Ramping dot data for single-pass monochrome printing at high speeds

Abstract

A method of printing an image from a printhead module having a plurality of horizontal nozzle rows. The method includes the steps of: allocating first dot data for an image line of the image to nozzles in a main row portion of a first nozzle row; allocating second dot data for the image line to nozzles in a dropped row portion of the first nozzle row; sending the first and second dot data to the printhead module and firing respective droplets. Some bits of the first dot data correspond to pixels of the image line aligned with the dropped row portion, and some bits of the second dot data correspond to pixels of the image line aligned with the main row portion.

Claims

1. A method of printing an image from a print chip having a plurality of horizontal nozzle rows, each nozzle row having a main row portion and a corresponding dropped row portion that does not overlap with the main row portion, the method comprising the steps of: allocating first dot data for an image line of the image to nozzles in a main row portion of a first nozzle row; allocating second dot data for the image line to nozzles in a dropped row portion of the first nozzle row; sending the first dot data to the print chip and firing droplets, based on the first dot data, from nozzles of the main row portion; sending the second dot data to the print chip and firing droplets, based on the second dot data, from nozzles of the corresponding dropped row portion, wherein: one or more bits of the first dot data correspond to pixels of the image line aligned with the dropped row portion; and one or more bits of the second dot data correspond to pixels of the image line aligned with the main row portion.

2. The method of claim 1, wherein the first nozzle row of the dropped row portion corresponds to the first nozzle row of the main row portion.

3. The method of claim 1, wherein the second nozzle row of the dropped row portion does not correspond with the first nozzle row of the main row portion.

4. The method of claim 1, wherein allocation of first and second dot data to nozzles of the main row portion and dropped row portion is performed in a printer controller communicating with the print chip.

5. The method of claim 1, wherein the print chip has redundant nozzle rows.

6. The method of claim 1, wherein the print chip is a monochrome print chip having all nozzle rows supplied with a same color ink.

7. The method of claim 1, wherein the nozzle rows of the dropped nozzle portion together are generally trapezoidal or triangular in plan view.

8. The method of claim 1, wherein: third dot data for the image line is allocated to a second nozzle row of the main row portion; fourth dot data for the image line of is allocated to the corresponding the second nozzle row of the dropped row portion; one or more bits of the third dot data correspond to pixels of the image line aligned with the dropped row portion; and one or more bits of the fourth dot data correspond to pixels of the image line aligned with the main row portion.

9. The method of claim 1, wherein the first and second dot data correspond to even pixels of the image line, and the third and fourth dot data correspond to odd pixels of the image line or vice versa.

10. The method of claim 1, wherein the second dot data is sent to the print chip subsequent to the first dot data.

11. The method of claim 1, wherein the dot data comprises a ‘1’ for an enabled firing nozzle and a ‘0’ for a non-enabled non-firing nozzle.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

(2) FIG. 1 shows a print chip having a dropped nozzle region;

(3) FIG. 2 is a magnified view of the dropped nozzle region;

(4) FIG. 3 is a schematic view of a printhead having multiple butting print chips;

(5) FIG. 4 shows dot placement errors resulting from dropped nozzle region artefacts in various print modes; and

(6) FIGS. 5A and 5B are simulated test prints using printing methods described herein.

DETAILED DESCRIPTION OF THE INVENTION

(7) Referring to FIG. 1, the printing methods described herein employ printhead modules, typically in the form of print chips as described in, for example, U.S. Pat. No. 7,290,852. Accordingly, each print chip comprises horizontal rows of nozzles extending parallel with a longitudinal axis of the print chip. Each nozzle row has a main row portion and a corresponding displaced (“dropped”) row portion, which is vertically offset from its main row portion.

