EP2444250A1 - Device and method for digital printing of decorative elements - Google Patents

Abstract

The method involves selecting a tarn-image file via a control device such that the file exhibits a pattern in an environment of a region that is re-painted by an incorrect color point-generator, so that a measure of histogram of a texture feature exceeds a predetermined limitation value. The measure of histogram is determined from the region with and without using the texture that is caused by incorrect function of the generator. A sensor detects faulty function of the color point-generator, which is provided in a print head (81) of a digital printer. An independent claim is also included for a device for digital printing of a decoration unit.

Description

invention

The invention relates generally to the technical field of digital printing. In particular, the invention relates to a method and a device for the production of decors on a surface to be printed by means of digital printing.

Background of the invention description

The production of aesthetically sophisticated multicolored patterned decors for the flooring industry, the furniture industry, the cladding of interior and exterior walls is increasingly no longer due to the elaborate and more suitable for mass production multicolor rotary gravure, but by in terms of the batch size much more flexible inkjet printing.

The quality standard of such decors, which often reproduce wood structures, marble and similar natural structures in great detail, is dictated by traditional and highly professional manufacturers of decorative paper printers. These are able to produce high-quality decorative paperboards cost-effectively in large quantities with a quality that is very close to that of the original.

In order to serve the market of small series of decorative papers, thermoplastic decorative films or directly printed panels, but increasingly ink jet decor printers are used, which are also able to produce small quantities up to the lot size EINS.

The requirements for an inkjet printer for the production of decors on a substrate (panel, foil, paper, etc.) differ significantly from the printing of classical print media such as advertising materials, packaging printing, advertisements. While the latter usually contain numerous unprinted areas, decors are large uninterrupted multicolored images, which may repeat themselves after a repeat, wherein the repeat border is graphically designed so that it is invisible to the eye.

• durch die direkte Bedruckung des oberflächenbehandelten Panels entfällt die getrennte Herstellung von dekorierten Papieren und Folien, welche in einem zweiten Arbeitsgang aufgepresst oder auflaminiert werden müssen,
• alle Dekore werden oft mit lediglich den vier Grundfarben CMYK gedruckt, während die Dekorpapierdrucker typischerweise spezielle Schmuckfarben verwenden, deren Fertigung und Handhabung komplex ist,
• das Dekor-Druckbild wird direkt aus der digitalen Vorlage abgeleitet; es werden keine Druckzylinder benötigt und die Umstellung auf neue Motive ist praktisch augenblicklich möglich.

Inkjet printing technology has a number of advantages over classic rotary decorative paper printing, including:

• the direct printing of the surface-treated panel eliminates the separate production of decorated papers and foils, which must be pressed or laminated in a second operation,
• all decors are often printed with only the four primary colors CMYK, while the decorative paper printers typically use special spot colors, whose manufacture and handling is complex,
• the decor print image is derived directly from the digital template; No printing cylinders are needed and switching to new motifs is possible almost instantly.

These printing processes, which include not only inkjet printing but also thermal transfer printing, electrostatic printing and, in general, all printing technologies that directly generate dots on a substrate, are therefore often referred to in the trade as "digital printing".

This term is also adopted in the present application, the focus of the description of the inventive concept being specifically concerned with inkjet printing.

However, digital printing, and in particular ink-jet printing technology, has a number of inherent weaknesses compared to traditional rotary gravure printing, especially when it comes to rapid printing of wood panels for the flooring, furniture and interior wall and exterior wall industries. Although nozzle head technology has greatly improved over the past few years, typical ink jet printer errors, such as nozzle stalling (nozzle does not open or nozzle open), or stuttering of a nozzle, are therefore nonexistent rare events.

There are known optical or electronic monitoring systems which monitor each individual nozzle and report their malfunction. But even if such an error can be detected and reported automatically, the decoration process for maintenance of the printer would have to be aborted for each error message. The productivity of the decoration process would suffer unduly.

To reduce the above problem, the decor designer strives to aesthetically decorate the décor (eg, a wood grain) so that the failure of a single nozzle is minimally noticeable visually. This can be partially achieved with light decors, but much less with highly contrasting decors. This approach also severely restricts the variety of possible decorations, as such a tolerant decor must ensure the suppression of the visibility of the most common ink jet printing errors (open nozzle, clogged nozzle) at any point on the decor image.

