Facsimile and static presentation processing – Static presentation processing – Dot matrix array
Reexamination Certificate
1998-03-16
2001-02-06
Coles, Edward L. (Department: 2722)
Facsimile and static presentation processing
Static presentation processing
Dot matrix array
C358S001100, C358S001180, C358S451000, C358S466000, C358S526000, C358S001900
Reexamination Certificate
active
06185002
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to machines and procedures for incremental printing of text or graphics on printing media such as paper, transparency stock, or other glossy media; and most particularly to a scanning thermal-inkjet machine and method that construct images from individual ink spots created on a printing medium, in a two-dimensional pixel array. The invention is also believed applicable to sister technologies such as the hot wax transfer method. The invention employs image-processing—as distinguished from printmode—techniques to optimize image quality, but can be used in conjunction with downstream printmode strategies including random and pseudorandom printmasks.
BACKGROUND OF THE INVENTION
A basic goal for these machines is very high quality in printed images, using a relatively inexpensive printer. This goal is implemented by the devices described below, but at the same time has been obstructed by certain characteristics—also described below—of these same devices.
Incremental printing nowadays is generally accomplished through digital manipulation of image data in one or another type of electronic digital microprocessor. All such manipulation, including the stages discussed below under the conventional designations of “image processing” and “printmasking”, can be performed in a host computer, e. g. in software that operates an attached printer, or can be built into the printer—but most commonly is shared between the two.
For operations performed within the printer, as is now well known, the printer may contain either a general-purpose digital processor running programs called “firmware”, or an application-specific integrated circuit (ASIC) manufactured to perform only specific functions of particular printers. In some cases the printer may use both a firmware subsystem and an ASIC.
Image processing—The fundamental task of all these devices is receiving data representing a desired image and developing from those data specific moment-by-moment commands to a printing mechanism. This task, for purposes of the present document, will be called “image processing”.
Such processing typically includes, at the outset, some form of darkness and contrast control or adjustment. In a color printer, this preprocessing stage analogously also includes color conversions and any needed color corrections. Such preprocessing can handle both user-desired color modification and any known mismatch between an input-image color specification and the operating color space and gamut of the printer.
Next downstream from contrast, darkness and color corrections—and particularly important for images other than text—image processing also includes rendering or rendition techniques (such as dithering of error diffusion). A rendition stage has two principal functions, both directed to making spatial assignments of color ink spots to particular pixels.
First, it attempts to implement the relatively continuous or very fine tonal gradations of a photograph-like image, in terms of the relatively limited number of gradations which a typical inexpensive printer can produce. A digital file in a computer ordinarily is able to represent fine tonal gradations quite accurately, since data formats—although digital—usually allow for at least 256 distinct tonal levels between, for instance, pure white and dead black.
Second, in a color printer, rendition also attempts analogously to implement the relatively huge number of colors which a computer can invoke. Rendition must accomplish this in terms of the relatively limited number of colors which a typical inexpensive printer can produce.
Banding—An obstacle to highest-quality printing is caused by repeating failure of particular elements of the print mechanisms to mark properly—or consistently with other elements. Periodic artifacts arise from constant or repeating errors of inkjet trajectory, pen positioning and speed, and printing-medium positioning and speed.
For instance malfunction or misalignment of a particular inking nozzle or the like can leave a generally consistent white or light pixel row across every image region where that particular element (e. g. nozzle) is supposed to mark. In the case of misalignment, the same problem also produces excess inking across some nearby region where the same element should not be marking.
Image regions are not all equally affected by such defects. The magnitude of banding problems, or more generally dot-placement errors, varies with the tonal level or in other words dot density within an image.
We can define three regions of a tonal ramp, based on the amount of white space:
(1) highlights: These areas have ample white space and to the naked eye exhibit little in the way of banding or other dot-placement artifacts. Such artifacts are of course present, but hard to see—because small differences in dot position can represent only a relatively small fractional change (or none) in the large amount of white space that is seen. Furthermore, because the dots that are present are so far apart, and usually irregularly located, they fail to form a visual frame of reference within which a person can detect placement errors directly.
(2) midtones: These parts of the tonal range are most sensitive to banding because they have small amounts of white space. Dot-placement errors are highly visible because small differences in dot position can have a large effect on how much white space is visible. Coalescence contributes further to the conspicuousness of banding and graininess because dots clump together.
(3) saturated areas: These segments of the tonal range have almost no white space showing through. The large amount of colorant on the printing medium hides dot placement errors—with the exception of print-medium advance problems. Interactions between the colorant and the printing medium, however, can lead to flood banding and coalescence.
As a practical matter, the boundaries of these tonal-range segments depend in part upon the nature of the image being printed, as well as the exact character of the dot-placement errors produced by a particular printhead. Therefore these regions of the tonal ramp can be defined neither sharply nor generally.
For a rule of thumb, however, for purposes of placement-error visibility the midtone region has very roughly more than one single printed dot per four pixels—but, at the saturated end of the range, very roughly more than one single dot subtracted from full coverage, per four pixels. For example in a four-level (including zero) system, since the maximum number of dots in each pixel is three, the maximum inking in four pixels is 3×4=12 and the upper limit of the midtone region is 12−1=11 dots per four pixels.
In other words, the high-visibility range lies above approximately twenty-five percent coverage in single dots, but below approximately twenty-five percent in single dots deducted from the maximum possible inking level. Again, in practice the range defines itself in a functional way and not exactly in numerical terms.
Inking and coalescence—To achieve good tonal gradations and (for color printers) vivid colors, and to substantially fill the white space between addressable pixel locations, ample quantities of colorant must be deposited. Doing so, however, generally requires subsequent removal of the water or other base—for instance by evaporation and, for some print media, absorption—and this drying step can be unduly time consuming.
In addition, if a large amount of colorant is put down all at substantially the same time, within each section of an image, related adverse bulk-colorant effects arise. These include so-called “bleed” of one color into another (particularly noticeable at color boundaries that should be sharp), “cockle” or puckering of the printing medium, and even “blocking” or offset of colorant in one printed image onto the back of an adjacent sheet. In extreme cases such blocking can cause sticking of the two sheets together, or of one sheet to pieces of the printer apparatus.
All these conditions of course—like the banding
Askeland Ronald A.
Doron Amir
Coles Edward L.
Hewlett--Packard Company
Lamb Twyler
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