(8) For the sake of convenience, the print chip is defined to have a nominal horizontal axis extending parallel with its length dimension and a nominal vertical axis extending perpendicular to the horizontal axis. As used herein, the terms “horizontal” and “vertical” are not intended to limit the orientation of print chips or nozzles rows in use. Furthermore, the term “dropped” (e.g. “dropped row portion”, “dropped nozzle region” etc) is not intended to limit the orientation of the print chip relative to a media feed direction—a “dropped row portion” merely means that a row portion is displaced, either upstream or downstream relative to a media feed direction, of a corresponding main row portion

(9) Nozzles in the main row portion extend along a majority of the length of the print chip, while nozzles in the dropped row portion are positioned at one end of the print chip. The total number of nozzles in each main row portion and corresponding dropped row portion is the same for all nozzle rows (e.g. 640 nozzles per row). However, the dropped row portions each have different lengths and, as shown in FIGS. 1 and 2, are together arranged in a generally trapezoidal shape in plan view. The multiple dropped row portions having a trapezoidal shape are together referred to as a “dropped nozzle region” of the print chip.

(10) The print chip shown in FIGS. 1 and 2 contains five ink planes, which are all supplied with a same color of ink for monochrome printing. Each ink plane contains two nozzle rows (“odd” and “even”) horizontally offset from each other by 1 dot pitch. Since the nozzles within the same nozzle row are spaced apart by 2 dot pitches, then the odd and even nozzle rows in one ink plane can print odd and even dots in one line of print. In the embodiment shown, the odd and even nozzle rows within the same ink plane are vertically offset from each other by 4 dot pitches, while each dropped row portion is offset from its corresponding main row portion by 10 dot pitches (at a nominal 1600 dpi).

(11) While one embodiment is described herein with reference to a Memjet print chip printing at a nominal 1600 (horizontal)×1600 (vertical) dpi, it will of course be appreciated that the present invention is not limited by way of print resolution or print speed.

(12) As best seen in FIG. 2, each dropped row portion is positioned to align horizontally with its corresponding main row portion such that a constant dot pitch is effectively maintained both along the print chip and between neighboring print chips. In this way, the dropped row portions can, in principle, compensate for printing in the join regions between neighboring print chips where nozzles cannot be fabricated due to a lack of available silicon at the edges of the print chips. Nevertheless, due to the problems foreshadowed above, the print chip described U.S. Pat. No. 7,290,852 is not ideally suited for fast printing (e.g. at a nominal 5× print speed) in monochrome for all printing resolutions. For example, as explained above and with reference to FIG. 4, when printing in monochrome at 1200 dpi and 5× print speed, errors of 2.5 DP occur between the main nozzle region and the dropped nozzle region. This error produces noticeable artefacts on the printed page.

(13) First Method (Ramped Dot Data)

(14) In order to print, for example, at 1200 dpi at 5× print speed using the Memjet® print chip 1, each nozzle row 3 has a main row portion 5 printing dots for a predetermined image line while the dropped row portion 7 prints dots for the next image line downstream. Although this method of printing produces an error of 2.5 DP, this is the closest alignment achievable in this particular print mode, since the print chip must fire its nozzles row-by-row (including the main nozzle region and the corresponding dropped nozzle region from each nozzle row).

(15) While the error of 2.5 DP is unavoidable in this instance, the noticeability of the consequent print artefact can be minimized by ramping dot data from the main nozzle region into dropped nozzle region. Accordingly, some nozzles of the main nozzle region proximal the dropped nozzle region receive dot data for part of the image line allocated to the corresponding main nozzle portion; and, likewise, some nozzles of the dropped nozzle region receive dot data for part of the image line allocated to the main nozzle region. Effectively, some of the dot data is swapped between the main nozzle region and the dropped nozzle region.

(16) Alternate nozzles may be used to ramp the dot data in this way. More sophisticatedly, the dropped nozzle region may be divided into a plurality of columnar zones with dot data swapped between predetermined zones of the dropped nozzle region and main nozzle region.

(17) Intuitively, one might suppose that ramping dot data in this way by swapping some of the dot data between the dropped nozzle region and the main nozzle region would have the effect of worsening print quality. After all, fewer droplets ultimately land at their intended pixel position on the media. However, ramping of dot data has the effect of smoothing the transition from the main nozzle region to the dropped nozzle region as opposed to a step jump between the two regions. In practice, the step jump manifests in a visible line down a printed page, whereas the ramped transition is far less visually noticeable. Therefore, the use of ramped dot data significantly improves overall print quality.

