Removal or mitigation of artifacts in incremental printing

Details Removal or mitigation of artifacts in incremental printing Removal or mitigation of artifacts in incremental printing

2002-09-05

2004-10-05

Nguyen, Lamson D (Department: 2861)

Incremental printing of symbolic information

Ink jet

Controller

C347S041000, C358S001900

Reexamination Certificate

active

06799823

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to machines and procedures for incremental printing of text or graphic images; and more particularly to removal of artifacts in liquid-colorant printing that constructs such images from individual colorant spots, formed in a two-dimensional pixel array on a printing medium such as paper, transparency stock, or other glossy media.
BACKGROUND
Incremental printing with liquid-base colorants is subject to several very subtle but undesired image defects. Although these arise in generally understood ways from operation of the associated mechanical components, they have nonetheless been very resistant to corrective action.
Many forms of incremental printing operate by creating inkdrop swaths. These swaths are formed in successively stepped positions, by iterated relative motion—along a print-medium advance direction—between the medium and an inking device. Such swath-based printing systems may be of a scanning type, operating by repeated operation of the inking devices across the medium, or a pagewide swath-height array type.
Some artifacts, even though due to different apparatus phenomena, are often designated by the generic term “banding”—meaning that they usually present the appearance of subtle bands, stripes or striations. This is particularly true for swath-based devices, or scanning devices—although artifacts of interest with respect to preferred embodiments of the invention are not limited to such systems.
The reason for this commonality with respect to banding artifacts is that almost any cyclical or near-cyclical perturbation of inking operations creates some form of correspondingly periodic visible pattern. Merely by way of example, features that appear in generally the same elevation across each swath naturally tend to produce a visual effect at the spatial periodicity of the printing-medium advance.
When printing with a single layer of swaths fitted edge-to-edge, that periodicity is in principle equal to the swath height. (Such an edge-to-edge arrangement is obtained through use of a so-called single-pass printing mode, for a scanning system—or more generally a single-inking-installment mode, to encompass a pagewide swath system.)
In plural-installment modes—i.e., with overlapping swaths—the spatial periodicity of the banding is finer. In these cases the print medium usually advances by some fraction of the swath height, giving rise to the finer banding.
Some types of banding—and, more generally, artifacts addressed by some forms of the present invention—are not fundamentally swath-related at all. Again merely by way of example are those artifacts arising from tiled relatively small printmasks (discussed below).
Such artifacts are most conspicuous in midtone regions of an image, where there is modulation range in either direction to exhibit extremely subtle patterns. Other artifact types, for example those associated with slight overinking effects (also discussed below), are instead most conspicuous in darker regions where colorant liquid effects can dominate.
(a) Boundary banding—One of the major contributors to banding is a thin, darker line that appears along one or usually all edges of a printed field. This is the so-called “boundary banding”; however, more generally it should be regarded as a boundary artifact.
In a swath-based system, some manifestations of this type of artifact appear to be concentrated particularly where two swath edges abut, or nearly abut. Boundary artifacts are very hard to remove—especially when printing in swaths, and especially at a low number of passes or installments.
Normally they appear only in image regions that are rather highly saturated calorimetrically. Such colorimetric saturation giving rise to boundary artifacts, however, occurs in either:
a single primary colorant (ordinarily cyan, magenta, yellow and black), or
a composite color formed from combinations of those colorants in various proportions.
Accordingly this localization of the banding is believed to be a liquid-loading effect—coalescence of the liquid in adjacent inkdrops, concentrated by surface tension at the edges of the just-deposited liquid field.
Boundary artifacts are hard to attack when associated with heavy inking in one primary. This difficulty is exacerbated when the overinking takes the form of an aggregation, with only modest contributions from plural colorants as in the case of composite colors.
Another form of banding—along horizontal boundaries only—arises from swath-height error (“SHE”, and sometimes “SWE”) rather than coalescence. It will be discussed below.
(b) Other swath-associated artifacts—One distinctly different kind of image defect, although it too strongly affects contiguous-swath abutments, relates to swath-height error. This type of error usually occurs when nominal relationships between swath height
135
(
FIG. 1A
) and printing-medium advance
133
fail.
In the nominal relationship, when the effective pen height
135
just matches the advance distance
133
, swaths
131
,
132
abut neatly
134
. For nominal advance, such relationships are maintained when ink-discharging nozzles near the inking-array edge are pointed straight toward the printing medium
130
along a normal to the surface.
When those end nozzles instead point outboard or inboard—along the print-medium advance direction—such misdirections cause the swath to be taller
135
′ or shallower
135
″, respectively, than its nominal height
135
. In the former case, for a nominal print-medium advance stroke
133
, excess lengths
136
,
137
at top and bottom, respectively, of overlong adjacent swaths
131
′,
132
′ then overlap slightly. The overlap forms a dark line
134
′ (
FIG. 1B
) along the swath boundary.
In the opposite case of inboard-pointing end nozzles causing shallower swaths
135
″, the foreshortened regions
138
,
139
at top and bottom, respectively, of the undersize adjacent swaths
131
″,
132
″ fail to abut at all. The failure
134
″ to abut, leaves a white line
134
″ (
FIG. 1C
) between the swaths.
In practice, these conditions arise also with nominal swath height, for short stroke and long stroke respectively. (If the only problem is inaccurate stroke, however, then correction is straightforward and easy.)
Through fine adjustment of the advance stroke, this kind of mismatch between stroke and swath height can be hidden, for some one particular magnitude of nozzle-direction error, but not entirely cured. The previously introduced patent documents of Cluet, Donovan and Doval, Vilanova '499 and particularly Subirada '652, introduce various techniques for attacking this problem—and other errors related to nozzle health, taken up shortly.
Generally speaking such techniques also require printing and measurement of test patterns designed to reveal details of the banding characteristics to be overcome. Measurements of this sort are facilitated by apparatus and methodology introduced in several related patent documents, particularly those of Baker, Bockman, Borrell (Ser. No. '858), Soler, Subirada and Vilanova '207—as well as the three others noted in the preceding paragraph.
Unfortunately, however, stroke adjustment either foreshortens or lengthens, respectively, the overall image. The image is typically made a significant fraction of one percent too tall or short.
Worse, such a corrective tactic cannot restore image detail that is either blurred or lost, respectively, as the abutment region is distorted in height relative to areas within the swath. Even worse yet is the inability of such strategies to accommodate more than just one particular magnitude of directional error, when in fact each color in an image is printed from a respective different inking array—i.e., printhead, or so-called “pen”.
For instance suppose that in a particular printer one of these pens prints, in one color, a swath
131
″ that is 0.3% too shallow
135
″—while another pen makes, in a second color, a swath
131
′ that is 0.4% too tall
135

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