Therefore, the inkjet decoration is often used only in low-priced goods with low quality standards, in which well visible printer errors are accepted. High-quality decors will continue to be produced using the classic decorative paper printed in gravure.

Obviously, these two approaches are unsatisfactory and constitute a major obstacle to the use of digital printers, especially inkjet printers, for decor printing.

It is also known from the prior art, when nozzle errors occur which essentially consist of elongated strips, to locally change the printed image in order to camouflage it or make it appear less visible.

For example, in the US 2005 / 0259296A1 describes a printing method for camouflaging faulty printing elements, in which errors occurring in printing - such as caused by failure of a nozzle - be camouflaged by means of a correction of the printed image in the vicinity of the defective pixels. For this purpose, a gray value is taken into account for each pixel of a pixel matrix to be printed in the form of an 8-bit word, ie a natural number between 0 and 255. By means of a mathematical error diffusion process, an error determined with regard to the gray value is distributed in a camouflaging manner over an area surrounding the error region. In this case, an output pixel is related to a target pixel via weighting factors. Errors occurring in an output pixel, such as a nozzle failure or unwanted black print, are compensated in the environment such that an averaged gray level of the camouflaged, ie modified print image resembles the desired gray level of the same area of the image to be printed.

From the US 2007/0070111 A1 For example, a method of compensating for a defective nozzle in an inkjet printer is described. Also in the process of US 2007/0070111 A1 An attempt is made to make the failure or incorrect color content of one of the primary colors black, cyan, magenta and yellow largely invisible due to an error camouflaging the image data to be printed in the vicinity of the defect. In this case, if one color nozzle fails for one pixel area, another color nozzle that does not precipitate in this area can be used or the Print data is exchanged accordingly so that instead of a non-printed pixel, a pixel printed with a different color appears adjacent to a modified printed pixel. As a result, a failure of color nozzles caused by white lines can be avoided and a contrast or a low-contrast texture concealing the error as far as possible can be achieved.

Both of the above methods are very complex to perform, since they concern a local change of the print file in the vicinity of the defective nozzle. To calculate a local camouflage image in a narrow corridor to the left and right of the faulty nozzle track, very complex algorithms are required. This proves to be particularly disadvantageous if the production downtime required for this purpose should be short.

Object of the invention

There is therefore a great economic and technical interest in having a simple, also for the not trained in the theory of camouflage and image synthesis trained printer understandable method and an arrangement for carrying out the method in which the decorations produced by ink jet printing in the Failure of individual nozzles can be continued without substantial production interruption by the nozzle cleaning cycles required during nozzle failure and / or required complex local changes in the RIP (RIP) image file (RIP).

Summary of the invention

The object is achieved by a method and a device for producing decors on a surface to be printed by means of digital printing with the features of method claim 1 or the device claim 12.

Advantageous embodiments and further developments are specified in the dependent claims.

The inventive idea of the present application relates to such a simple measure for increasing the productivity of digital printers, in particular for the production of decorated flat substrates. This is summarized, shortened, achieved in that in the case of a printing error by failure of at least one color pixel generator (an ink droplets from pushing nozzle) of the current print job is canceled and that from the list of pending print jobs with their respective different decors the Production is performed with that print job welter in which the decor to be printed has such an appearance that the caused by the erroneous function of the color dot generator oblong strip is only slightly visible, so this new print job as opposed to the previous the required visual quality also without repair of the defective color point generator.

The change in the decor image consists here in an exchange with another decor image.

Accordingly, the invention provides a method for decorating surfaces with planar, multicolor patterned decors by means of digital printing, wherein the surface to be printed moves relative to at least one equipped with a plurality of color dot generators printhead of a digital printer, wherein the printhead is controlled by a control device is, which control data corresponding to a stored print pattern transmits to the print head, so that it generates a decor on the passing surface, wherein by means of at least one sensor, a faulty function of the color point generator is detected. In this case, in response to the detection of a faulty function of at least one color point generator, the active print job is canceled. An arithmetic unit determines the printing pattern of the at least one defective color dot generator. A comparison unit compares this print pattern with print patterns of the still to be produced decors. Furthermore, the control unit transmits that printing pattern to the print head which has the smallest deviation in the region of the at least one defective color dot generator, so that the error of the at least one color dot producer appears least visible to the human viewer in the decor of the surface to be printed.

