EP0916491A1 - Method for the determination of colorimetric gradients - Google Patents

Abstract

Image elements are photo-electrically scanned over both visible and infrared spectra. From the scanned signals, color coordinates approximating to a sensitivity-wise equal separation color system and at least one infrared value are determined. From the color coordinates and the infrared value, a color value gradient is determined.

Description

The invention relates to a method for determining the color value gradients of a Image element of a printed image with changes in the layer thicknesses of the print involved inks according to the preamble of the independent claim.

The regulation of coloring in modern printing machines, especially in Offset printing, is carried out with advantage color distance controlled. A typical one Color-distance-controlled control method is described for example in EP-B2-0 228 347 and described in DE 195 15 499 C2. In this method, a with the Printing press printed sheet in a number of test areas with respect of a selected color coordinate system measured colorimetrically. From the thereby obtained color coordinates become the color space vectors to the same Calculated color coordinate system related desired color coordinates. These Color difference vectors are determined by means of color gradient in Layer thickness change vectors converted, and the control of the color guide of Printing machine is due to the converted from the color space vectors Layer thickness change vectors made. The fields are used as test areas used by printed with the actual printed image color control strip.

Meanwhile i.a. Scanners known as scanners have become known, which allow the entire image content of a sheet in large numbers relatively small picture elements with reasonable effort and in a very short time to measure colorimetrically or spectrophotometrically. These scanners provide the basic metrological conditions for the regulation of Farbfürhung a printing press to use not only printed test strips, but the color information from all the picture elements of the entire actual Print image for this purpose. A difficulty with this than so-called. Measurement indicated in the picture, however, is by the in the Given four color printing problem of the black component, to which as you know, not only the printing ink black itself, but also the contributing to overprinted bright colors. A reliable determination of the for the calculation of the input variables required for color control Color gradient for all in a printed image occurring, very different pressure situations is not possible by the usual methods. Another difficulty arises from the required enormously high Computing and associated with the unacceptably long for practice Computing times.

Based on this prior art, it is an object of the present invention Invention to improve a method of the generic type in that it is also applicable to the so-called measurement in the image. In particular, should by the Invention the determination of Farbwertgradienten on arbitrary picture elements of a Print image and thereby the influences of all participating inks, especially the printing ink black, can be safely separated. A Another object of the invention is to determine the Farbwertgradienten with practically reasonable effort and high speed to enable and thus the conditions for the computational feasibility of the To create press control due to measurements in the print image.

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

In the following the invention will be explained in more detail with reference to the drawing, the only Figure shows a schematic diagram of the control or regulation of a printing press.

As shown, a printing machine 1, in particular a multi-color offset printing machine, produces Printed sheets 3, which the desired print image and possibly additionally have pressure control elements. The signatures 3 are the taken current printing process and a spectrophotometric Scanning 2 fed. This scans the signatures 3 substantially over the entire surface imagewise. The size of the individual Image elements 4 is typically about 2.5 mm x 2.5 mm, corresponding to about 130,000 Picture elements 4 in a printed sheet 3 of conventional dimensions. The of the Sampler 2 generated samples-typically spectral Remissionswerte- are in a substantially consisting of a computer Evaluation device 5 analyzed and to input variables for one of Printing machine 1 associated control device 9 processed, which in turn the Coloring organs of the printing press 1 according to these input variables controls. The input variables are, at least in the case of an offset printing machine 1, typically around zonal layer thickness changes for the individual inks involved in printing. The determination of said Input variables or layer thickness changes are made by comparing the Samples or derived therefrom sizes, in particular color measurements (Color or color vectors) of a so-called OK sheet 3 with the corresponding sizes of the current printing process taken Sheet 3 in the sense that by the input variables or Layer thickness changes caused changes in the settings of the Farbgebungsorgane the printing press 1 a good approximation of the color impression of the continuously generated sheets 3 to the OK sheet 3 to Episode. For comparison, instead of an OK sheet 3, another Reference be used, for example, about corresponding default values or corresponding values obtained from pre-presses.

