Abstract
A method for carrying out a printing operation on an inkjet printing machine compensates for failed printing nozzles by using a computer. After a screening process in which a halftone image is created for an image to be printed, failed printing nozzles are compensated for by increased ink application from neighboring printing nozzles by precalculating at least one corrective halftone image for the adjacent printing nozzles by using the computer, replacing the halftone image created in the screening process by the corrective halftone image in the at least one column of the neighboring printing nozzles, and carrying out the printing operation on the inkjet printing machine using the corrected halftone.
Claims
1. A method for carrying out a printing operation on an inkjet printing machine while compensating for failed printing nozzles by using a computer, the method comprising the following steps: after a screening process in which a halftone image is created for an image to be printed, compensating for failed printing nozzles by increased ink application from adjacent printing nozzles, by: precalculating at least one corrective halftone image for the adjacent printing nozzles by using the computer; replacing the halftone image created in the screening process with the corrective halftone image in at least one column of the adjacent printing nozzles; replacing at least two columns of the adjacent printing nozzles both left and right of the failed printing nozzle in the halftone image as viewed in a printing direction with at least two different precalculated corrective halftone images; and carrying out the printing operation on the inkjet printing machine using the corrected halftone image.
2. The method according to claim 1, which further comprises both left and right of the failed printing nozzle as viewed in the printing direction: using the at least one column immediately adjacent the failed printing nozzle to compensate for the failed printing nozzle by using at least one first corrective halftone image with increased ink application, and using the at least one column being farther away from the failed printing nozzle to prevent potential overcompensation by using at least one second corrective halftone image with reduced ink application.
3. The method according to claim 2, which further comprises generating the respective increased or reduced ink application in the precalculated corrective halftone image by increased or reduced ink drop volumes of the adjacent printing nozzles.
4. The method according to claim 1, which further comprises using respective precalculated corrective halftone images having been adapted in a suitable way for failed printing nozzles having a deviating print dot and for failed printing nozzles not printing at all or printing to a very reduced extent.
5. The method according to claim 1, which further comprises using columns respectively positioned left and right of the failed printing nozzle as viewed in a printing direction to carry out the compensation by using different precalculated corrective halftone images.
Description
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
(1) FIG. 1 is a diagrammatic, longitudinal-sectional view of an example of an inkjet printing machine;
(2) FIG. 2 is a top-plan view of a printing substrate having a halftone image with a defective printing nozzle;
(3) FIG. 3 is a top-plan view of a printing substrate having a correction width of five nozzles in the halftone;
(4) FIG. 4 is a top-plan view of a printing substrate having a first corrective halftone with increased ink application;
(5) FIG. 5 is a top-plan view of a printing substrate having a second corrective halftone with decreased ink application;
(6) FIG. 6 is a top-plan view of a printing substrate having a corrected halftone image; and
(7) FIG. 7 is a flow chart of the method of the invention.
DETAILED DESCRIPTION OF THE INVENTION
(8) Referring now in detail to the figures of the drawings, in which mutually corresponding elements have the same reference symbols, and first, particularly, to FIG. 1 thereof, it is seen that the field of application of the preferred embodiment is in an inkjet printing machine 7. FIG. 1 shows an example of the fundamental structure of such a machine 7, including a feeder 1 for feeding a printing substrate 2 to a printing unit 4, where it receives an image printed by print heads 5, and a delivery 3. The machine is a sheet-fed inkjet printing machine 7 controlled by a control unit 6. While this printing machine 7 is in operation, individual printing nozzles 8 in the print heads 5 in the printing unit 4 may fail as described above. Such a failure results in white lines 9 seen in FIG. 2 or, in the case of multicolor printing, in distorted color values.