Thus, in particular, by comparing histograms of the textures, a texture image is selected which is visually similar to that of the texture image generated by the defective color point generator (s).

The corresponding device for carrying out the method described above accordingly comprises at least one print head of a digital printer equipped with a multiplicity of color dot generators for generating the decors, a feed device for moving the surface to be printed relative to the print head, a control unit for controlling the print head with control data a stored print image, at least one sensor for detecting an erroneous function of individual color point generators, an arithmetic unit for determining the print pattern of at least one erroneous color point generator, a comparison unit for comparing this print pattern with print patterns of the decors yet to be produced, wherein the control device is set up, in response to the detection of an erroneous function of at least one color dot generator, canceling the active print job and transferring the print pattern to the printhead, which is in Ber eich of the at least one faulty color point generator has the slightest deviation, so that the error of the at least one color point producer in the decor of the printed surface for the human observer appears least visible.

In a further preferred method step of the method according to the invention for the production of decors, the control unit transmits only that print pattern to the print head which falls below a threshold value which results from a permissible deviation of the print pattern in the region of the at least one defective color dot generator to the print patterns of the still to be produced Decors is determined. In this case, the threshold value can be set. With the help of histograms print patterns (textures) can be characterized. These are particularly important for calculating the extent of visibility of the defect of the print pattern. Thus, the deviation of the printing pattern in the region of the at least one defective color dot generator from the print patterns of the decors still to be produced can advantageously be determined with the aid of a histogram.

• das Histogramm H1 der vorkommenden Grau-, beziehungsweise Helligkeitswerte p(I),
• das Histogramm H2 der Kontur-Richtungen p(α),
• das Histogramm H3 der Krümmungsradien p(r) der Maserungen.

There are a number of histograms that can be used to characterize textures. Particularly suitable are the following histogram types:

• the histogram H1 of the occurring gray or brightness values p (I),
• the histogram H2 of the contour directions p (α),
• the histogram H3 of the radii of curvature p (r) of the grains.

Preferably, histograms of different histogram types, such as in particular all three aforementioned histogram types, are determined and the similarity of the image files with the texture containing the defect is determined.

• der Korrelationskoeffizient zwischen den Histogrammen Hi einer Umgebung in der Bilddatei ohne den Fehler und Hi* der Umgebung mit dem Fehler,
• das Komplement des Betrags der Abweichung zwischen den Histogrammen Hi, Hi* einer Umgebung ohne den Fehler, beziehungsweise mit dem Fehler.

In a further embodiment of the method according to the invention, at least one of the following similarity dimensions for an environment of the error in the image file generated by the malfunction of the color point generator can be determined:

• the correlation coefficient between the histograms Hi of an environment in the image file without the error and hi environment with the mistake
• the complement of the amount of deviation between the histograms Hi, Hi * of an environment without the error, or with the error.

An image file can be created for a selection from the print images still to be printed, wherein the print pattern is at least partially replaced by the image file which generates an altered local print pattern at least in the vicinity of the at least one defective color dot generator, the control device thus producing the image file selects or creates such that it has a pattern in the vicinity of the region swept by the defective color dot generator that is visually similar to a printed pattern generated by the defective color dot generator.

Such a change may include a crop, a new compilation of patches of the image, a scaling, a reflection, a rotation, etc.

The selected or created image data thereby has local texture smears which are as similar as possible to the local texture properties produced by the erroneous color point generators.

Thereby, a visually similar structure as the error generated by the failure of the nozzle can be generated and thus a reduction or possibly compensation of the printing error for the human visual system can be effected.

According to one embodiment of the invention, the image files available for selection are stored in a memory of the device according to the invention. Likewise, it is alternatively or additionally possible, in addition to change the image file from existing image data, such as those that are currently intended for decor printing.

In both cases, the image file may contain, at least in the vicinity of the pixels which are erroneously printed by the at least one defective color dot generator, such patterns which, by utilizing optical illusion effects, correct the defective printing of the at least one nozzle in shape and color human visual system does not or only very weakly recognize.

