Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
2000-11-21
2003-04-08
Nguyen, Thinh (Department: 2861)
Incremental printing of symbolic information
Ink jet
Controller
C347S043000
Reexamination Certificate
active
06543871
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to methods and apparatus for incremental printing of images or text on printing media such as paper, transparency stock, or other glossy media. More particularly, the invention relates to methods and apparatus that construct images or text from individual ink drops deposited on a printing medium in a two-dimensional pixel array.
2. Description of Prior Art
In the prior art of inkjet printers there is an inverse relationship between printed image quality and speed, such that the higher the quality of a printed image the lower the throughput speed, and vice versa. At the same time, there has been a direct correlation of high quality and high speed with high cost. The tradeoff between image quality, throughput speed, and cost has been the fundamental dilemma in design and construction of printers.
An image formed by an inkjet printer is built up incrementally on the surface of a recording medium from a myriad of dots of ink.
Unfortunately, there has been a persistent problem with undesirable visual artifacts which result from the process of such incremental printing.
Many attempts have been made in prior art to minimize or avoid incremental printing artifacts through mask design. It is known throughout the industry that the quality of the masks has an immediate and a very visible effect on quality of the output. In certain industries and markets, such as publishing and graphic design, not having any visible artifact from the way the image was printed, and uniformity of color, is essential. Such industries also are at the cutting edge of the use of large- and wide-format color printers, in which artifacts of incremental printing with masks are particularly noticeable, especially when attempting to achieve photographic-quality printed images.
In general, the more regular and uniform the mask the more the artifacts, so various attempts have been made in prior art to introduce randomness, or at least pseudo-randomness, in generation of masks and in printing with the use of masks. Previously, the improvement available through randomization has been limited because printmasks were only effective at reducing undesirable artifacts within the dimensions of each individual mask. Such printmasks are necessarily replicated horizontally across the image and vertically down the image. The result has often been a repeating pattern artifact resembling a grid that is especially apparent in midtones.
Another repeating pattern artifact is known as “banding,” in which horizontal bands are apparent where printed swaths abut. Although it is known that some banding artifacts can be reduced by printing highly staggered/overlapping bands, doing so reduces overall throughput proportionately.
In addition, to achieve good tonal gradations and/or vivid colors, and to substantially fill the space between addressable pixel locations, sufficient quantities of colorant must be deposited. After being deposited, however, some period of time is generally required for removal of the water or other base from the colorant, for instance by evaporation and, for some print media, absorption. If too much colorant is deposited this “drying” period can be unduly time long. Further, if a large amount of colorant is deposited all at substantially the same time within each section of an image, other related adverse bulk-colorant effects arise. These include so-called “bleeding” of colors into each other (especially noticeable at color boundaries that should be sharp), “cockle” or puckering of the printing medium, “smudging” if the printed image is handled before it is dry, and “blocking” or transfer of colorant from one printed image onto the back of an adjacent sheet. In extreme cases such blocking can cause two sheets to stick together, or one sheet to a slipcover used to protect the imaged sheet, or even one sheet to components of the printer.
Ironically, many attempts to solve banding and liquid-loading problems actually contributed to pattern artifacts. Printmode techniques are all highly systematic and repetitive, and thus vulnerable to patterning defects. For example, some printmodes such as square or rectangular checkerboard-like patterns tend to create objectionable pattern effects when frequencies or harmonics generated within the patterns are close to the frequencies or harmonics of interacting subsystems. Such interfering frequencies may arise in dithering subsystems sometimes used to help control the paper advance or the pen speed.
Pixel structures called “superpixels” have previously been used in the early image-processing stages for various purposes. However, in all known uses of superpixels, the superpixel selection for each situation is repetitive and consistent, not random. The same is true of dither cells, generally used in the rendition stage to produce a random appearance. Their cells, like random printmasks are randomly or pseudo-randomly derived, but once derived are used repetitively, and are therefore prone to periodic and/or pattern defects.
Even with spatial dithering, or for continuous-tone input images error diffusion, heretofore some patterning persists even in images printed under conditions which should yield the highest possible image quality. It has been suggested that in theory such patterning cannot be eliminated through dither or error-diffusion redesign exclusively, and that further improvement in image quality must be sought elsewhere (see, for example, EP 0 718 105 B1, in the name of Yen et al., U.S. patent application Ser. No. 363,447).
The persistent problems outlined above, balanced against pervasive concerns of throughput, reliability and cost have continued to impede optimization of inkjet printing. Repetitive pattern artifacts arising from systematic dot-placement errors, even in the presence of internally randomized printmask patterns, have continued to impede such optimization of high quality inkjet printing at high throughput rates on all printing media for a competitive price. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement. What is needed is a technique for introducing as much randomness as possible in generating masks to minimize artifacts of incremental printing, combined with selection of various constraints to optimize speed for a selected level of quality.
SUMMARY OF THE INVENTION
The invention disclosed herein overcomes the limitations of prior art by providing a pseudo-random approach to mask production, combining random selection of a pass number for printing each pixel of a mask with application of various constraints to optimize quality, throughput speed and reliability. Along with other factors discussed more fully below, speed is optimized by depositing substantially a single drop of ink per pixel, thereby minimizing both the number of passes required to completely render all pixels and the drying time; image quality is optimized by maximizing the time and distance between deposition of individual ink drops, thereby minimizing coalescence, bleeding, cockling, blocking, pen temperature and pen-firing frequency (which affect uniformity and consistency of ink drop size); and, reliability and durability are optimized by maintaining pen-firing frequency and carriage velocity below maximum limits set by manufacturers thereof.
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Mishra Chandan
Rosen Ziv
Electronics for Imaging, Inc.
Nguyen Thinh
LandOfFree
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