Typewriting machines – Sheet or web – Including programmed-control-system for record-medium feed
Reexamination Certificate
1999-09-29
2002-07-30
Hirshfeld, Andrew H. (Department: 2854)
Typewriting machines
Sheet or web
Including programmed-control-system for record-medium feed
C347S016000
Reexamination Certificate
active
06425699
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to machines and procedures for printing text or graphics on printing media such as paper, transparency stock, or other glossy media; and more particularly to incremental machines and methods that construct text or images from individual ink spots formed progressively on a printing medium, in a pixel array—as for example by a scanning inkjet printer, or most other forms of matrix printing. The invention is directed to mitigation of several kinds of printing artifacts.
BACKGROUND OF THE INVENTION
(a) Uneven graininess—One class of overall image-quality problems in incremental printing may take the curious form of image regions in which
local
image quality is overly
good
. This can be a problem because local image quality can be so much better than nearby image quality that the contrast between the two becomes conspicuous in itself.
This phenomenon has two adverse consequences. First, two adjacent regions can be so different in image character as to present a sort of banded appearance—a band of generally acceptable quality set next to a band of strikingly fine quality. The difference simply destroys the illusion of looking at a unitary image, calling attention instead to some unknown peculiarities of the reproduction process.
Second, the higher quality in one region calls into question what would otherwise be considered acceptable quality in the other region. In other words, a “good” region and a “better” region add up to dissatisfaction with the good region.
In some incremental-printing technologies such appearances arise, in particular, very near the top and bottom ends of a page, because distinctly different modes of printing are applied in those two regions as compared with all the image portions between. This behavior is a result of the inability of some incremental technologies to print a swath that is partially on the printing medium and partially off.
If the edge of the sheet lies partway along an inkjet printhead, for instance, and the printhead scans along the edge, ink applied by the head is likely to be smeared erratically by upward curling of the edge of the sheet. In some cases it is also possible to damage the nozzles.
To avoid such effects, the end zones of the sheet are printed with the printhead or “pen” entirely on the sheet. Since several passes are needed to print an image in any region, such operation requires that the several printing passes be made in sequence, but with no print-medium advance relative to the printhead.
Pure printmask rotation of the sweep type is used, instead of advance-type rotation, to complete the image in these end zones. The process is taken up in the previously mentioned patent of Cleveland.
Generally such operation produces somewhat clearer or less-grainy image quality than the more-typical advance rotation—in which a corresponding “normal” advance occurs between printing passes—employed in other parts of the sheet. This difference can give rise to the banding effects discussed above.
More generally, however, such banding effects can occur in any composite region that bridges subregions with and without normal advance. More generally still, such effects can occur in any printing process which uses distinctly different printing modes or techniques in immediately adjacent image regions.
Although elimination of these effects is highly desirable, the regions in which they appear are after all rather small, and literally peripheral. Hence it is extremely important that any methodology adopted to mitigate these artifacts pose a very minimal degree of disruption to the overall printing process—and also to the overall structure of the programming which controls that process.
(b) Better-known artifacts—Several other types of image-quality defects are well documented in the patent and other literature, and have been the subject of extensive corrective efforts. Yet as the modern competitive push continues toward ever higher photographic-quality images and ever faster but less expensive machines, such artifacts persist stubbornly—and newer, more economical and less disruptive techniques are always at a premium.
Some such artifacts arise very directly from defects in nozzle geometry or firing characteristics. These first-order defects include white space and double-printing—due to nozzles that are not printing or are misdirected.
Other artifacts, harder to understand and still harder to uproot, come from unfortunate combinations of those simple nozzle-to-nozzle defects with
regularity
in the printing process. These include portions of the progressive migrating patterns which develop in error diffusion.
They can also include the repetitive stepping of printmasks that are not large enough to escape from the repetition-sensitive angular range of the eye, as taught in the previously mentioned patent document of Garcia. On the other hand, as Garcia has also pointed out, the opposite of excessive regularity—namely, excessive
randomness
—can also lead to a different sort of artifact, namely undesirable visible granularity in an image.
Although all these defects can be managed very effectively by multipass printmode techniques and related tactics, those techniques and tactics in general levy a large price in terms of printing throughput. Such a penalty is increasingly less acceptable in the marketplace.
(c) Repetitive overprinting—Returning now to somewhat more-subtle difficulties, a problem that persists even in some sophisticated randomized printmasking schemes is repetitive overprinting of particular nozzles by specific other nozzles. In such situations the first-order problem of nozzle outages or misdirections is already eliminated by mixing inking by different nozzles in a single pixel row—and even within an individual pixel.
This stratagem, however, may be to no avail if it happens that two or three nozzles used in conjunction, in a given row or pixel,
all
happen to be malfunctioning similarly or complementarily. Of course such a result is statistically less prevalent, but still significant.
(d) Software-generated beats—Some repetitive visible effects originate in software-generated interferences. These include, for example, interactions between dither-mask and printmask periodicities as explained in the previously mentioned patent document of Borrell.
(e) Unrelated use of small advances—For completeness it is mentioned here that it has been known heretofore to advance the printing medium, in addition to the normal advance used between printing passes, by supplemental very small amounts. These supplemental advances—most typically many pixel rows, but in any event substantially greater than a single row—are strictly for the purpose of compensating known errors in mechanical positioning, and have not been employed to address any of the problems discussed above.
(f) Conclusion—These several difficulties have continued to impede achievement of uniformly excellent inkjet printing—at high throughput. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.
SUMMARY OF THE DISCLOSURE
The present invention introduces such refinement. In its preferred embodiments, the present invention has several aspects or facets that can be used independently, although they are preferably employed together to optimize their benefits.
In preferred embodiments of a first of its facets or aspects, the invention is apparatus for printing desired images on a printing medium, by construction from individual marks formed in pixel row-and-column arrays. The apparatus includes a printhead mounted for scanning motion to form marks in a multiple-pixel-row swath on the printing medium.
The apparatus also includes a printing-medium advance mechanism providing relative motion between the printhead and printing medium. This relative motion is in a direction substantially orthogonal to the scanning motion.
A
normal
advance of the mechanism is equal to the height of at least several pixel rows; however, the apparatus also includes some means for stepping the print
Bartolome Emiliano
Doval Jose J
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