Further, with an ink F1, the image file may be selected or created to display a local print image having predominantly oblong, background low-contrast patterns with a dominant color F1 at least in the vicinity of the at least one defective color dot generator At least one color point generator permanently printing with the ink F2, the image file is selected or generated such that it displays a local print image which predominantly has elongated patterns contrasting with the background, dominating at least in the vicinity of the pixel having at least one defective color dot generator Color F2 has.

In a further method step, in the case of an erroneous function of at least one color point generator, the print patterns of the decors still to be produced can be displayed on a display unit. In the respective printing pattern, this or these printed pattern shown on the display unit can be simulated the occurred printing error of the at least one color dot generator,

A difference of an error range descriptive feature vector between the print pattern without simulated error of the at least one color dot generator and the printed pattern with simulated error of the at least one color dot generator can be displayed numerically.

By way of exemplary explanation of the spirit of the invention, a digital decor printer for natural wood decors will be discussed based on ink-jet printing in which typical errors such as non-closing nozzle, non-opening nozzle, or irregular nozzle will occur, resulting in objectionable stripe-shaped defects.

For this purpose, the following figures are used, with only the high-contrast contours of the wood decor are sketched, but not the color information can be reproduced. The same reference numerals hereby refer to the same structures.

• Fig. 1 zeigt schematisch ein Dekor 30, welches durch einen Druckkopf 10 eines Tintenstrahldruckers fehlerfrei erzeugt wird. Der Druckkopf 10 umfasst vier Düsenstöcken 12 bis 15. Die Düsenstöcke 12 bis 15 sind jeweils einer von vier Primärfarben CYMK zugeordnet, welche in Form kleinster Tintenstrahltröpfchen auf einem hellen Hintergrund 20 einer Laminatdiele 8 im Durchlauf ein Dekor 30 erzeugen.
• Fig. 2 zeigt schematisch das Dekor 30, welches durch den Druckkopf 10 fehlerhaften auf dem hellen Hintergrund 20 der Laminatdiele 8 erzeugt wurde. Dabei wurde durch eine nicht schließende Düse 16 (dargestellt in Fig. 1) ein dunkler Streifen 21 erzeugt, dessen Farbe einem der in dem Düsenstock 12 bis 15 zugeordneten Primärfarbe (C oder M oder Y oder K) entspricht. Die Breite des Streifens 21 ist aus Gründen der besseren Verdeutlichung des Erfindungsgedanken stark vergrößert dargestellt.
• Fig. 3 zeigt beispielhaft anhand des fehlerhaften dunklen Streifens 21 nach Fig. 2, wie die Sichtbarkeit des durch die nicht schließende Düse 16 erzeugten dunklen Streifens 21 stark reduziert wird, indem der laufende Druckauftrag abgebrochen und durch einen neuen Druckauftrag mit einem andersartigen Holzdekor weitergeführt wird, welcher in der Umgebung der nicht schließenden Düse 16 starke kontrastreiche und längliche Holzmaserungs-Strukturen besitzt. So ist trotz Fortbestehen des Fehlers der Düse 16 der fehlerhafte dunkle Streifen 21 nicht mehr störend sichtbar. Der Dekor 30 der Laminatdiele 8 kann somit ohne zeitaufwändige Wartung der fehlerhaften Düse 16 des Druckkopfes 10 produziert werden.
• Fig.4 zeigt beispielhaft ein Druckauftrag 40 eines Druckmusters, bei welchem durch den Ausfall einer Düse 16 (Düse öffnet nicht mehr) ein störender heller Streifen 22 erscheint, sowie die Warteschlange von vier anstehenden Druckaufträgen 41 bis 44. Das Dekor 30 des Druckauftrags 44 ist geeignet, den Druck trotz des Fehlers der Düse 16 fortzusetzen, da an der Stelle des Fehlers der Düse 16 und des damit einhergehenden hellen Streifens 22 das Dekor 30 eine helle Zone aufweist und damit der Fehler der Düse 16 nicht oder nur wenig auffällt.
• Flg. 5 zeigt eine Anordnung zum Bedrucken einer Laminatdiele 8 mit vier Düsenstöcken 12 bis 15, welche jeweils die Farben Y (Yellow = Gelb), C (=Cyan), M (Magenta) und K (Black = Schwarz) in Form kleinster Tröpfchen auf das Substrat bringen.
  Durch einen Sensor 50 wird der Ausfall wenigstens einer Düse 16 erkannt und an eine Steuereinheit 52 weitergeleitet. Diese vergleicht den durch den Sensor 50 erkannten fehlerbehafteten Ausschnitt des Dekors 30 mit den n sich in einer Warteschlange 60 befindlichen weiteren Druckaufträgen 41 bis 44 und wählt für das Weiterdrucken denjenigen Druckauftrag 41 bis 44 aus, in welchem sich der Fehler am wenigsten visuell erkennbar darstellt. Über ein Netzwerk 54 werden die Druckaufträge 41 bis 44 in die Warteschlange 60 fortlaufend nachgeladen.
• Fig. 6 verdeutlicht beispielhaft, wie durch die Berechnung von drei lokalen Textur-Histogrammen H1 bis H3 in der Umgebung des Fehlers, nämlich:

