Spot color replacement method

Abstract

The present invention relates to a method for substituting spot colors when performing color setting for a printing process on a printing press (7) by means of a computer (6), the method comprising the following steps: calculating, by the computer, substitute color values (14a) for the selected spot colors; calculating the color space of the printing process, and presenting, by means of a display (8), the color space with a restricted color volume (9, 11) of the printing process and the calculated substitute color values (14a) in the color space; checking and correcting, by the user, the color coordinates of the substitute color values and optionally also the color coordinates of further target color values (14b, 14c); performing, in the printing press (7), a printing task with the corrected substitute color values (14b, 14c) and the optionally corrected further target color values.

Description

Technical Field

[0001] The present invention relates to a method for substituting spot colors during color setting (Farbeinstellung) of a printing press.

[0002] The present invention belongs to the technical field of color control of printing presses. Background Art

[0003] Currently, it is necessary to reproduce the color impression of spot colors on a printing press without using spot colors. Especially in digital printing presses, this is important because the ink sets have been typically identified here and no spot colors can be injected. If the color model value / Lab value (Lab-Wert) of a spot color lies outside the color solid (i.e., the presentable color space of the printing process), it is necessary to reproduce a substituted Lab value instead of the original Lab value. The mapping from the original Lab value to the substituted Lab value typically occurs through the ICC color profile (ICC-Profil) of the printing process. Especially for those colors that are far outside the color solid, substituted Lab values that vary from user to user are typically selected. The reason for this is that different users have different expectations for the substituted colors, for example: the color is as rich as possible, the brightness remains unchanged, or the color fits well perceptually into the rest of the design.

[0004] Currently, on the one hand, users can output spot colors and iteratively select suitable colors if necessary, by proof-printing (Ausdruck) different variations (Variationen) of the substituted Lab values pre-given by the color profile. This process is on the one hand time-consuming and largely depends on a good pre-selection of the variations.

[0005] Another possible solution is to manually adapt (anpassen) the color values of the substituted colors. Here, only the Lab values can be meaningfully adapted. The color formulation itself (especially in the inkjet printing process) cannot be manually adapted but must be determined by the output color profile of the machine. In the case of using a manually adapted color formulation, it cannot be ensured that all technically necessary restrictions of the machine are followed (e.g., maximum color application). In addition, this process requires expert knowledge. For example, the user must have a hunch or a feeling about the influence of the change in color coordinates caused by the change in color separation in multicolor printing.

[0006] Because of this color solid It has an irregular pattern in the Lab color space, so it is difficult for users to anticipate what kind of impact the adaptation of the original Lab value will have when the original Lab value is outside the color solid. For users, it is not easy to think about what adaptations need to be made to the Lab value to move it into the color solid. In practice, when manually adapting colors, it always happens that the reproduced color changes strongly and seemingly randomly due to the slightest change in the output parameters. Thus, when analyzing these problems, it has been repeatedly confirmed that for the spot color in question, such Lab color values are defined that are far from all physically printable Lab color values. Now, if a modified nominal *Lab value located outside the color solid is set, this modified nominal *Lab value is mapped to a substitute *Lab value by running the color profile. If a nominal *Lab value far outside the color solid is set, the method has a high probability of error. Although the nominal *Lab value is mapped to a substitute *Lab value that meets the user's wishes, however, the slightest change (e.g., caused by changing the substrate) during the printing process (Druckprozess) requires a new ICC color profile, and thus this slightest change may still result in a significantly changed substitute *Lab value.

[0007] Regarding this problem, from the prior art, US Patent Application US 20160352972 A1 is known. This US patent application discloses the output method of the variant of the spot color and the selection method of the best substitute in the user software. However, in this document, the process of proof printing and selecting the printed color patches for each spot color is time-consuming and may have to iteratively "explore" on the color solid.

[0008] In addition, it is known from the prior art to manually edit the color separation in the color control of the corresponding printing process. However, manually adapting the nominal Lab value is a kind of blind action (Blindflug) because it is not clear where the user is relative to the color solid. Therefore, both of the above two known methods carry the risk of setting a nominal value that does not match the original expectation but is more or less randomly mapped to an acceptable color value through the ICC color profile of the process. Summary of the Invention

[0009] Therefore, the object of the present invention is to provide an improved method for determining substitute colors for spot colors, which is faster, more effective, and less error-prone than the processing methods known from the prior art so far.

