DE4410228A1 - Method and device for the freely selectable substitution of the uncoloured component in multicolour printing by black printing ink - Google Patents

Abstract

This is a method for the freely selectable substitution of the uncoloured component in multicolour printing by black printing ink on the basis of a spectrophotometrically scanned original, reference points in a colour space being obtained during the spectrophotometric scanning of the original. In accordance with a preselected degree, the uncoloured component or the complementary colour component of the colours of a printing colour space, constructed in a multicolour fashion, is substituted by black printing ink. Reference point data are registered photometrically from a colour scale which is specific to the printing process. The printing colour space is classified in a colour model (HSL model, Fig. 1) which is independent of printing colour and is represented in this model. A two-dimensional, nonlinear darkening field is formed using lightness values (Fig. 2) and a two-dimensional nonlinear desaturation field is formed with saturation values (Fig. 3), both of which are taken from the original, and the substitution of the uncoloured component by black printing ink is undertaken by means of iterative searching within the darkening field and the desaturation field. <IMAGE>

Description

The invention relates to a method and an apparatus for freely selectable Ren substitution of the achromatic portion in multi-color printing with the printing ink Black on the basis of a previously spectrophotometrically scanned template, where in the spectrophotometric scanning of the template reference points in a color space.

The object of the invention is a freely selectable replacement of the achromatic portion in the Multi-color printing through the printing ink black. This task is accomplished by the characteristic features of claims 1 and 4 solved. Advantageous training Developments of the invention are specified in subclaims 2 and 3.

The invention is described below with reference to FIGS. 1 to 6 ben. Show it:

Fig. 1 a hue with representation of the color structure,

Fig. 2 is a schematic representation of a darkening field,

Fig. 3 is a schematic representation of a desaturation field,

Fig. 4 a constant hue to represent GCR generation,

Fig. 5 shows an example of a darkening field for GCR and

Fig. 6 shows an example of a desaturation field for GCR.

Prerequisite for performing the procedure, which accordingly a pre-selected and set degree the achromatic portion or complementary Color proportion of all colors in a multicolored printing space replacing the black ink are:

• - A spectrophotometric acquisition of suitable support data from a print process-specific color scale,
• - a classification and representation of the printing color space in a printing-independent color model (here: HSL model, see Fig. 1),
• - a two-dimensional non-linear darkening field with brightness values (see Fig. 2) and
• - A two-dimensional non-linear desaturation field with saturation values (see Fig. 3), which are also obtained from the scanned template.

The following also applies approximately:

• - The influence of black on the hue (hue shift) is neglected sigbar. The darkening by black is independent of saturation, i. H. the following applies:
• ΔL = f (K, L) (two-dimensional darkening field).
• - The desaturation by black is independent of the brightness, i. H. the following applies:
• ΔS = f (K, S) (two-dimensional desaturation field).

The procedure includes the inclusion of a skeletal black for sub support of the drawing and the contrast (depth expansion). Except for the Setting the GCR level (0 ... 100%) and the minimum printable black value (phase-out value), the process works fully automatically and extends there with printing space that can be realized over the entire multicolor.

The pressure-independent HSL color model, which only calculates, follows from that set by the "International Lighting Commission (CIE)" ten LAB model, which is described in DIN standard no .: 6174.  

The right-angled (Cartesian) lab coordinate system is inserted into the cylindri HSL coordinate system transformed as follows:

The HSL model can be extended to 128 levels in all three axes (expandable as required) equally quantized and correspondingly for H to 2 * π for L to 100. and for S to the maximum possible level (usually <100) through the printing process standardized. The sensed distance in the usable area is therefore in the spanned grid in hue (H), saturation (S) and brightness (L) less than 1 Δ E and is therefore below that which can be perceived by the human eye Threshold. Due to the hue-independent influence of black, it becomes egg only one hue at a time (grid in S, L with 128 * 128 dots) the GCR process applied. This reduces the degrees of freedom for CMY significantly. The description of the GCR process can therefore be representative tend to happen in one hue for all other hues. For transformation of the tri-color printing color space in the HSL model is based on the German Pa tent registration P 40 12 905.5.