  p(I)=

  Wahrscheinlichkeitsverteilung der Intensitätswerte

  p (α) =

  Wahrscheinlichkeitsverteilung der Konturrichtungen

  p(r)=

  Wahrscheinlichkeitsverteilung der Krümmungsradien

  das visuelle Aussehen des fehlerhaft bedruckten Ausschnittes mathematisch beschrieben werden kann und als beschreibender Merkmalsvektors für das Aussehen dieses Fehlergebietes genutzt werden kann.
• Fig. 7 zeigt den Ablauf der Bestimmung des am besten geeigneten Folge-Druckauftrags beim Auftreten eines Fehlers der Düse16 auf der zu bedruckenden Laminatdiele 8.

In the drawing:

• Fig. 1 schematically shows a decor 30, which is generated by a printhead 10 of an inkjet printer error-free. The printhead 10 comprises four nozzle sticks 12 to 15. The nozzle sticks 12 to 15 are each associated with one of four primary colors CYMK, which in the form of minute inkjet droplets on a light background 20 of a laminate board 8 in the passage create a decor 30.
• Fig. 2 schematically shows the decor 30, which was generated by the print head 10 faulty on the light background 20 of the laminate flooring 8. It was characterized by a non-closing nozzle 16 (shown in FIG Fig. 1 ) produces a dark strip 21 whose color corresponds to one of the primary colors (C or M or Y or K) assigned in the nozzle 12 to 15. The width of the strip 21 is shown greatly enlarged for reasons of better clarity of the inventive concept.
• Fig. 3 shows by way of example with reference to the defective dark strip 21 after Fig. 2 in that the visibility of the dark strip 21 produced by the non-closing nozzle 16 is greatly reduced by aborting the current print job and changing it by a new print job Wood decor is continued, which has strong contrast-rich and elongated woodgrain structures in the vicinity of the non-closing nozzle 16. Thus, despite the failure of the nozzle 16, the defective dark strip 21 is no longer objectionable. The decor 30 of the laminate flooring 8 can thus be produced without time-consuming maintenance of the defective nozzle 16 of the print head 10.
• Figure 4 2 shows by way of example a print job 40 of a printing pattern in which a disruptive bright strip 22 appears due to the failure of a nozzle 16 (nozzle no longer opens) and the queue of four pending print jobs 41 to 44. The decor 30 of the print job 44 is suitable for printing Pressure to continue despite the error of the nozzle 16, since at the location of the error of the nozzle 16 and the accompanying light stripe 22, the decor 30 has a bright zone and thus the error of the nozzle 16 is not or only slightly noticeable.
• Flg. 5 shows an arrangement for printing a laminate plank 8 with four nozzle sticks 12 to 15, each of which has the colors Y (yellow), C (= cyan), M (magenta) and K (black = black) in the form of very small droplets Bring substrate.
  By a sensor 50, the failure of at least one nozzle 16 is detected and forwarded to a control unit 52. This compares the faulty cut-out of the decor 30 detected by the sensor 50 with the n further print jobs 41 to 44 in a queue 60 and selects, for further printing, the print job 41 to 44 in which the defect is the least visually recognizable. Via a network 54, the print jobs 41 to 44 are loaded into the queue 60 consecutively.
• Fig. 6 illustrates, for example, how by calculating three local texture histograms H1 to H3 in the environment of the error, namely:

  p (I) =

  Probability distribution of the intensity values

  p (α) =

  Probability distribution of the contour directions

  p (r) =

  Probability distribution of radii of curvature

  the visual appearance of the faulty printed section can be mathematically described and used as a descriptive feature vector for the appearance of this defect area.
• Fig. 7 FIG. 12 shows the process of determining the most appropriate subsequent print job upon the occurrence of a defect of the nozzle 16 on the laminate board 8 to be printed.