[0010] This task is solved by a method for substituting spot colors when setting the color of a printing press for a printing process by means of a computer. The method includes the following steps: calculating, by the computer, a substitution color value for the selected spot color; calculating the color space of the printing process; presenting, by means of a display, the color space having a limited color volume of the printing process, and presenting the calculated substitution color value in the color space; having the user check and correct the color coordinates of the substitution color value (and optionally additional target color values); and executing, in the printing press, a printing task with the corrected substitution color value (and optionally corrected additional target color values). The method according to the invention basically follows the practice of pursuing the best substitution color value for the required spot color. By the method according to the invention, the steps of searching for an optimized substitution color value (or color coordinates) within the color volume can be greatly reduced. By means of the computer's calculation of the color volume and its presentation of the color volume boundaries in the color space, the user is accurately shown the position of the substitution color value in the color space with its substitution color coordinates, and is pointed to the route where the substitution color value must go in order to reach the optimal value inside the color volume to be bounded with its substitution color coordinates from the current value. In this way, the user can efficiently adapt the color coordinates of the substitution color value with the least possible cost, and then execute as quickly as possible, without unnecessary waste, a printing task with this corrected substitution color value. Here, the color coordinates of the substitution color value are usually given as target Lab values. Next, these three concepts are used interchangeably, but all refer to the color coordinates of the substitution color value as target Lab values.

[0011] Advantageous and thus preferred developments of the method according to the invention result from the dependent claims belonging thereto and from the description and the drawings belonging thereto.

[0012] Here, a preferred development of the method according to the invention is that the computer presents to the user the color space of the printing process having a limited color volume, and presents the calculated substitution color value on the display in two independent two-dimensional displays. On these two displays, it is accurately presented to the user the position where the currently calculated substitution color value is located in the limited color volume as color coordinates (or whether the currently calculated substitution color value is outside the limited color volume), and where and by which adaptations of the Lab values of the substitution color value the substitution color value can reach the optimally corrected substitution color value as quickly as possible, this optimally corrected substitution color value not only substituting the spot color as accurately as possible, but also being within the limited range of the color volume (i.e., the printable color space of the printing press used).

[0013] Here, in a further preferred development of the method according to the invention, the computer presents this color space as the Lab color space (Lab color space) via the display unit, and the user corrects the color coordinates of the replacement color value as a change to the Lab value of the color coordinates. Here, this change is presented in the machine-independent Lab color space and is carried out by the user, and is then transformed back into the process color space of the printing press used after the corrected replacement color value has been determined for the spot color to be replaced.

[0014] Here, in a further preferred development of the method according to the invention, the two independent two-dimensional displays each present different cut planes of the Lab color space. Since the Lab color space, like most color spaces, is a multi-dimensional color space (more precisely, a three-dimensional color space here), a always defined cut plane of the Lab color space to be presented is thus presented by these two independent two-dimensional displays.

[0015] Here, in a further preferred development of the method according to the invention, the user checks the color coordinates of the replacement color value: whether the replacement color value lies outside the limited color body of the printing process, and corrects the color coordinates of the replacement color value such that the color coordinates of the replacement color value are moved inside the limited color body of the printing process. Under normal circumstances, when automatically creating replacement color values for the spot colors to be presented, such replacement color values are calculated by means of color regulation methods, which lie inside the limited color body of the printing process and thus inside the printable range of the printing press used. However, if there are changes in the printing conditions, it is possible for color values (or the replacement color coordinates of the replacement color values) to move out of the limited color body of the color space. In this case, the user must manually adapt the replacement color value. If the current position of the replacement color coordinates of the replacement color value and the position of the limited color body of the printing process are not accurately recognized, this can be an extremely error-prone and time-consuming process. By means of the method according to the invention, the user of the printing press is enabled to accurately target these corrections of the replacement color coordinates of the replacement color value and to obtain the desired replacement color values for the spot colors much faster and more effectively than in the prior art.

