Method of dithering for high-speed single-pass printing

Abstract

A method of single-pass printing uses a printing system having first and second aligned printheads supplied with a same ink. The method includes the steps of: receiving first and second halftone images at the first and second printheads, respectively; printing the first halftone image from the first printhead; and printing the second halftone image from the second printhead. The first halftone image is based on a first dither pattern and the second halftone image is based on a second dither pattern, the first dither pattern being different than the second dither pattern.

Claims

1. A method of single-pass printing using a printing system comprising at least first and second aligned printheads supplied with a same ink, the second printhead being downstream of the first printhead, the method comprising the steps of: receiving first and second halftone images at the first and second printheads, respectively; printing the first halftone image from the first printhead; and printing the second halftone image from the second printhead such that a resulting printed image contains the second halftone image interleaved with the first halftone image, wherein: the printed image is printed at a first resolution in a printing direction; the first and second halftone images have a second resolution in the printing direction, the second resolution being less than the first resolution; the first halftone image is based on a first dither mask and the second halftone image is based on a second dither mask, the first dither pattern mask being different than the second dither mask.

2. The method of claim 1 further comprising the steps of providing a full contone image for one ink channel of the printing system at the first resolution in a printing direction.

3. The method of claim 2 further comprising the steps of: dividing the full contone image at the first resolution into at least first and second contone images at the second resolution in the printing direction, the second resolution being less than the first resolution; dithering the first contone image using the first dither mask to provide the first halftone image at the second resolution; dithering the second contone image using the second dither mask to provide the second halftone image at the second resolution.

4. The method of claim 3, wherein the first and second contone images contain respective first and second sets of lines of the full contone image.

5. The method of claim 3, wherein the first and second contone images contain respective alternate lines of the full contone image.

6. The method of claim 3, wherein the first and second contone images have a same resolution as the full contone image in a direction perpendicular to the printing direction.

7. The method of claim 1 further comprising the steps of: dithering the full contone image using a combined dither mask to provide a full halftone image at the first resolution; dividing the full halftone image into at least first and second halftone images at the second resolution in the printing direction, the second resolution being less than the first resolution; wherein the combined dither mask is a combination of a first dither mask for the first printhead and a second dither mask for the second printhead, the first dither mask being different than the second dither mask.

8. The method of claim 7, wherein the first and second halftone images contain respective first and second sets of lines of the full halftone image.

9. The method of claim 7, wherein the first and second halftone images contain respective alternate lines of the full halftone image.

10. The method of claim 7, wherein the first and second halftone images have a same resolution as the full halftone image in a direction perpendicular to the printing direction.

11. The method of claim 2, wherein the step of providing the contone image for one ink channel comprises one or more of: rasterizing; calibrating to a target printing resolution; and generating, from a color contone image, a plurality of contone images corresponding to respective ink channels of the printing system.

12. The method of claim 1, wherein the printing system comprises n aligned printheads supplied with the same ink and the second resolution is 1/n of the first resolution in the printing direction.

13. The method of claim 1, wherein the printing system comprises two aligned printheads supplied with the same ink and the second resolution is half of the first resolution.

14. The method of claim 1, wherein the first and second dither masks are independently selected from the group consisting of: blue noise dither masks and green noise dither masks.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) One or more embodiments of the present invention will now be described with reference to the drawings, in which:

(2) FIG. 1 is a schematic plan view of a printing system comprising first and second aligned printheads;

(3) FIG. 2 is a perspective of a print engine comprising four print modules;

(4) FIG. 3 is a schematic plan view of a printing system comprising an 8×2 array of printheads;

(5) FIG. 4 shows conventional processing steps for generating first and second halftone images for printing using the printing system shown in FIG. 1;

(6) FIG. 5 shows processing steps for generating first and second halftone images according to a first embodiment; and

(7) FIG. 6 shows processing steps for generating first and second halftone images according to a second embodiment.

DETAILED DESCRIPTION

(8) Referring to FIG. 1, there is shown schematically a printing system 1 comprising a first printhead 2 and a second printhead 4 positioned downstream of the first printhead relative to a media feed direction indicated by arrow F. The second printhead is aligned with the first printhead in the media feed direction insofar as both printheads are capable of printing onto a same portion of media 6 (e.g. cut-sheet media or a roll-to-roll fed media web). An extent of alignment between the first and second printheads 2 and 4 may vary according to individual printhead conformations, printhead placement accuracy etc, as described above.

