔD, strictly speaking, a separate one Sensitivity matrix S to use.

Provided that the sensitivity matrices S are known, the matrix equation ΔF = S * ΔD can be solved according to the known rules of the matrix calculus according to ΔD (ΔD = S ^-1 * ΔF).

The visual color impression (technically the color value, color locus or color vector) of a picture element 4 is in offset raster printing by the percentage Raster values (area coverage) of the printing inks involved and, to a lesser extent Mass determined by the layer thickness of the printing inks. The grid values or Area coverage (0-100%) is due to the underlying printing plates fixed and practically unchangeable. Influenced the color impression and thus can only be regulated under the given pressure conditions via the Layer thicknesses of the printing inks involved. The terms "grid value" and "Area coverage" is used synonymously below. The entirety of all possible combinations R of percentage grid values of the involved Printing inks (usually cyan, yellow, magenta, black) are in the following as Grid space (four-dimensional) called.

Under given printing conditions (characteristic curves of the printing press, nominal Layer thicknesses, material to be printed, printing inks used etc.) corresponds to each Raster value combination R a precisely defined color impression or color vector F the picture element 4 printed with this raster value combination R; so it exists a clear assignment of raster value combination R to color location or Color vector F; the grid space can be clearly mapped onto the color space, whereby however, the color space is not completely occupied because it is also not printable Contains color locations. Conversely, there is generally no clear relationship. The Color vector F belonging to any raster value combination R can be empirically determined determined by test prints or by means of a suitable model, which the Printing process sufficiently accurate under the given printing conditions describes, can be calculated. A suitable model is e.g. through the well-known Neugebauer equations for four-color offset printing. The model sets knowledge of the reflectance spectra of single-color full tones, some Overprinting full tones and some grid fields all on the print involved inks with the nominal layer thicknesses of the inks ahead. These reflectance spectra can be measured very easily using a test print. If the characteristics of the printing press 1 are known, simple ones are sufficient Measurements on solid tones.

With the help of the model mentioned, it is possible for everyone in a manner known per se any raster value combination R the (16) coefficients of this To determine the raster value combination belonging to the sensitivity matrix S. Is to only necessary in the model the nominal layer thicknesses of the printing inks involved preferably individually by e.g. 1% change and with these changed Layer thicknesses the corresponding color vectors and corresponding Color difference vectors compared to that resulting from the nominal layer thicknesses to calculate the resulting color vector. These color distance vectors ΔF and the underlying layer thickness change vectors ΔD are included in the equation ΔF = S * ΔD and this according to the coefficients of the sensitivity matrix S dissolved.

According to a further aspect of the invention, the mentioned model to a limited number of possible Raster value combinations R the associated color vector F and the associated Sensitivity matrix S calculated in advance and stored in a table. This the entirety of all sensitivity matrices S and color vectors F calculated in this way containing table is referred to below as a raster color table RFT.

For the calculation of the layer thickness change vectors ΔD from the equation As explained above, ΔF = S * ΔD is the knowledge of the respective color location or color vector F or sensitivity matrix generally associated with each picture element 4 S required. To get to this, according to another aspect the invention from the color vector F of the respective picture element 4 after a further Particularly advantageous calculation methods explained in more detail below associated raster value combination R is calculated and based on this Raster value combination R the associated sensitivity matrix S from the pre-calculated raster color table. That way it is without Excessive computational effort possible for each pixel 4 very quickly determine the required sensitivity matrix.

According to a further idea of the invention, a number of, for example, 1296 equally spaced discrete halftone value combinations R _iR (6 discrete halftone percentage values A _C , A _G , A _M , A _S for the printing colors cyan, yellow, magenta, black) are defined in the raster space as follows : i 0 1 2 3 4 5 A _C 0 20 40 60 80 100% A _G 0 20 40 60 80 100% A _M 0 20 40 60 80 100% A _S 0 20 40 60 80 100%

These 1296 discrete raster value combinations R _iR are numbered according to the following formula with a unique raster index iR: iR = i (A C ) * 5 0 + i (A G ) * 5 1 + i (A M ) * 5 2 + i (A S ) * 5 3

I (A _C ) .... is the value of the index i for the respective discrete screen value of the respective printing ink. For each of these 1296 discrete raster value combinations R _iR , a sensitivity matrix S _{iR is} calculated and stored in the raster color table. The calculated color vector F _iR belonging to the discrete raster value combinations R _iR is also stored in the table. The raster color table RFT thus contains a total of 1296 color vectors F _iR and 1296 associated sensitivity matrices S _iR .