[0016] Here, a further preferred development of the method according to the invention is that the distance of the corrected color coordinates of the substitute color value within the limited color space of the printing process is as close as possible to the original color coordinates of the substitute color value outside the limited color space of the printing process. Thus, the processing method according to the invention also enables the user to find the corrected color coordinates of the substitute color value, which are as close as possible to the original color coordinates of the substitute color value located outside the limited color space of the printing process. Here, the distance should be as small as possible, because the further the color coordinates of the substitute color value are from the original value, the less likely it is that the color value can reproduce the desired spot color as well as possible. The method according to the invention presents the current color coordinates of the substitute color value and the limited color space, which makes it easy to find the new color coordinates of the substitute color value with this corresponding minimum distance.

[0017] Here, a further preferred development of the method according to the invention is that, in the case of each change in the color settings of the printing press (including the color coordinates of the substitute color value), the computer calculates and displays on the monitor: the color space updated to represent the limited color space of the printing process and the substitute color value calculated in this color space. Since the method according to the invention does not guarantee that the new color coordinates are within the limited color space during the guided manual correction of the color coordinates of the substitute color value, naturally, in the case of each corresponding change in the color coordinates of the substitute color value, the computer recalculates the limited color space in the color space and the new color coordinates of the substitute color value, and updates the presentation to the user accordingly, so that the user can immediately identify what caused the change in the color coordinates of his substitute color value. Thus, the user can also perform further changes (if necessary) in order to optimize the setting of the new color coordinates accordingly. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Next, the invention itself and its structurally and / or functionally advantageous developments will be explained in more detail with reference to the attached drawings according to at least one preferred embodiment. In the drawings, corresponding elements are provided with the same reference numerals. The drawings show:

[0019] Figure 1 showing: an inkjet printing press on which the method according to the invention is carried out,

[0020] Figure 2 showing: a first 2D view of the color coordinates of the limited color space and the substitute color value in the LC plane,

[0021] Figure 3Shows: A first 2D view of color coordinates in the ab plane with respect to a restricted color volume and alternative color values.

[0022] Figure 4 Shows: A second 2D view of color coordinates with a first change in alternative color values in the LC plane.

[0023] Figure 5 Shows: A second 2D view in the ab plane with the first changed alternative color values.

[0024] Figure 6 Shows: A third 2D view of color coordinates with a second change in alternative color values in the LC plane.

[0025] Figure 7 Shows: A third 2D view of color coordinates with a second change in alternative color values in the ab plane.

[0026] Figure 8 Shows: A schematic view of the flow of the method according to the invention. Detailed Description

[0027] The application field of the preferred embodiment of the method according to the invention is printing press 7. Generally, the type of printing press is not decisive for the method according to the invention, as long as they have a process color space and can perform color changes for color adjustment in a Lab color space independent of the printing machine. In Figure 1 An example of the basic construction of such a machine 7 (here in the form of an inkjet printing press) is shown, which includes: a feeder 1 for feeding a printing substrate 2 into a printing mechanism 4 up to a stacker 3, where the printing substrate 2 is printed by a print head 5 at the printing mechanism 4. Here, this relates to a sheet-fed inkjet printing press 7, which is controlled by a control computer 6 with a display 8.

[0028] In Figure 8 The process of the method according to the invention is schematically shown. The method according to the invention gives the user the possibility to achieve orientation in the color space and to adapt the target Lab values of the color coordinates of the alternative color values 14a to the surface of the pattern. Here, the boundaries 10a, 10b of the color volumes 9, 11 are presented for orientation. In this way, the user obtains a preset representation of the available color volumes 9, 11 and can perform a targeted adaptation on the target Lab values 14a in order to reach the color volumes 9, 11. Next, this will be explained with an example, where only the method steps are generally valid, and the quantities used here are only related to any arbitrary example.

[0029] Here, in the following example, the nominal Lab values 14a for the printing process in accordance with the German Institute for Printing and Media Technology 39 (Fogra39) should be given: L = 38, a = 63, b = -16.

[0030] As a starting point, Figure 2 A menu in the display section 8a is shown, which shows the nominal Lab values 14a at a hue angle (Bunttonwinkel) of the nominal Lab values 14a in the LC plane (lightness - chroma, Lightness - Chroma). Additionally, the boundary 10a of the color solid 9 (in the form of a dark / blue line) is shown as a part at constant chroma. The nominal Lab values 14a are located outside the color solid 9.

