Facsimile and static presentation processing – Static presentation processing – Attribute control
Reexamination Certificate
1999-02-18
2004-02-10
Williams, Kimberly A. (Department: 2626)
Facsimile and static presentation processing
Static presentation processing
Attribute control
C358S451000, C358S296000, C358S451000, C358S533000, C358S534000, C358S536000, C358S535000, C358S451000, C358S451000, C347S043000, C347S012000, C347S009000
Reexamination Certificate
active
06690484
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 a scanning machine and method that construct text or images from individual ink spots created on a printing medium, in a two-dimensional pixel array. Thermal-inkjet printers and processes are of greatest interest.
The invention is applicable, however, in other types of units such as, for example only, piezodriven inkjet printers and hot-wax transfer printers. The invention coordinates dither masking and printmode techniques in such a way as to optimize image quality with minimal disturbance of preestablished data structures and control programming.
BACKGROUND OF THE INVENTION
This section introduces the basis and history of a particularly persistent category of undesirable printing artifacts that impair the quality of images made with incremental printers. Those artifacts are very peculiar-appearing and repetitive patterns that appear superimposed on, usually, rather uniform colorant fields in the middle tonal range.
These patterns are particularly undesirable because they repeat, and therefore often manifest themselves as spurious banding or tiling within the image. (Some curious shapes may appear even in the absence of repetition or other distracting systematic character, but such shapes generally go unnoticed or accepted.)
(a) Rendition—Incremental printers are generally capable of creating only a relatively small number of colors at each picture-element (pixel) position—particularly as compared with the millions of colors that can be developed on a computer or television screen, or the virtually continuous gradations available through photography. To enable incremental printers to simulate the finer gradations provided by such other technologies, workers in the incremental-printing field have developed techniques known as “rendition”.
Prominent rendition methods include scattered dither and error diffusion. Such techniques aim to reproduce midtones of colors by, in effect, averaging colors—over a number of pixels that is relatively large in comparison with just a single pixel, but still rather small as compared with the spatial resolving power of the human eye.
Scattered dither in printed images thus smoothly spreads fine dots on the printing medium in such a way that the average reflected light per unit of surface—taking into account the unprinted space on the medium, between dots, as well as the printed dots—matches the light intensity of the desired tone. It is important to spread the dots smoothly because unevenness in spatial distribution of the dots creates grainy or noisy images that are often objectionable to the viewer. Other methods such as error diffusion can assign locations to dots to be printed without generating repeating patterns—but require much more computing power in the computer or host, or both. Error diffusion, furthermore, although producing no repetitive patterns does often generate systematically propagating patterns. These patterns too can be obtrusive when seen in nominally uniform fields. Error diffusion is therefore most typically reserved for photograph-like pictures or other images having many small details that obscure the propagating patterns.)
Heretofore it has been recognized by many workers in this field that unevenness in dither-type rendition may be associated with particular dither matrices used. Some such efforts are discussed below.
It has also been recognized by many, however, that unevenness may be associated with a separate function known as “printmasking”. Printmasks are used to allocate or distribute the selected dots among successive printhead passes over the printing medium. These efforts too are outlined below.
(b) Dither—As to dither matrices, unevenness in spatial distribution of dots may arise from the quality of the matrices themselves, or from the interaction of such dither matrices with other parts of the writing system that affect the final placement of dots on the print medium. More specifically, consistent dot-placement error (DPE) when combined with the use of dither matrices creates unevenness that repeats consistently throughout an image, creating intrusively unpleasant patterning and banding.
For present purposes it is important to understand precisely how such patterns develop so that they physically appear in a finished, printed image on a physical printing medium. The prime mechanism for development of a pattern is the repetitive recurrence of particular dot-placement errors (DPEs) in the same positions of a particular tiled mask.
DPE is basically a fingerprint of each infinitesimal irregularity of firing direction and speed, and drop volume as well, for each of the many different nozzles in a particular printhead. Image dithering is performed using threshold matrices that provide the spatial distribution of dots to be printed for every tonal value.
Now, when a certain DPE characteristic is registered in a particular way with a certain dither mask (or print-mask), and used to print a midtone field, the unique printhead DPE characteristic and unique mask characteristic in combination produce, potentially, a distinctive set of shapes.
The qualifying term “potentially” is used here because it has not yet been shown how the dither-mask pattern is preserved through the printing process and expresses itself through the printhead DPE pattern to form noticeable patterns on the printing medium. That will be demonstrated shortly. For the moment, to enable further intermediate discussion, that demonstration may be taken on faith.
Since each matrix has limited size, it must be stepped and repeated, over and over in both directions, to entirely fill all the space to be printed with the tone. Thus if the midtone field is uniform or roughly uniform, throughout an area that extends over multiple units of the same mask, then those distinctive shapes appear tiled across and down that area. The resulting appearance is sometimes quite conspicuous, and in extreme cases even distracts from the subject matter of the image.
Accordingly some previous workers have striven to provide dither masks having an ideal degree of randomness. The objective of such work has been to avoid both the appearance of patterns due to excessive regularity within earlier dither masks, on the one hand, and the appearance of graininess due to excessive randomness on the other hand—while at the same time achieving a desired level of vividness in resulting colors. Some advances in this area are due to Qian Lin, Paul Dillinger and Alexander Perumal as reported in their above-mentioned documents.
Such work has been extremely useful and successful in many regards, including virtual elimination of systematic-looking patterns within individual dither masks. These innovations, however, have fallen far short of eliminating larger patterns.
This is because the pattern of an individual mask, even though not itself systematic-looking or even noticeable when considered internally and singly, does look conspicuously systematic when stepped and repeated—in its entirety—multiple times across or down a page. The contorted, random crawling patterns (
FIG. 1
) are sometimes reminiscent of inkblots, gray matter, or, appropriately, a can of worms. They are at least as disturbing as the systematic patterns created with earlier, internally more-regular masks.
The above-mentioned Askeland documents report methods for attacking this repetition of dither-mask/DPE interactions. One of Askeland's techniques provides plural “superpixels” (related to dither masks) and introduces randomness into the selection of the superpixel to be applied at each pixel position.
The other Askeland document defines plural, colorimetrically equivalent tonal levels that can be used in randomized selection of either dither masks or printmasks. Askeland displays results that appear to represent significant improvement.
For purposes of the present invention it is noteworthy that the dimension
Borrell Ramón
Gomez Jordi M.
Viñals Lluís
Ghee Ashanti
Hewlett--Packard Development Company, L.P.
Lippman Peter I.
Williams Kimberly A.
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