The grid space is preferably quantized in two stages. In the first stage, for only 256 discrete halftone value combinations (corresponding to four discrete halftone percentage values 0%, 40%, 80%, 100% for each of the printing colors cyan, yellow, magenta, black), the associated color vectors and the are based on the offset printing model associated sensitivity matrices. In the second stage, the associated color vectors and sensitivity matrices for the missing raster percentage values 20% and 60% are then calculated by linear interpolation from the color vectors and sensitivity matrices of the 16 nearest discrete raster value combinations. This again results in a total of 1296 discrete raster value combinations R _iR with 1296 associated discrete color vectors F _iR and 1296 associated sensitivity matrices S _iR . Of course, the grid space could also be reduced to a different number of discrete grid combinations, for example about 625 or 2401, but the number 1296 represents an optimal compromise between accuracy and computational effort in practice.

A sensitivity matrix S _iR whose associated discrete raster value combination R _{iR is} closest to the raster value combination R calculated from the color vector F is now assigned to a color vector F determined for a picture element 4. In other words, the calculated raster value combination is replaced by R each closest discrete halftone value combination R _iR and receives associated with the precalculated to this discrete halftone value combination R _iR sensitivity matrix S _iR.

In a variant of the method, the screen value combinations (R _iR ) and the color value gradients (S _iR ) can be determined by interpolation from the screen color table (RFT).

According to a further aspect of the invention, the (including infrared value I four-dimensional) color space is also subjected to quantization, ie divided into a number of subspaces, for determining the raster value combination R from the color vector F. For this purpose, a number of discrete color locations F _iF , each with discrete coordinate values, are defined in the color space. For example, the four-dimensional color space can be quantized such that each dimension L, a, b, I of the color space can only assume 11 discrete values, resulting in a total of 14641 discrete color locations F _iF : i 0 1 2 3 4 5 6 7 8th 9 10 L 0 10 20 30 40 50 60 70 80 90 100 a -75 -60 -45 -30 -15 0 15 30 45 60 75 b -45 -30 -15 0 15 30 45 60 75 90 105 I 0 10 20 30 40 50 60 70 80 90 100

These 14641 discrete color loci F _iF are numbered according to the following formula with a unique color locus index iF: iF = i (L) * 11 0 + i (a) * 11 1 + i (b) * 11 2 + i (I) * 11 3

For these discrete color locations F _{iF of} the color space, the associated raster value combinations R _{iF are} calculated using the particularly advantageous calculation method explained below and, if they do not coincide with a discrete raster value combination R _iR , are replaced by the closest discrete raster value combination R _iR . This results in a clear, pre-calculated mapping of the 14641 discrete color locations F _{iF of} the (four-dimensional) color space to the 1296 discrete raster value combinations R _{iR of} the (also four-dimensional) raster space. As already mentioned, this mapping is calculated in advance and stored in an assignment table referred to below as the raster index table RIT.

For the purposes of determining the raster value combinations R from the color vectors F determined for the picture elements 4, each color vector F determined for a picture element 4 is replaced by the closest discrete color location F _iF . The discrete raster value combination R _iR assigned to this discrete color location F _iF is then taken from the raster index table RIT, and the corresponding sensitivity matrix S _iR and the color vector F and thus the image element 4 are read out from the raster color table RFT assigned. In this way, the sensitivity matrix S can be determined with comparatively little computation effort and accordingly quickly for any picture element 4 on the basis of the color vector F determined for it, with sufficient accuracy for practice.

For the above it was assumed that from the color vectors F the associated raster value combinations R can be calculated. Like this according to another aspect of the invention can be carried out particularly advantageously can be the subject of the explanations below.