[0031] Figure 3 A second view 12a is shown. Here, simultaneously with Figure 1 the first view Figure 8 a, the nominal Lab values 14a are shown to the user in the ab plane at the L value of the nominal Lab values 14a. Additionally, the boundary 10a of the color solid 9 (here as a filled surface) is shown as a part at constant lightness. Here, the nominal Lab values 14a are also located outside the color solid 11.

[0032] Now, the first user A adapts in terms of lightness using this information and obtains the modified nominal *Lab values 14b (L = 42, a = 63, b = -16). For this purpose, this nominal Lab value 14b can be touched and moved in the LC view.

[0033] Then, the computer 6 adapts the two views Figure 8 a, 12a presented to the user via the display 8 according to the changed parameters. Correspondingly, Figure 4 the display view Figure 8 b is shown, in which the modified nominal *Lab values 14b and the original nominal Lab values 14a are shown updated in the LC plane. Since the hue angle H has not changed, the same color solid boundary 10a as in Figure 8 is shown. Now, the modified nominal *Lab values 14b are located within the color solid 9. Figure 2 Correspondingly,

[0034] the updated second 2D view 12b is shown. Here, simultaneously with Figure 5 the first view Figure 4 the first view Figure 8b Simultaneously, the modified rated *Lab value 14b and the original rated Lab value 14a are shown in the ab plane with the L value of the modified rated *Lab value 14b. Since the lightness has changed compared to the old second view 12a from Figure 3 a larger boundary of the color solid 11 is now shown as a portion at constant lightness. Now, the modified rated *Lab value 14b lies inside the color solid 11. Since a and b have not been modified, the modified rated *Lab value 14b and the original rated Lab value 14a are superposed on each other in this view 12b, so that the original rated Lab value 14a cannot be seen.

[0035] As an alternative, another user B now adapts the a and b components based on the data presented in the display 8 instead of adapting the lightness L. This other user B obtains a modified rated *Lab value 14c of (L = 38, a = 55, b = -10). For this purpose, this rated Lab value 14c can be touched and moved in the ab view.

[0036] Here, the computer also adapts the two 2D views Figure 8 a, 12a accordingly. Thus, Figure 6 shows the view Figure 8 c: how the modified rated *Lab value 14c and the original rated Lab value 14a are shown in the LC plane. The hue angle H corresponds to the modified rated *Lab value 14c. Since this hue angle has changed compared to Figure 2 different boundaries 10b of the color solid 9 are presented. Now, the modified rated *Lab value 14c also lies inside the color solid 9 here. Since in addition to the hue angle H the chroma C has also changed due to the change in a and b, the modified rated *Lab value 14c and the original rated Lab value 14a do not superpose on each other this time in this view Figure 8 c.

[0037] Simultaneously with Figure 6 the first view Figure 8 c, Figure 7 the second 2D view 12c is shown. Here, the modified rated *Lab value 14c and the original rated Lab value 14a are shown in the ab plane with the L value of the modified rated *Lab value 14c. Since the lightness has not changed with respect to Figure 3 the same color space boundaries as in Figure 3 are also shown here. The moved rated *Lab value 14c also lies inside the color solid 11 in this view 12c.

[0038] Move step by step through the rated *Lab value 14a in the LC plane and the ab plane, so that the rated *Lab value can be depicted in the color space of the process according to the wishes that vary from user to user. Because these visual Figure 8 a, 12a are automatically updated after each movement, so that the user can accurately achieve orientation in the color space. Therefore, in the example shown, it becomes clear at first glance that a slight movement towards higher lightness is sufficient to reach the color space. In addition, the user immediately sees that without changing the lightness, these color bodies 9, 11 cannot be reached only by changing the hue angle alone, but only by reducing the chroma / chroma C.

[0039] The presented method is equally suitable for correcting those rated Lab values 14s located inside the color bodies 9, 11. In this way, a very simple adaptation can be achieved, for example: "The user wants to move a red spot color slightly towards the orange direction."

[0040] A possible alternative to the method according to the invention lies in presenting a full 3D presentation of the color bodies 9, 11 and the original rated Lab values 14a (or adapted rated *Lab values 14b, 14c) of the spot color. It is also possible to show three 2D views (i.e., the La plane, the Lb plane, the ab plane) in the Lab color space. Of course, in the full 3D view, the movement of the rated *Lab value 14a is significantly more difficult to perform accurately. In addition, the user must place the view itself in the view state that is correctly rotated for its correction. Compared with what is originally required, there are more operation possibilities with the three 2D views. This will result in the clarity being impaired due to this.