1. A) die Berechnung der drei lokalen Textur-Histogramme [p(I); p(α) ; p(r)] aus, welche das längliche Gebiet um den Düsenfehler in einem mathematischen Texturmodell beschreiben.
2. B) Die Berechnung der lokalen Textur-Histogramme der Druckaufträge 41, 42, 43, 44 für alle sich in der Warteschlange 60 befindlichen Druckaufträge 41 bis 44 an der Stelle der fehlerhaften Düse 16.

The error detection unit triggers a nozzle 16 when an error occurs:

1. A) the calculation of the three local texture histograms [p (I); p (α); p (r)] which describe the oblong area around the nozzle error in a mathematical texture model.
2. B) The calculation of the local texture histograms of the print jobs 41, 42, 43, 44 for all print jobs 41 to 44 in the queue 60 at the location of the faulty nozzle 16.

As a follow-up pressure, the control unit 52 selects that print job 41, 42, 43, 44 whose texture histogram H1 to H3 has the smallest deviation from that of the faulty print.

The idea of the invention should follow Fig.1 the example of one with a wood grain of a decor 30 in the run with a 4-color ink jet print head 10 consisting of the nozzle sticks 12,13,14 and 15 to be printed laminate plank 8 explained. The decor 30 is in the subtractive color of the four primary colors cyan, magenta, yellow, black (CYMK) formed on the brightly prepared background 20 of the laminate flooring 8 in the form of smallest ejected droplets.

Fig. 2 shows the typical error case of a color point generator (hereinafter also referred to as nozzle 16), in which, for example, no longer closes a nozzle 16 for black color, thus creating a visually disturbing continuous dark streaks 21. This error is visually so troublesome that production must be stopped immediately and the black ink nozzle 16 serviced by a cleaning process. This means a typical production stoppage of 15 to 30 minutes and a corresponding loss of productivity.

Fig. 3 shows the concept of the invention in its simplest form. If next to the print job 40 with a decor 30 after Fig. 2 also laminate flooring 8 with a decor 30 (see Fig. 3 ) are to print, in this decor 30 of the nozzle defect in the form of the dark strip 21 would hardly disturb visually, since he changed the local texture due to the already dense there linear line grain image only insignificantly.

Fig. 4 illustrates the concept of the invention in another example, the reverse error of a nozzle 18, in which no longer opens a faulty nozzle for black color and thus causes a disturbing unprinted light stripe 22. Again, there is in the queue of still to print jobs 41- to -44 a suitable decor 30 of the print job 44, in which this bright stripe disturbs little visually, since in this position, the pattern to be printed is patterned very small.

Further printing with the application decor 30 of the print job 44 can therefore be continued immediately, ie without production standstill by cleaning cycles of the print head 10, with this new print job 44. The switching of print jobs 40 is very fast, because essentially only to another file stored in RIP memory (Raster Image Processor Graphics Memory).

This rapid re-printing means a significant increase in product value compared to the known method of turning on cleaning cycles or calculating new local stealth files.

1. a) eine hochauflösende Farbzeilenkamera mit einer Mustererkennung zur Detektion von streifigen Fehlern
2. b) eine oder mehrere benachbart angeordnete monochrome Kameras mit stroboskopischer Mehrfarben Beleuchtung und einer Detektionseinheit von streifigen Fehlern

Fig. 5 illustrates the signal flow according to the invention for carrying out the method. Upon exiting the 4-color printing unit, the print result is checked with an imaging sensor 50 and the presence of nozzle errors detected. Such sensors 50 are known to those skilled in the image processing and may be, for example:

1. a) a high-resolution color line camera with a pattern recognition for the detection of streaky errors
2. b) one or more adjacent arranged monochrome cameras with stroboscopic multi-color illumination and a detection unit of streaky errors

The sensor 50 identifies the location of the faulty nozzle 16 as well as the affected nozzle 12-12. At the same time, it provides a color image of the faulty elongated section to the control unit 52, which uses color texture analysis techniques to mathematically describe the visual appearance of that patch.