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

A first essential aspect of the present invention is the inclusion of the ink black in the determination of the Farbwertgradienten and calculated with their help input variables for the printing press control. For this purpose, the printed sheets 3 are measured not only in the visible spectral range (about 400-700 nm), but also at at least one point in the near infrared, where only the ink black has a significant absorption. This makes it possible to selectively detect the influence of the ink black on the color impression. The remission spectra of the individual picture elements 4 thus consist of remission values in the visible spectral range, typically 16 remission values at intervals of 20 nm each, and a remission value in the near infrared range. From the remission values of the visible spectral range, color values (color coordinates, color vectors, color loci) with respect to a selected color space are calculated. Preferably one selects for it a sentimentally equidistant color space, typically about 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 remission values of the visible spectral range is standardized by CIE and therefore needs no explanation. The remission value in the near infrared is converted into an infrared value I, which qualitatively corresponds to the brightness value L of the color space. This is done analogously to the calculation formula for L according to the relationship: I = 116 3 ii Iin -16 wherein I _i mean the infrared remission measured in the relevant pixel 4 and I _in the infrared remission measured at an unprinted position of the printed sheet 43. The infrared value I can therefore assume values of 0-100 just 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 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 by means of suitable colorimeters.

The color and infrared values L, a, b and I present after scanning a printed sheet 3 for each individual picture element 4 form the starting point for the calculation of the color value gradients and with their help the input variables for the printing machine control device 9. These calculations also take place in the evaluation device 5. For the following description, the quadrant of values determined for each picture element 4 and comprising the three color values L, a, b (or the corresponding values of a different color system) and the infrared value I is simplistic as a (four-dimensional) color vector F of the relevant pixel 4, ie: F = (L, a, b, I) The term "color location" in the four-dimensional color space accordingly means a point in the color space whose four coordinates are the four components of the color vector. The color difference of a picture element 4 to a reference picture element 4 or to the corresponding picture element 4 of a reference, typically an OK sheet 3, is referred to as the color difference vector ΔF, which differs according to 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 ) where the values provided with the index i are those of the considered picture element 4 and the values provided with the index r are the components of the color vector of the reference picture element 4 and of the corresponding picture element 4 of the OK sheet 3, respectively. The color vectors of the picture elements of the OK sheet or another reference are often referred to as desired color vectors. The color difference ΔE of two picture elements 4 or of a picture element 4 and of the corresponding picture element 4 of the OK sheet 3 is understood to be the absolute value of the relevant color space vector ΔF, ie ΔE = | ΔF | = {(L i - L r ) 2 + (a i - a r ) 2 + (b i - b r ) 2 + (I i - I r ) 2 } 0.5 wherein the indices i and r again have the meaning given. The computer of the evaluation device 5 calculates the color difference vector ΔF for each picture element 4 of the current print sheet 3 from the color vectors F determined at this and the OK sheet 3.

The input variables to be determined for the printing machine control device 9, ie the zonal relative layer thickness changes 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 layer thickness changes for the ink indicated by the index. The actual layer thicknesses themselves can be represented as layer thickness vector D: D = (D c , D G , D m , D s ) wherein the indices have the same meaning.

According to the teaching of, for example, the above-mentioned EP-B2 0 228 347, the relative layer thickness changes ΔD of the individual participating inks required for the compensation of a color deviation from the reference (OK sheet 3) can be determined from the color difference vectors ΔF determined for a reference (3). OK-arc 3) according to the equation ΔF = S * ΔD in which 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 for the four components D _c , D _g , D _m , D _{s of} the layer thickness vector D. : S = dL DDC there DDC db DDC dI DDC dL DDG there DDG db DDG dI DDG dL DDM there DDM db DDM dI DDM dL DDS there DDS db DDS dI DDS The coefficients of the sensitivity matrix S are commonly referred to as color gradient. In the following explanations, the summary term sensitivity matrix is used for each of these 16 color value gradients.