(18) Second Method (Mis-Matched Main Nozzle Rows and Dropped Nozzle Rows)

(19) As described above, the main nozzle region 13 and dropped nozzle region 11 are designed to provide a constant dot pitch across the print chip 1 and between neighboring print chips by delaying data for each dropped row portion 7 such that its printed dots join with a line of dots printed by its corresponding main row portion 5.

(20) The problem of monochrome printing at high speed in certain print modes may be further addressed by using one or more mis-matched (i.e. non-corresponding) nozzle rows from the dropped nozzle region. As explained above, an image line printed by Row 0 and 1 (even and odd dots) in the main nozzle region cannot be adequately compensated by Rows 0 and 1 of the dropped nozzle region (positioned 10/1600.sup.th inch away from the corresponding rows of the main nozzle region) when printing at 5× print speed at 1200 dpi, because the media has moved by 7.5 image lines during one fire cycle or 15 image lines during two fire cycles. However, Rows 6 and 7 of the dropped nozzle region (positioned 40/1600.sup.th inch from Row 0 of the main nozzle region) can compensate perfectly by delaying data by 30 image lines (corresponding to four fire cycles at 1200 dpi). In this example, Rows 6 and 7 of the dropped nozzle region have a greater number of nozzles than Rows 0 and 1 of the dropped nozzle region and so only those nozzles from Row 6 and 7 which are aligned with Rows 0 and 1 can be used to compensate. Thus, the line of dots printed from the Rows 6 and 7 of the dropped nozzle region can join seamlessly with the image line printed by Rows 0 and 1 of the main nozzle region.

(21) However, it is not always possible to compensate using non-corresponding nozzle rows from the dropped nozzle region that are longer than the nozzle rows actually corresponding with the main nozzle region. If, for example, an image line is printed in the main nozzle region by Rows 7 and 8, but by Rows 9 and 0 of the dropped nozzle region then there will missing dots when Row 0 (dropped nozzle region) joins with Row 8 (main nozzle region) and, potentially, extra dots when Row 9 (dropped nozzle region) joins with Row 7 (main nozzle region). In this scenario, it is preferable to double print some dots using aligned nozzles from Row 9 (dropped nozzle region) and Row 7 (main nozzle region) in order to maintain, as far as possible, ink density across the join. In practice, ink bleed reduces the noticeability of this artefact. Thus, the step error caused by the dropped nozzle region in certain print modes can be reduced at the expense of some double-printing and/or unprintable dots in the dropped nozzle region.

(22) Once again, intuitively one might suppose that this method of compensation would produce worse visual artefacts than the aforementioned step errors. However, if the non-corresponding nozzles rows from the dropped nozzle region are selected carefully, then the overall visual effect is much less noticeable than the step errors produced in certain print modes by the dropped nozzle region. Typically, as exemplified above, the non-corresponding even and odd rows in the dropped nozzle region are selected such that one is longer and one is shorter than the actual corresponding nozzle rows in the dropped nozzle region in order to maintain, as far as possible, a printed ink density.

(23) The first and second methods described hereinabove may be used in combination in order to further minimize the noticeability of visible print artefacts relating to the dropped nozzle region, which arise from high-speed monochrome printing in certain print modes.

(24) FIGS. 5A and 5B are simulated test prints showing the combined effects of the first and second methods described above when printing at 400 dpi at a nominal 5× print speed. In FIG. 5A, using the method described in U.S. Pat. No. 7,290,852, the join region between two neighboring print chips is visible as a hump due to imperfect dot placement in the dropped nozzle region. However, as shown in FIG. 5B, with the use of ramped dot data and mismatched dropped nozzle rows, the join region becomes much less noticeable in the same print mode.

(25) It will, of course, be appreciated that the present invention has been described by way of example only and that modifications of detail may be made within the scope of the invention, which is defined in the accompanying claims.