The control unit 52 compares this description with the corresponding descriptions of the subsequent jobs in the queue 60 with the print jobs 41 to 44 for best match. The queue 60 is usually replenished continuously by the parent production computer via a network 54.

The description of the visual impression of a local nozzle error is carried out according to the invention by a mathematical texture model. As in Fig. 6 2, it is sufficient, upon detecting a defect of a nozzle 16 in the form of a dark stripe 21, to describe a narrow elongated area around this defect stripe by a perceptual texture model. This is a model which describes the human perception of color textures such as woodgrains by mathematical models in such a way that the properties important for the human visual system, such as the dominant colors, dominant contour directions, occurring directional angles of the grain lines, are approximated by statistical models.

By way of example, this is done by image-wise evaluation of the image taken with the sensor 50 along an elongated area centered around the defect of the nozzle 16, such that three local histograms H1 to H3 for the brightness values p (I) occurring in this area. , the contour directions p (α) occurring there and the curvature radii p (r) of the wood grain lines occurring there are calculated.

These texture descriptors describing the appearance are exemplary; the literature of color textures has a number of other mathematical models for describing natural or synthetic texture images.

Often, these individual statistical features are combined into a feature vector M with the components [M1, M2, M3].

Fig. 7 describes the last method step according to the invention for the exemplary nozzle error "black elongated strip due to no longer closing K nozzle" in a block diagram.

After the sensor 52 has detected a nozzle error and has triggered the calculation of the feature vector M from the oblong region around the nozzle error, the control unit 52 initiates the calculation of the same feature vector M for all print jobs 41 through 44 in the print queue 60. The comparison unit 56 compares the feature vector of the defective area with the n feature vectors of the decors 30 in the queue 60 and selects for further printing that print job whose feature vector is most similar, the difference must not exceed a predetermined visibility threshold. If a matching job can not be found, production must be stopped and a printhead cleaning cycle started.

The mentioned feature vectors are to be understood as examples; The concept of the invention encompasses all mathematical models known to the person skilled in the art of color textures, which are suitable for describing the visual impression and the deviation between two monochrome or colored textures.

According to the invention, the decision whether and with which print job from the queue in the case of a nozzle error is further printed, not only the automatically running process of determining the mathematically similar decoration, but at the request of the operator of the system those print pattern for further printing on a color screen higher Quality visual comparison and visual assessment presented. This allows the operator to override the automatic selection of the subsequent print job.

According to the present invention, this or these follow-on print jobs represented on the monitor are simulated in the respective image file and the nozzle defect that has occurred is displayed for better assessment of the visual quality of the subsequent print job.

Optionally, the difference between the feature vector describing the error zone and the selected follow-up print job with simulated nozzle error and the selected follow-on print job with simulated nozzle error is superimposed numerically to give the user, in addition to the visual impression, also a numerical measure of the residual deviation of the decoration in the document Environment of remaining nozzle error displayed.

The idea of the invention is not restricted to the printing of laminate flooring designs discussed by way of example, but relates to all substrates which are printed over large areas with pictorial decors by means of digital printing in an analogous manner.

It also encompasses alternative digital printing techniques for ink jet printing, which operate by spot coloring a substrate under electronic control, such as electrophoresis on charged toner particles or on digital printers operating on thermochromic processes.

LIST OF REFERENCE NUMBERS

8th

laminate plank

9

wood grain

10

printhead

12, 13

14, 15

nozzle assemblies

16

jet

20

bright background

21

dark stripes

22

bright stripes

30

decor

41-44

Print job, image file

50

sensor

52

control unit

54

network

56

comparing unit

60

queue

71-73

texture histogram

Claims (13)