(9) The preferred embodiment will be described below. FIG. 2 illustrates a halftone image 10 with a defective printing nozzle 8. It is clearly visible how the corresponding halftone dots in the corresponding printing direction are missing, resulting in a corresponding white line 9. The preferred exemplary embodiment uses a correction width 11 of five nozzles 5. This means that two columns 12 of adjacent printing nozzles disposed to the left and right of the defective printing nozzle 8 are used for compensation purposes. This is shown very clearly in FIG. 3. In order to compensate for the failed printing nozzle 8, the immediately neighboring printing nozzles in the columns 12 need to apply more ink. For this purpose, a different halftone 13 is used, which has been precalculated in accordance with the invention during the screening process at the preprint stage to avoid a re-screening of the print image when a defective nozzle 8 occurs. Another advantage of using a precalculated corrective halftone 13, 14 is that it may be adapted to the original image halftone 10. This prevents the occurrence of new artifacts in the print image due to a non-adapted corrective halftone that violates the screening rules. Such a corrective halftone 13 with increased ink application is shown by way of example in FIG. 4. In this case it is clearly shown how the increased ink application is achieved in accordance with the screening rules. For instance, at every location at which there was a pixel in the original halftone image 10, a pixel of the same or of a greater drop size is set in the first corrective halftone image 13. As is shown in FIG. 4, the first corrective halftone image 13 is exclusively used for the immediately neighboring printing nozzles 12 or rather the print columns 12 that are printed by these printing nozzles.
(10) In order to prevent overcompensation by increased ink application, a second corrective halftone 14 is used for the adjacent printing nozzles or rather print columns 15 that are correspondingly further away. This is shown in a corresponding way in FIG. 5. In this case, the screening rules are likewise respected. For instance, no dot that did not have a pixel in the original screen 10 has a pixel now. In addition, it is shown how the respective outer adjacent printing nozzles in the columns 15 in the applied correction width 11 of five use the corresponding corrective halftone image 14 with reduced ink application.
(11) FIG. 6 shows the resultant corrected halftone image 16, which uses the original halftone 10 for the remainder of the print image. In order to correct the white line that has been created by the missing printing nozzle 8, the first precalculated corrective halftone 13 with increased ink application is used in the columns 12 immediately to the left and right of the missing printing nozzle 8. In order to avoid overcompensation, the second precalculated corrective halftone image 14 with decreased ink application is used at the respective outer locations of the columns 15 of the correction width 11.
(12) FIG. 7 is a flow chart of the method of the invention. The first step is to detect a missing nozzle 8. If the position thereof is known, the control unit 6 of the inkjet printing machine 7 mayin accordance with the preset, i.e. preconfigured correction width 11insert the first and second corrective halftone images 13, 14 into the existing halftone image 10 that has already been screened. Then the printing operation is continued using this corrected halftone image 16 to compensate for the missing printing nozzle 8. For this purpose, in addition to the actual corrective algorithm for compensating for the missing printing nozzle 8, both the precalculated corrective halftone images 13, 14 and the corresponding correction width 11 to be applied need to be saved on the control unit 6 of the inkjet printing machine 7. Potential variations of the method of the invention, for instance in terms of the use of varying screens for the printing nozzles in the columns 12, 15 to the left and right including the specification when they need to be applied, must also be known on the control unit 6 by configuration. The precalculation of the corresponding corrective halftones 13, 14 preferably occurs at the same time as the screening of the actual print image. Thus, the precalculated corrective halftone images 13, 14 may be adapted to the corresponding print image. However, it is conceivable to use universal corrective halftone images that are independent of the print image to be produced. If this is the case, the latter need to be available in a memory that may be accessed by the control unit 6 of the inkjet printing machine. The precalculation of the corrective halftone images 13, 14 as well as the actual screening process may theoretically be carried out by the control unit of the inkjet printing machine itself. In general, however, this is done by a computer in the preprint department, also known as the raster image processor, and both the halftone print image 10 and the precalculated corrective halftone images 13, 14 are made available to the control unit 6 of the printing machine 7 by using a network or data memory.
(13) The following is a summary list of reference numerals and the corresponding structure used in the above description of the invention: 1 feeder 2 printing substrate 3 delivery 4 inkjet printing unit 5 inkjet print head 6 computer 7 inkjet printing machine 8 defective printing nozzle 9 white line 10 original halftone 11 correction width 12 columns of immediately neighboring printing nozzles 13 first corrective halftone 14 second corrective halftone 15 columns of adjacent printing nozzles that are farther away 16 corrected halftone image