The sensitivity matrix S is a linear substitute model for the relationship between the changes in the layer thicknesses of the printing inks involved in the printing and the resulting changes in the color impression of the image element 4 printed with the changed layer thickness values. The sensitivity matrix S is not the same for all color loci in the color space, but applies strictly speaking, only in the immediate vicinity of a color locus, ie for each measured color vector F of the individual picture elements 4 in the equation ΔF = S * ΔD strictly speaking, use a separate sensitivity matrix S.

Assuming that the sensitivity matrices S are known, the template equation can be used ΔF = S * ΔD solve for ΔD according to the known rules of the matrix calculus ( ΔD = S -1 * .DELTA.F ).

The visual color impression (metrologically the color value, color location or color vector) of a pixel 4 is the offset raster printing by the percentage Grid values (area coverage) of the participating inks and, in lesser Mass, determined by the layer thicknesses of the printing inks. The grid values or Area coverage (0-100%) are due to the underlying printing plates fixed and virtually invariable. Influence on the color impression taken and thus regulated under given pressure conditions only over the Layer thicknesses of the involved inks are. The terms "grid value" and "Area Coverage" is used synonymously below. The totality of all possible combinations R of percentage grid values of the participating Printing inks (usually cyan, yellow, magenta, black) is hereafter referred to as Raster space (four-dimensional) called.

Under given pressure conditions (characteristics of the printing machine, nominal Layer thickness, material to be printed, used printing inks, etc.) corresponds to each Grid value combination R a well-defined color impression or color vector F of the pixel 4 printed with this raster value combination R; it exists a clear assignment of raster value combination R to color location or Color vector F; the grid space can be clearly mapped to the color space, where However, the color space is not fully occupied, as this is not printable Color locus contains. Conversely, there is generally no clear relationship. The to any raster value combination R belonging color vector F can be empirical determined by trial prints or by means of a suitable model, which the Printing process under the given pressure conditions sufficiently accurate describes, be calculated. A suitable model is e.g. by the well-known Neugebauer equations given for the four-color offset printing. The model is set the knowledge of the remission spectra of single color solid tones, some Overprinting full tones and some grids of all at the print involved in the nominal coating thicknesses of the printing inks. These remission spectra can be measured very easily by means of a test print. If the characteristics of the printing press 1 are known, simple enough Measurements on solid tones.

With the aid of the abovementioned model, it is possible in a manner known per se to determine the (16) coefficients of the sensitivity matrix S belonging to this grid value combination for any desired grid value combination R. For this purpose, it is only necessary to change the nominal size thicknesses of the inks in the model, preferably by 1%, for example, and to calculate the associated color vectors and corresponding color distance vectors with respect to the color vector resulting from the nominal layer thicknesses with these changed layer thicknesses. These color space vectors ΔF and the underlying layer thickness change vectors ΔD are in the equation ΔF = S * ΔD and resolved according to the coefficients of the sensitivity matrix S.

According to a further essential aspect of the invention will now be described with reference to the mentioned model to a limited number of possible Grid value combinations R, the associated color vector F and the associated Sensitivity matrix S calculated in advance and stored in a table. These the totality of all thus calculated sensitivity matrices S and color vectors F The table below is referred to below as the raster color table RFT.

For the calculation of the layer thickness change vectors ΔD from the equation ΔF = S * ΔD is, as stated above, the knowledge of the respective color location or color vector F and generally to each pixel 4 associated sensitivity matrix S required. In order to achieve this, according to a further aspect of the invention, the associated grid value combination R is calculated from the color vector F of the respective picture element 4 according to a particularly advantageous calculation method explained below, and the associated sensitivity matrix S is calculated from the predicted value using this grid value combination R Raster color table taken. In this way it is possible without undue computational effort to determine the required sensitivity matrix for each picture element 4 very quickly.