(9) Each of the first and second printheads 2 and 4 is a monochrome printhead supplied with a same ink so as to enable double-speed printing. For double-speed printing, each printhead prints half an image at half the target resolution (in the media direction F). For example, a full halftone image may be generated at a target resolution of 1600×790 dpi and each of the first and second printheads 2 and 4 is configured to print at a resolution of 1600×395 dpi. Typically, each printhead prints respective alternate lines (row) of the full halftone image. Since printheads have a maximum drop ejection frequency, it will be appreciated that halving the resolution in the media feed direction F enables printing at twice the speed that would otherwise be obtainable.

(10) Each of the first and second printheads 2 and 4, is typically a component of a print module, which may additionally comprise a printhead mounting structure, electronics for supply of data and power to the printhead, ink couplings, pressure regulator(s) etc. Examples of suitable print modules are described in U.S. Pat. Nos. 10,457,075 and 10,081,204, the contents of which are incorporated herein by reference.

(11) By way of example, and referring to FIG. 2, there is shown a print engine 10 having four aligned print modules 12 as described in US 2019/0118537, the contents of which are incorporated herein by reference. Each print module 12 comprises a respective printhead (not visible in FIG. 2) as well as ink couplings, PCBs etc. It will be appreciated that the print engine 10 having four aligned printheads supplied with a same ink potentially enables quadruple-speed printing by allocating a quarter of a full halftone image to each of the four print modules 12. For example, every 4.sup.th line of the full halftone image may be allocated to a respective print module 12, such that each printhead prints at one ¼ resolution in the media feed direction. A full halftone image generated at a resolution of 1600×800 dpi would be printed at a resolution of 1600×200 dpi by each printhead, thereby enabling the print engine 10 to print at quadruple speed compared to a single print module 12.

(12) In principle, any number of n printheads may be used to print at n times speed by allocated 1/n of a full halftone image (e.g. every nth line of the halftone image) to a respective one of the n printheads.

(13) Of course, the aligned printheads may be part of a matrix of printheads arranged for color and/or wideformat printing. FIG. 3 shows schematically a print engine 20 containing sixteen print modules 12 in an 8×2 array for full color printing. The print modules 12 are arranged in sets of four for printing each of four colors (KCMY). In each of the color channels (KCMY), there are two pairs of aligned print modules 12, each pair of aligned print modules overlapping to print onto a different portion of the media. For example, as shown in the black channel, the pair of print modules 12A are aligned and the pair of print modules 12B are aligned along the media feed direction F. Thus, each ink (color) channel is capable of printing at double speed in the manner described above in connection with FIG. 1.

(14) Referring to FIG. 4, there is shown a simple method of processing a contone image at a first resolution for high-speed printing using the first printhead 2 and the second printhead 4 shown in FIG. 1. The method is performed in a raster image processor (RIP) although, for the sake of clarity, not all processing steps performed by the RIP are shown in FIG. 4. The contone image is a contone (grayscale) bitmap for a single ink channel at the target printing resolution of 1600×790 dpi. The skilled person will understand that typical upstream processing steps in the RIP (e.g. rasterizing, color space conversion, ink channel separation, calibration to the target printing resolution etc) are not shown in FIG. 4.

(15) Still referring to FIG. 4, in a first step, the contone image is dithered using a conventional single dither pattern (e.g. a blue noise dither as described in U.S. Pat. No. 5,111,310 or a green noise dither as described in U.S. Pat. No. 6,493,112 etc.) to generate a full halftone image. The full halftone image is then divided into a first halftone image and a second halftone image, each at a second resolution, in a process known as “deinterleaving”. Alternate rows of the full halftone image are allocated to respective printheads, such that each of the first and second halftone images resulting from the deinterleaving process has a resolution of 1600×395 dpi. For example, the first halftone image may comprise odd lines (1, 3, 5, 7 etc.) of the full halftone image and the second halftone image may comprises even lines (0, 2, 4, 8 etc.) of the full halftone image or vice versa. The first and second halftone images are then sent to respective first and second printheads of a print engine and for printing.

(16) In the single-pass printing process using a first printhead 2 and a downstream second printhead 4, as shown in FIG. 1, the full printed image 8 contains the first and second halftone images interleaved on the media 6 to represent the full halftone image generated from the dithering process. The half-density image 9 printed by the upstream first printhead 2 is based on the first halftone image only and is therefore printed at half density (alternate lines of the full halftone image).