First, the (four-dimensional) color space is divided into 81 sub-areas T _iT as follows: i 0 1 2 L (0..120) 0..20..40 40..60..80 80..100..120 a (-90 .. + 90) -90 ..- 60 ..- 30 -30..0 .. + 30 +30 .. + 60 .. + 90 b (-60 .. + 120) -60 ..- 30..0 0 .. + 30 .. + 60 +60 .. + 90 .. + 120 I (0..120) 0..20..40 40..60..80 80..100..120

The total of 81 sub-areas T _iT are clearly numbered using a sub-area index iT defined according to the following formula: iT = i (L) * 3 0 + i (a) * 3 1 + i (b) * 3 2 + i (I) * 3 3

Within each sub-area T _iT , the relationship between the color vector F and the associated raster value combination R, written as raster vector A, is approximated linearly by the following matrix equation: A = U iT * F

In it A means the raster vector with the raster percentage values A _C , A _G , A _M , A _{S of} the four printing inks involved as components and U _iT a conversion matrix with 16 coefficients, which shows the partial derivatives (gradients) of the components of the raster vector according to the components of the color vector are. If the conversion matrices U _{iT of} the individual partial areas T _{iT are} known, the associated raster vector A or the associated raster value combination R can thus be calculated for each color vector F.

The problem is therefore reduced to the calculation of the conversion matrices U _iT for the individual partial areas T _iT or more precisely for the color vectors F _iT from their centers. The conversion matrices are calculated using a weighted linear compensation calculation using the values from the raster-color table RFT explained above, that is to say the 1296 discrete raster value combinations R _iR and the associated discrete color vectors F _iR . For the compensation calculation, essentially only the inversion of a 4x4 matrix is required for each sub-area T _iT . The weight of the support points, ie the discrete color locations F _{iR of} the raster color table, for the compensation calculation is determined according to a suitable function with the color distance between the support points and the respective color vector F _iT as parameters. The compensation calculation is linear, ie there are discontinuities at the transitions of the individual sub-areas T _iT , which are insignificant in practice.

The according to the above explanations for the individual picture elements 4th ascertained sensitivity matrices S or the color value gradients forming them can now be used for the calculation of the input variables for the Control of the coloring of the printing press can be used.

Claims (3)

1. Method for the determination of the color value gradients of a picture element of a printed image when there are changes in the layer thicknesses of the inks used in the printing, whereby the picture element (4) is photo-electrically scanned in the visible region of the spectrum, and the color value gradients are derived from the scanning signals thereby obtained,
   and whereby from the scanning signals of the visible region of the spectrum, color coordinates (L,a,b) of an approximately subjectively equidistant color system are formed,
   characterized in that the picture element (4) is also scanned photo-electrically in the near-infrared region of the spectrum,
   that from the scanning signals of the infrared range, at least one infrared value is formed (I),
   and that the color value gradients (S) are calculated from the color coordinates and the at least one infrared value,
   that for a specified first number of discrete half-tone value combinations (R_iR), color value gradients (S_iR) corresponding to the colors used in the printing are calculated and stored in a half-tone color table (RFT),
   that for the picture element (4), the corresponding half-tone value combination (R) of the colors used in the printing is calculated from the color coordinates (L,a,b) and the at least one infrared value (I),
   and that the color value gradients (S_iR) from the half-tone color table (RFT) are assigned to the picture element (4), when the corresponding discrete half-tone value combination (R_iR) of the gradients in question is closest to the half-tone value combination (R) calculated for the picture element (4).
2. Method as claimed in claim 1,
   characterized in that a four-dimensional color space is formed, the coordinates of which are the color coordinates (L,a,b) and the infrared value (I),
   that in this four-dimensional color space, a specified second number of discrete color sites (F_iF) are defined, for each of these discrete color sites the corresponding half-tone value combination (R) of the colors used in the printing is calculated, this half-tone combination (R) is replaced by the closest discrete half-tone value combination (R_iR) taken from the half-tone color value table (RFT) and the discrete color sites (F_iF) are stored in correspondence to the discrete halftone value combinations (R_iR) in a half-tone index table (RIT),
   and that for the determination of the color value gradients of the picture element (4) from the color coordinates (L,a,b) and the infrared value (I) of this picture element (4), the coordinates of a color site are formed in the four-dimensional color space, this color site is replaced by the closest discrete color site (F_iF), the discrete half-tone value combination (R_iR) corresponding to this discrete color site (F_iF) is taken from the half-tone index table (RIT), the color value gradient (S_iR) corresponding to this discrete half-tone value combination (R_iR) is taken from the half-tone color table (RFT), and this color value gradient (S_iR) is assigned to the picture element (4).
3. The method according to claim 2,
   characterized in that the half-tone value combination (R_iR) and the color value gradients (S_iR) are determined by interpolation from the half-tone color table (RFT).

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