According to a further aspect of the invention, a number of, for example, 1296 equidistant discrete grid value combinations R _iR (6 discrete grid percentage values A _C , A _G , A _M , A _S for the cyan, yellow, magenta, black inks) are determined 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 grid 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 Here, i (A _C ) .... is understood to be the value of the index i for the respective discrete grid value of the respective printing ink. For each of these 1296 discrete grid value combinations R _iR , a sensitivity matrix S _{iR is} calculated and stored in the raster color table. The to the discrete raster value combinations R _iR belonging calculated color vector F _iR is also stored in the table. Overall, the raster color table RFT thus contains 1296 color vectors F _iR and 1296 associated sensitivity matrices S _iR .

The quantization of the grid space is preferably carried out in two stages. In the first stage, for only 256 discrete grid value combinations (corresponding to four discrete grid percentages 0%, 40%, 80%, 100% for each of the cyan, yellow, magenta, black inks), the associated color vectors are determined using the offset-pressure model associated sensitivity matrices calculated. In the second stage, for the missing halftone percent values 20% and 60%, the associated color vectors and sensitivity matrices are calculated by linear interpolation from the color vectors and sensitivity matrices of the 16 closest discrete grid value combinations. This results in a total of 1296 discrete grid 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 be reduced to a different number of discrete grid co-ordinates, such as about 625 or 2401, but the number 1296 represents an optimal trade-off between accuracy and computational effort.

A color vector F determined for a picture element 4 is then assigned the 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. In other words, the calculated raster value combination is replaced by R each closest discrete halftone value combination R _iR and obtains the assigned to this discrete halftone value combination R _iR predicted sensitivity matrix S _iR.

In a variant of the method, the raster value combinations (R _{: r} ) and the color value gradients (S _{: R} ) can be determined by interpolation from the raster color table (RFT).

According to a further aspect of the invention, for the determination of the grid value combination R from the color vector F, the color space (including the infrared value I four-dimensional) is also subjected to quantization, ie divided into a number of subspaces. For this purpose, a number of discrete color locations F _iF are defined in the color space, each with discrete coordinate values. For example, the quantization of the four-dimensional color space can take place such that each dimension L, a, b, I of the color space can assume only 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 locations F _iF are numbered according to the following formula with a unique color location index iF: iF = i (L) * 11 0 + i (a) * 11 1 + i (b) * 11 2 + i (I) * 11 3 Calculation method For these discrete color loci F _iF of the color space according to the yet explained further below, particularly advantageous keep calculates the associated raster value combinations R _iF and, if they do not coincide with a discrete raster value combination R _iR replaced by the respective closest discrete halftone value combination R _iR. This results in a unique, precalculated mapping of the 14641 discrete color loci F _{iF of} the (four-dimensional) color space to the 1296 discrete raster value combinations R _{iR of} the (also four-dimensional) raster space. This mapping is, as already said, precalculated and stored in an allocation table referred to below as the raster index table RIT.

For the purpose of determining the grid 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 nearest discrete color locus F _iF . The discrete grid value combination R _iR assigned to this discrete color locus F _iF is then taken from the raster index table RIT and the corresponding sensitivity matrix S _iR read from the raster color table RFT and the color vector F and thus the picture element 4 assigned. In this way, the sensitivity matrix S can be determined with sufficient accuracy for practice with comparatively low computational effort and accordingly fast for any pixel 4 on the basis of the color vector F determined for it.

For the above, it was assumed that of the color vectors F associated raster value combinations R can be calculated. As according to a further aspect of the invention are carried out particularly advantageous can, is the subject of the following statements.