Process for the production of decors on a surface to be printed by means of digital printing, wherein the surface to be printed moves relative to at least one printhead of a digital printer equipped with a plurality of color dot generators,
in which - The printhead is controlled by a control device, which transmits control data corresponding to a stored print pattern to the printhead, so that it creates a decor on the moving past surface, wherein by means of at least one sensor an erroneous function of the color point generators is detected,
characterized in that - in response to the detection of an erroneous function of at least one color point generator, the active print job is aborted, a computing unit determines the printing pattern of the at least one defective color dot generator, a comparison unit compares this print pattern with print patterns of the decors still to be produced and - The control unit transmits the one printing pattern to the printhead, which has the smallest deviation in the region of the at least one defective color dot generator, so that the error of the at least one color point generator in the decor of surface to be printed appears least visible to the human observer. A method for the production of decors according to claim 1, characterized in that the control unit transmits only the one printing pattern to the print head, which falls below a threshold value, from an allowable deviation of the print pattern in the region of the at least one erroneous color point generator to the print patterns still to producing decors is determined. Method for the production of decors according to any preceding claim, characterized in that the threshold value is adjustable. Method for the production of decors according to any preceding claim, characterized in that for the determination of the deviation, in particular for a calculation of the extent of the visibility of the defect of the printing pattern, at least one, preferably all, of the following histogram types are determined: the histogram H1 of the occurring gray values p (I), the histogram H2 of the contour directions p (α), the histogram H3 of the radii of curvature p (r) of the grains. Method for producing decors according to any preceding claim, characterized in that at least one of the following similarity measures for an environment of the error in the image data set generated by the malfunction of the color point generator is determined: the correlation coefficient between the histograms of an environment in the image file without the error and hi environment with the error, - the complement of the amount of deviation between the Histogrammen Hi, Hi * an environment without the error, or with the error. Method for the production of decors according to one of the preceding claims, characterized in that the printed pattern is at least partially replaced by an image file which generates at least in the vicinity of the at least one defective color dot generator, a changed local print pattern, the control means so selecting the image file or is made to have a pattern in the vicinity of the area swept by the defective color dot generator, which pattern is visually similar to a printed pattern generated by the defective color dot generator. A method according to claim 6, characterized in that the decor printing is continued with the image file whose decor is better suited for reducing the visibility of the printing error. Method for the production of decors according to claim 6, characterized in that with an ink F1, the image file is selected or created so that it displays a local print image, which at least in the vicinity of the at least one defective color point generator predominantly elongated, the background contrast weak Pattern having a dominant color F1, or
in the presence of at least one color point generator permanently printing with the printing ink F2, the image file is selected or generated such that it displays a local print image which predominantly has elongated patterns contrasting with the background, at least in the vicinity of the pixels corresponding to at least one defective color dot generator having a dominant color F2. Method for the production of decors according to one of the preceding claims, characterized in that in case of an erroneous function of at least one color point generator, the print patterns of the still to be produced decors are displayed on a display unit Method for the production of decors according to claim 9, characterized in that in the respective printing pattern this or these printing patterns shown on the display unit of the occurred printing errors of the at least one color point generator is simulated. A method of producing decors according to claim 10, characterized in that the difference in a feature area describing the defect pattern between the print pattern without simulated error of the at least one color dot generator and the simulated error print pattern of the at least one color dot generator displayed numerically. Device for the production of decors on a surface to be printed by means of digital printing, with at least one printhead of a digital printer equipped with a plurality of color dot generators for producing the decors, a feed device for moving the surface to be printed relative to the print head, a control unit for controlling the print head with control data corresponding to a stored print image, at least one sensor for detecting a faulty function of individual color dot generators, an arithmetic unit for determining the printing pattern of at least one defective color dot generator, a comparison unit for comparing this print pattern with print patterns of the decors yet to be produced,
wherein the control device is set up,
in response to detection of an erroneous function of at least one color dot generator, canceling the active print job and transferring the print pattern to the printhead having the least deviation in the region of the at least one defective color dot generator such that the error of the at least one color point generator appears least visible to the human viewer in the decor of the printed area. The apparatus of the preceding claim, further comprising an inspection unit connected to the controller which optically captures a surface of the substrate to be printed prior to passing the surface on the printhead and creates images of the surface, wherein an error detection unit detects surface imperfections and wherein the controller is adapted to respond on the detection of a surface defect to replace the print pattern in whole or in part by an image file, which generates a changed local decor at least in the vicinity of the corresponding by the erroneous function of the color point generator or pixels.

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