[0041] The important advantages of the present invention are summarized as follows: When adapting the Lab value 14a of the spot color, the color body is directly presented, and the user can adapt the rated Lab value 14a by moving a point in this view. Only a small amount of specialized knowledge about the relationship between the numerical value of the Lab value and the color impression is still required. Moreover, the user does not have to know the shape of the color bodies 9, 11 of the printing process, because the relevant contours will be continuously shown to the user during the adaptation. This increases the difficulty of unreasonable settings for the user, and the adaptation can be achieved with just a few mouse clicks.

[0042] List of reference numerals

[0043] 1 Feeder

[0044] 2 Current printing substrate / current printed sheet

[0045] 3 Reclaimer

[0046] 4 Inkjet printing mechanism

[0047] 5 Inkjet print head

[0048] 6 Computer

[0049] 7 Inkjet printer

[0050] 8 Monitor

[0051] 8a First display section / view in the LC plane

[0052] 8b Second display section / view in the LC plane

[0053] 8c Third display section / view in the LC plane

[0054] 9 Color solid in the LC plane

[0055] 10a Color space boundary of the original color solid in the LC plane

[0056] 10b Color space boundary of the adapted color solid in the LC plane

[0057] 11 Original color solid in the ab plane

[0058] 12a First display section / view in the ab plane

[0059] 12b Second display section / view in the ab plane

[0060] 12c Third display section / view in the ab plane

[0061] 13 Current color setting

[0062] 14a Original color coordinates

[0063] 14b L value of the modified color coordinates

[0064] 14c ab value of the modified color coordinates

Claims

1. A method for substituting spot colors when performing color setting on a printing press (7) for a printing process by means of a computer (6), the method comprising the following steps: · Calculating, by the computer, a substitute color value (14a) for the selected spot color; · Calculating the color space of the printing process and presenting, by means of a display (8), the color space of the printing process having a restricted color volume (9, 11) and the calculated substitute color value (14a) in this color space; · Having the user check and correct the color coordinates of the substitute color value; · Executing, in the printing press (7), a printing task with the corrected substitute color value (14b, 14c); · The user checks the color coordinates (14a) of the substitute color value: whether the substitute color value is outside the restricted color volume (9, 11) of the printing process, and corrects the color coordinates (14a) of the substitute color value such that the color coordinates (14b, 14c) of the substitute color value are moved inside the restricted color volume (9, 11) of the printing process.

2. The method according to claim 1, wherein, the computer (6) presents to the user, on the display (8), in two independent two-dimensional display sections (8a, 12a), the color space of the printing process having a restricted color volume (9, 11) and the calculated substitute color value (14b, 14c).

3. The method according to claim 2, wherein, the computer (6) presents the color space as a Lab color space through the display section, and the user implements the correction of the color coordinates (14a) of the substitute color value as a change in the Lab values (14b, 14c) of the color coordinates.

4. The method according to claim 3, wherein, the two independent two-dimensional display sections (8a, 12a) respectively present different cross-sections of the Lab color space.

5. The method according to claim 1, wherein, the distance of the corrected color coordinates (14b, 14c) of the substitute color value that are inside the restricted color volume (9, 11) of the printing process is made as close as possible to the original color coordinates (14a) of the substitute color value that are outside the restricted color volume (9, 11) of the printing process.

6. The method according to any one of claims 1 to 5 above, wherein, in the case of each change in the color setting of the printing press (7) including the color coordinates (14a) of the substitute color value, the computer (6) calculates and displays on the display (8): an updated presentation (8b, 8c) of the color space having the restricted color volume (9, 11) of the printing process and the calculated substitute color value (14b, 14c) in this color space.

7. The method according to any one of claims 1 to 5 above, It is characterized in that the method further comprising: When checking and correcting the color coordinates of the replacement color value, additional nominal color values are checked and corrected.

8. The method according to claim 7, It is characterized in that The method further includes: Using the corrected additional nominal color values when performing a printing task with the corrected replacement color value.

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