This method of replacing the achromatic portion of a brightly colored color (F3 in Fig. 1) in a selectable range of 0%. . 100% extends the three-colored space (F3) by the four-colored space (F4), which together with F3 represents the entire scope of the printing process. In a second step, the requested GCR level is calculated over the entire pressure space (F3 + F4). In the first step, the four-color space is generated to such an extent that in each raster point (L, S) of F4 it is known which tri-color triple CMY border and which black value the color of the point can be realized. The four colored lower edge is determined by adding black to the lower three-colored edge. All colors between the tri-color lower edge and the four-color lower edge can thus be mixed from the tri-color edge colors and a certain black value. In this step, it is important for the actual GCR process to save the L, S coordinate of the colored border color triple, since the GCR process works with brightness and saturation values and not with pressure-dependent CMYK values.

After the scope has been determined in the first step, the GCR process starts in lowest point of the hue with the lowest saturation level. In the further ver The entire hue is processed in such a way that first the saturation dex incremented to maximum saturation with constant brightness index and then the brightness index, while resetting the satti index to the first level, is incremented. According to the previous one noted L, S coordinate can be determined for the four-color area which tricolor border color was used for the construction. The first identified Black level (short black level) is unimportant in that the GCR-Pro a new black value is calculated, which implicitly for the F4 area contains the most black level (not additive). From the minimum CMY value and the set GCR grade 0. . The algorithm calculates 100% for the GCR Process required black level (GCR-K). An iterative Ap Proximation procedure with the help of two-dimensional darkening and sowing a field in F3 that overprints with the required GCR-K exactly reproduces the tri-color border color. This point is due to the darkening the and desaturating properties of black ink in greater brightness speed and saturation as the tri-color border color. For the tri-color area is here the GCR process ends and the original three-color structure is the iterated lighter and more saturated color in combination with the GCR-K replaced. For the four-color area, the non-linear Abdun fields and desaturation fields determined a stronger black value, which in com combination with the CMY triple of the bright and saturated substitute color the original color F4. In cases where the maximum black level of 100% is exceeded or the theoretical replacement color outside of the print there is a gradual reduction in the GCR-K. The procedure ren works up to the minimum printable value (threshold) for GCR-K. The minimum printable value (e.g. 2%) is set at the beginning.  

In Fig. 4 is intended for the point PO (SO, LO) ([.. 0th 100%] = X), an X-owned GCR be performed. The following is known for PO:

• - the brightness and saturation.
• - The brightness and saturation of the tri-color border color P1 (S1, L1),

which simulates PO combined with a certain black value.

It is made from the minimal printing ink of the three-color P1 and the the GCR level set, the GCR-K to be demanded is calculated

GCR-K = GCR grade * MinCMY (P1),

GCR grade: = X / 100;
0 ⇐ GCR grade ⇐ 1.0
0 corresponds to colored structure including skeleton black
1.0 corresponds to 100% GCR.

The darkening compensation is shown in Fig. 5. The required black value (GCR-K) specifies the starting point (Kn, G = 0) for the iteration of L2. Through an iteration process with subsequent post-interpolation between the coordinates (Kn, Gn) and (Kn, Gn + 1), where L (Kn, Gn) <L (P1) <L (Kn, Gn +1), with constant K- Index (= Kn) a coordinate point determined for which:

L (Kn, Gi) = L (P1).

The plumb line on the G axis then results in (0, Gi) the brightness (L (P2)), that with GCR-K overprinted on L (P1) or with total K on L (PO). It contains total K found in that, starting from point (Kn, Gi), parallel to the K axis another point is iterated with Ks <Kn, for which the following applies:

L (Ks, Gi) = L (PO).

The plumb line on the K axis gives the total K in point (Ks, O), which is on P2 prints the brightness L (P2) darkens to L (PO).

In cases where L (P1) <L (Kn, O) the GCR-K will automatically incrementally reduced, in cases where L (P1) <L (Kn, 100) increments automatically. The results of this darkening compensation are:

the brightness for P2
the K value to darken L (P2) to L (P1) or L (PO).