First, the (four-dimensional) color space is subdivided into 81 subregions 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 subareas T _iT are unambiguously numbered by a subarray 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 subarea T _iT , the relationship between the color vector F and the associated raster value combination R written as raster vector A is linearly approximated by the following matrix equation: A = U iT * F In this A denotes the raster vector with the grid percent values A _C , A _G , A _M , A _{S of} the four participating inks as components, and U _iT a conversion matrix with 16 coefficients representing 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 subareas 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 subregions T _iT or more precisely for the color vectors F _iT of their midpoints. The calculation of the conversion matrices is carried out by a weighted linear compensation calculation with the values of the raster color table RFT explained above, ie the 1296 discrete raster value combinations R _iR and the associated discrete color vectors F _iR . For the compensation calculation, only the inversion of a 4x4 matrix is necessary for each subarea T _iT . The weight of the support points, ie the discrete color coordinates F _{iR of} the raster color table, for the compensation calculation is determined by a suitable function with the color difference between the support points and the respective color vector F _iT as a parameter. The compensation calculation is linear, ie discontinuities occur at the transitions of the individual subregions T _iT , but these are insignificant in practice.

The according to the above statements for the individual pixels 4 ascertained sensitivity matrices S or the color value gradients forming them can now for the outlined outlined calculation of the input quantities for the Control of the coloring of the printing press can be used.

Claims (6)

Method for determining the color value gradients of a picture element of a printed image with changes in the layer thicknesses of the printing inks involved in the printing, wherein the picture element is scanned photoelectrically in the visible region of the spectrum and the color value gradients are derived from the scanning signals obtained thereby,
characterized,
in that color coordinates (L, a, b) of an approximately sensually equidistant color system are formed from the scanning signals of the visible region of the spectrum, that the image element (4) is additionally photoelectrically scanned in the near infrared region of the spectrum, from the scanning signals of the infrared At least one infrared value is formed (I) and that the Farbwertgradienten (S) are calculated from the color coordinates and the at least one infrared value. Method according to claim 1,
characterized,
in that color value gradients (S) are assigned to the color coordinates and the infrared value from a predetermined table. Method according to claim 2,
characterized,
the table is calculated by means of a mathematical model of the printing machine 1 underlying the production of the printed image from measured values on solid areas printed with the printing press 1 and taking into account the characteristics of the printing press 1. Method according to one of the preceding claims,
characterized,
in that, for a given first number of discrete grid value combinations (R _iR ), the color value gradients (S _iR ) associated with the print are calculated and stored in a raster color table (RFT), that for the picture element (4) from the color coordinates ( L, a, b) and the at least one infrared value (I), the associated raster value combination (R) of the inks involved in the printing is calculated, and that the pixel (4) those Farbwertgradienten (S _iR ) from the raster color table (RFT) whose associated discrete raster value combination (R _iR ) is closest to the raster value combination (R) calculated for the picture element 4. Method according to claim 4,
characterized,
that a four-dimensional color space is formed, whose coordinates are the color coordinates (L, a, b) and the infrared value (I), that in this four-dimensional color space a predetermined second number of discrete color locations (F _iF ) is set, for each of these discrete color coordinates, the corresponding raster value combination (R) of the inks involved in the printing, this raster combination (R) replaced by the nearest in the raster color value table (RFT) discrete raster value combination (R _iR ) and the discrete color loci (F _iF ) in association to the discrete grid value combinations (R _iR ) in a raster 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 pixel (4) the coordinates of a color locus in the four-dimensional color space are formed, this color locus is replaced by the nearest discrete color locus (F _iF ), from the raster Ind ex table (RIT) that this discrete color locus (F _iF) associated discrete raster value combination (R _iR) taken from the halftone color table (RFT) are taken on this discrete halftone value combination (R _iR) associated with the color value gradients (S _iR) and the Pitch (4) these Farbwertgradienten (S _iR ) are assigned. Method according to claim 5,
characterized,
the raster value combination (R _iR ) and the color value gradients (S _iR ) are determined by interpolation from the raster color table (RFT).

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