(17) With perfect alignment of the first printhead 2 and second printhead 4, the process described in connection with FIG. 4 provides excellent print quality and enables double-speed printing compared to a single printhead printing the full halftone image. However, small misalignments between the first printhead 2 and the second printheads 4 (e.g. resulting from small mechanical misplacements and/or mis-matching of printhead conformations) results in a significant decline in print quality. In particular, print quality defects are exacerbated by interference effects between the first and second halftone images, which are not otherwise present when printing one color of ink from one printhead. Increasing misalignments between the first printhead 2 and the second printhead 4 result in a rapid decline in print quality, which is generally unacceptable to users.

(18) Referring to FIG. 5, there is shown a method of processing a contone image according to a first embodiment of the invention. In the method shown in FIG. 5, the contone image is deinterleaved prior to dithering. Deinterleaving of the contone image is performed similarly to the deinterleaving process described above, whereby alternate lines of the full contone image are allocated to a first contone image and a second contone image. The first contone image is then dithered using a first dither pattern to generate the first halftone image and the second contone image is dithered using a second dither pattern to generate the second halftone image. Crucially, the first and second dither patterns are different.

(19) When the first and second halftone images are printed using respective printheads, the printed image 8 generally has acceptable print quality. Advantageously, print quality is relatively tolerant of misalignments between the first printhead printhead 2 and the second printhead 4 compared to the method described above in connection with FIG. 4. In particular, it is understood by the present inventors that using different dither patterns for the first and second halftone images results in a somewhat less rapid decline in print quality with increasing misalignments between the printheads when compared to the method described above using a single dither pattern. Nevertheless, with good alignment between the first printhead 2 and the second printhead 4, the method according to the first embodiment produces lower print quality than the method described above.

(20) It would be desirable for users to substitute between the two different processes described above in order to optimize print quality for different extents of alignment between printheads. For example, an initially perfect alignment between the first and second printheads 2 and 4 may change over time, or replacement of one or both printheads may result in misalignments. In this scenario, it would be desirable to change from the process shown in FIG. 4 to the process shown in FIG. 5. However, since the two processes involve different datapaths, it is impractical to reconfigure the RIP so as to substitute between these two processes.

(21) Referring now to FIG. 6, there is shown a method of processing a contone image according to a second embodiment. The method shown in FIG. 6 has the same datapath as the method shown in FIG. 4—that is, the ordering of processing steps is identical in each case. However, the dithering step in the method shown in FIG. 6 employs a combined dither pattern comprising the first dither pattern and the second dither pattern. In other words, alternate lines of the combined dither pattern (or dither mask) are based on different dither patterns. For example, odd lines of the combined dither pattern may be based on the first dither pattern and even lines of the combined dither pattern may be based on the second dither pattern, which is different than the first dither pattern.

(22) Accordingly, dithering using the combined dither pattern results in first and second halftone images, which are identical to the first and second halftone images described above in connection with FIG. 5. The method according to the second embodiment, therefore, enjoys the same advantages as the method according to the first embodiment.

(23) Moreover, an additional advantage of the method according to the second embodiment is that the datapath uses the same sequence of processing steps as those shown in FIG. 4. Therefore, by simply substituting a conventional single dither pattern applied to the full contone image with the combined dither pattern, the RIP can readily switch between these two methods in order to optimize print quality for a given scenario.

(24) For example, a user may provide empirical qualitative feedback on print quality and the dither may be switched accordingly. Alternatively, a printhead alignment test pattern may provide quantitative printhead alignment data, which can be used to select the most appropriate dither pattern.

(25) The dither pattern may be selected automatically based on a printhead alignment measurement relative to a predetermined threshold. For example, if the printheads are determined to be aligned to within one dot pitch or less (in the printing direction) at the resolution of the first and second halftone images (i.e. within 64 microns for a 1600×395 dpi halftone image), then a single dither pattern may be employed, as shown in FIG. 4. However, if the printheads are aligned only to greater than one dot pitch (in the printing direction) at the resolution of the first and second halftone images (i.e. greater than 64 microns for a 1600×395 dpi halftone image), then the combined dither pattern may be employed, as shown in FIG. 6. The predetermined threshold may be variable depending on printing parameters (e.g. print speed, print media type, ink type etc.). In this way, print quality can be optimized for both well-aligned and somewhat misaligned printheads.

(26) The foregoing describes only some embodiments of the present invention, and modifications of detail may be made thereto without departing from the scope of the invention, the embodiments being illustrative and not restrictive.