Desaturation compensation is shown in Fig. 6.

The desaturation of the unknown point P2 depends on the saturation S (P2) and the black value to be applied. In the case of the GCR in the three-color range (F3) is the black value just the GCR-K, in the four-colored area (F4) the Ge velvet-K.

The black value gives the starting point in the two-dimensional desaturation field for the iterative search of the S (P2). The iteration process followed by Post-interpolation between the coordinates (Sb0, Kn) and (Sb1, Kn), where S (SbO, Kn) <S (P1) [or S (P₀)] <S (Sb1, Kn), determined at constant K index (Kn) a coordinate point for which the following applies:

S (Sbi, Kn) = S (P1) [or S (P₀)].

The S-colored axis here denotes a gray axis, which is exclusively determined by the colorful printing inks (CMY) is built up.

The plumb line on the S-colored axis provides the saturation (S (P2)) that occurs when overprinting P2 with GCR-K or total purchase gives the saturation S (P1) or S (PO).

The result of the desaturation compensation is that in point P2 = (S2, L2) changing saturation S2.  

Darkening compensation and desaturation compensation determine the Brightness and saturation for the new point P2 (L2, S2). Through a two-dimensional nales gradient process in the monotone hue is now very much quickly the required point P2. This point is within the triple remaining pressure space, the pressure values CMY (K) of the starting point for which the GCR was carried out by the colorful pressure triple of P2 and the GCR-K or total-K replaced. P2 is outside the tri-color print room the GCR-K is gradually decremented until the GCR process provides a printable result or the minimum printable black value for GCR-K is undercut. This only arises at the lower four-colored border, where be 100% skeleton black is already added, or with highly saturated bright colors the hue (upper edge of the tri-color printing area F3), in which the The achromatic portion is naturally very low, or is completely absent. In cases of the F4 The range is only with skeleton black (= 100%) and the tri-color Border color built up, the color remains in the rich three-color area colorfully structured.

Claims (4)

1. A method for the freely selectable replacement of the achromatic portion in multicolor printing by the printing ink black on the basis of a spectrophotometrically scanned template, ge during the spectral photometric scanning of the reference points in a color space being obtained, characterized in that

• - According to a preselected degree, the achromatic portion or the complementary color portion of the colors of a multicolored printing ink are replaced by the printing ink black,
• - Photometric data can be recorded from a printing process-specific color scale,
• - the printing ink space is classified into a printing ink-independent color model (HSL model, Fig. 1) and displayed in this model,
• - a two-dimensional non-linear darkening field with brightness values ( Fig. 2) and
• - a two-dimensional non-linear desaturation field is formed, with saturation values ( Fig. 3), which are taken from the template,
• - And that the replacement of the achromatic portion by the printing ink black is carried out by iterative search within the darkening field and the desaturation field.

2. The method according to claim 1, characterized in that after the iterative Looking for a post-interpolation procedure is performed. 3. The method according to claim 1 or 2, characterized in that for the purpose Support the drawing and increase the contrast with a skeleton is worked in black. 4. Device for carrying out the method according to claim 1, characterized by:

• Means for spectrophotometric scanning of a color template, a color space memory for storing support points obtained during the scanning of the template being provided in the spectrophotometric scanning of the template,
• Computing and storage means for replacing the colors of a multicolored printing color space with the printing ink black depending on a preselected degree of the achromatic portion or the complementary coloring portion of the colors of the multicolored printing color space,
• - Means for photometric scanning of a printing process-specific color scale, which are connected to a further memory for storing the support point data obtained when scanning the printing process-specific color scale,
• Means for converting the ink space into an ink-independent color model (HSL model, Fig. 1),
• - Means for generating a two-dimensional non-linear darkening field with brightness values ( Fig. 2) and for generating a two-dimensional non-linear desaturation field with saturation values ( Fig. 3), which are taken from the template and
• - Means for replacing the achromatic portion by the printing ink black by evaluating the darkening field and the saturation field.